Water Alarm Schematic

The LM1830 fluid detector IC from National Semiconduc tor is designed to be able to detect the presence of fluids using a probe. This chip requires a relatively high supply voltage and is not the most frugal power consumer. It is also quite specialised so unless you are buying in bulk the one-off price is not cheap.

An alternative circuit show n her e uses a standard CMOS IC type 74HC14. It has the advantage of operating with a 3 V supply and consumes less than 1 µA when the alarm is not sounding, this makes it ideal for use with batteries.

Water Alarm Schematic Circuit Diagram

The 74HC14 has six inverters with hysteresis on their input switching thresholds. A capacitor (C1) and a feedback resistor (R1) is all that’s necessary to make an inverter into a square wave signal generator.

In the water alarm circuit the feedback resistor consists of R1 and the water sensor in series. R1 prevents any possibility of short-circuit between the inverter’s input and out-put. Resistor R2 defines the inverter’s input signal level when the sensor is not in water. Any open-circuit (floating) input can cause the inverter to oscillate and draw more current.The remaining inverters in the package (IC1.B to IC1.F) drive the piezo buzzer to produce an alarm signal. Capacitor C2 ensures that no DC current flows when the circuit is in monitoring mode (with the alarm silent) this helps reduce the supply current.

A micro-switch can also be substituted for the water sensor to make the circuit a more general purpose alarm generator.

Author: Roland Heimann - Copyright: Elektor

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One Transistor Code Lock Schematic

This is of course the simplest electronic code lock circuit one can make. The circuit uses one transistor, a relay and few passive components. The simplicity does not have any influence on the performance and this circuit works really fine.

One Transistor Code Lock Circuit Schematic

The circuit is nothing but a simple transistor switch with a relay at its collector as load. Five switches (S0 to S4) arranged in series with the current limiting resistor R2 is connected across the base of the transistor and positive supply rail. Another five switches (S5 to S9) arranged in parallel is connected across the base of the transistor and ground. The transistor Q1 will be ON and relay will be activated only if all the switches S0 to S4 are ON and S5 to S9 are OFF. Arrange these switches in a shuffled manner on the panel and that it. The relay will be ON only if the switches S0 to S9 are either OFF or ON in the correct combination. The device to be controlled using the lock circuit can be connected through the relay terminals. Transformer T1, bridge D1, capacitor C1 forms the power supply section of the circuit. Diode D2 is a freewheeling diode. Resistor R1 ensures that the transistor Q1 is OFF when there is no connection between its base and positive supply rail.

Notes.

• This circuit can be assembled on a Vero board.
• Switch S1 is the lock’s power switch.
• The no of switches can be increased to make it hard to guess the combination.
• Transistor 2N2222 is not very critical here. Any low or medium power NPN transistor will do the job.

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Simple Light Sensitive Alarm

The circuit detects a sudden shadow falling on the light-sensor and sounds the bleeper when this happens. The circuit will not respond to gradual changes in brightness to avoid false alarms. The bleeper sounds for only a short time to prevent the battery running flat. Normal lighting can be used, but the circuit will work best if a beam of light is arranged to fall on the light-sensor. Breaking this beam will then cause the bleeper to sound. The light sensor is an LDR (light-dependant resistor), this has a low resistance in bright light and a high resistance in dim light.

Simple Light Sensitive Alarm Circuit Diagram

The light-sensitivity of the circuit can be adjusted by varying the 100k preset.

The length of bleep can be varied from 0.5 to 10 seconds using the 1M preset.

Using the 7555 low-power timer ensures that the circuit draws very little current (about 0.5mA) except for the short times when the bleeper is sounding (this uses about 7mA). If the circuit is switched on continuously an alkaline PP3 9V battery should last about a month, but for longer life (about 6 months) you can use a pack of 6 AA alkaline batteries.

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555 Timer Travel Touch Alarm

The Travel Touch Alarm can be used to provide a audible alarm  when someone touches the door knob or handle of your hotel room. The door knob or handle must be made of metal for the circuit to work. The main chip in the circuit is a 555 timer which will be triggered if a hand comes close to or touches the door knob.

555 Timer Travel Touch Alarm Circuit Diagram

The circuit attaches to the door knob at the end of the 1 meg ohm resistor. Once the timer is triggered the LED will light and the UJT will output a tone to the speaker. The timer will time out in 5 seconds. The sensitivity of the trigger can be changed by changing the 1 meg ohm resistor to another value. The 5 second time out can be adjusted by changing the value of the resistor connected between pin 8 and pin 7. The output tone can be changed by changing the RC values on the base of the UJT.

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Tent Alarm

Although this alarm is designed to protect valuables left in a tent, it can also be used as a baggage alarm (either on or in one‘s bags) and in similar situations.

The tent alarm can be triggered by many different sensors. One is a current loop, connected to pin PB4 of an ATtiny13 micro-controller: this could be a thin wire which would be broken by a prospective pilferer caught off his guard. Alternatively, it can be a reed switch contact normally held closed by a magnet, arranged so that the budding burglar will accidentally move the magnet and thus open the contact. This could be used to protect a door or a zip fastener securing a tent.

Another sensor connected to PB4 is an LDR (light dependent resistor). If the LDR is left in a dark place (such as under a sleeping bag) the thief will trigger the alarm if he moves the bag to expose the sensor to light. The resistance of the LDR is about 100 kΩ in the dark and just a few ohms in the light. If only the light sensor is to be used, the alarm wire (or reed contact) socket can be shorted using a jumper. If the LDR is not to be used, it can either be (temporarily) taped over to exclude light from it or (more permanently) replaced by a 100 kΩ resistor.

Tent Alarm Circuit Diagram:

A third sensor which can trigger the alarm is a vibration detector (S6), which is wired in series with a tilt sensor. The tilt sensor allows the vibration sensor to be disabled when the alarm unit is left upside-down. When the tilt sensor contacts are open, PB1 cannot be pulled low and so no alarm can be triggered.

The unit also features a number of push-buttons and switches connected to PB2. The arrangement and labelling of these buttons and switches is described below.On the left of the device lies switch S1 with the (deliberately misleading) legend ‘Power on/off ’. Of course, this does not turn the alarm on and off. On the right of the device is switch S2 with the legend ‘Speaker on/off ’, which, naturally, does nothing of the sort. As you have probably already guessed, the red and green but-tons also have nothing to do with arming or disarming the alarm.

These decoys should be enough to annoy and delay all but the most resourceful of robbers. Naturally, once the alarm has been triggered by uncovering the LDR, it will not turn off again if the LDR is then covered.The only way to disable the alarm is to set S1 and S2 in the correct positions (namely, ‘Power on’ and ‘Speaker on’) and hold down the two buttons simultaneously for five seconds. More complicated deactivation procedures can be programmed into the software, in case you are worried that some light-fingered Elektor reader (not that such a person exists) will be able to steal your valuables after having seen this article.The circuit requires a supply voltage of between 3.6 V and 5 V. In the circuit diagram we show a power supply made using a 9 V battery and a 5 V voltage regulator.

The ATtiny13 microcontroller belongs to Atmel’s AVR family, and can be programmed using BASCOM. Source and object code files, including fuse settings, are available in a ZIP archive that is available for free download from the Elektor website. The source code can be modified to suit your own application and then recompiled using the free version of BASCOM. The software arranges matters so that the processor enters sleep or power-down mode when the alarm is correctly deactivated; there is no other way to turn the device off. To wake the device up the switches must be set correctly (both to ‘on’) and the unit shaken briefly. The LED blinks twice to con-firm that the device has woken up; after a brief delay of approximately three seconds the alarm is armed. This state is indicated by three flashes of the LED. While the alarm remains in the armed state the LED blinks briefly once every few seconds.

When the alarm is triggered the red LED lights immediately. If it is not disarmed, the alarm sounds after a short pause.

To disarm the unit, both switches again need to be in the ‘on’ position as described above and both buttons must be pressed. After a double flash, whether the LED is on or off indicates whether the buttons must then be pressed again or not.

Author :Stefan Hoffmann - Copyright: Elektor

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Simple Electronic Water Alarm

Aburst water-supply hose of the washing machine, a bathroom tap that you forgot to close, or a broken aquarium wall may turn your house into a pond. You can avoid this mess by using an electronic water alarm that warns you of the water leakage as soon as possible.

The acoustic water alarm circuit presented here takes advantage of the fact that the tap water is always slightly contaminated (or has salts and minerals) and thus conducts electricity to a certain extent. It is built around IC LMC555 (IC1), which is a CMOS version of the bipolar 555 timer chip. IC1 is followed by a complementary pair of emitter followers (T1 and T2) to drive a standard 8-ohm speaker (LS1). Power is supplied by a compact 9V PP3 battery.

 Simple Electronic Water Alarm Circuit Diagram :

 
Power is applied when power switch S1 is closed. The reset input (pin 4) of IC1 is held low by resistor R1 (2.2-kilo-ohm). The astable oscillator wired around IC1 is in disabled mode. When probes P1 and P2 become wet, these conduct to reverse the state of IC1’s reset terminal. As a result, the astable multivibrator starts oscillating at a frequency determined by resistor R2 and capacitor C3. The output of IC1 drives the complementary pair of transistors T1 and T2.

Although this combination causes significant crossover distortion, it doesn’t have any adverse effect on the square-wave audio signal processing. A 10-kilo-ohm potentiometer (VRI) is inserted between output pin 3 of IC1 and the bases of transistors T1 and T2 for volume control. The probes can be made using two suitable copper needles or small pieces of circuit board with the copper surface coated with solder. Fit these at the lowest point where water will accumulate. After construction, place the alarm circuit well away from the point of possible leakage. Use a pair of thin twisted flexible wires to connect the probes to the circuit.

Capacitor C1 connected across IC1 input (pin 4 and GND) keeps the alarm circuit from responding to stray electrostatic fields. Similarly, twisting the wires together makes the relatively long connection between the probes and the circuit less sensitive to false alarms due to external electromagnetic interference. Finally, if you want to lower the probe sensitivity, reduce the value of grounding resistor R1.

Author : T.K. Hareendran - Copyright : EFY

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Small Personal Alarm

Small, portable, anti-bag-snatching unit. Also suitable for doors and windows control

This circuit, enclosed in a small plastic box, can be placed into a bag or handbag. A small magnet is placed close to the reed switch and connected to the hand or the clothes of the person carrying the bag by means of a tiny cord. If the bag is snatched abruptly, the magnet looses its contact with the reed switch, SW1 opens, the circuit starts oscillating and the loudspeaker emits a loud alarm sound.

The device can be reverse connected, i.e. the box can be placed in a pocket and the cord connected to the bag. This device can be very useful in signalling the opening of a door or window: place the box on the frame and the magnet on the movable part in a way that magnet and reed switch are very close when the door or window is closed.

Personal Alarm Circuit Diagram :

Personal Alarm Circuit Diagram

Circuit operation:

A complementary transistor-pair is wired as a high efficiency oscillator, directly driving a small loudspeaker. Low part count and 3V battery supply allow a very compact construction.

Parts:

R1____________330K   1/4W Resistor
R2____________100R   1/4W Resistor

C1_____________10nF   63V Polyester or Ceramic Capacitor
C2____________100µF   25V Electrolytic Capacitor

Q1____________BC547   45V 100mA NPN Transistor
Q2____________BC327   45V 800mA PNP Transistor

SW1____________Reed Switch and small magnet (See Notes)

SPKR___________8 Ohm Loudspeaker (See Notes)

B1_____________3V Battery (two A or AA cells wired in series etc.)

 Notes:

• The loudspeaker can be any type, its dimensions are limited only by the box that will enclose it.
• An on-off switch is unnecessary because the stand-by current drawing is less than 20µA.
• Current consumption when the alarm is sounding is about 100mA.
• If the circuit is used as anti-bag-snatching, SW1 can be replaced by a 3.5mm mono Jack socket and the magnet by a 3.5mm. mono Jack plug having its internal leads shorted. The Jack plug will be connected to the tiny cord etc.
• Do not supply this circuit at voltages exceeding 4.5V: it will not work and Q2 could be damaged. In any case a 3V supply is the best compromise.

Source : www.redcircuits.com

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5-Zone Alarm System

Each zone uses a normally closed contact. These can be micro switches or standard alarm contacts (usually reed switches). Zone 1 is a timed zone which must be used as the entry and exit point of the building. Zones 2 - 5 are immediate zones, which will trigger the alarm with no delay. Some RF immunity is provided for long wiring runs by the input capacitors, C1-C5. C7 and R14 also form a transient suppresser.

 5-Zone Alarm System Circuit Diagram : 

The key switch acts as the Set/Unset and Reset switch. For good security this should be the metal type with a key. At switch on, C6 will charge via R11, this acts as the exit delay and is set to around 30 seconds. This can be altered by varying either C6 or R11. Once the timing period has elapsed, LED6 will light, meaning the system is armed. LED6 may be mounted externally (at the bell box for example) and provides visual indication that the system has set.

Once set any contact that opens will trigger the alarm, including Zone 1. To prevent triggering the alarm on entry to the building, the concealed re-entry switch must be operated. This will discharge C6 and start the entry timer. The re-entry switch could be a concealed reed switch, located anywhere in a door frame, but invisible to the eye. The panic switch, when pressed, will trigger the alarm when set. Relay contacts RLA1 provide the latch, RLA2 operate the siren or buzzer.
 

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Simple Security Alarm

Thwart any attempt of burglary in your house using this alarm circuit. When someone opens the door of your room, it sounds an alarm intermittently and flashes light as well. The circuit can also be used as an audio/visual alarm in case of fire or other emergency by momentarily pressing switch S3.

The circuit (refer Fig.1) is built around transformer X1, a standard bar magnet, reed switch S2, timer IC NE555 (IC1), opto-coupler IC MOC3020 (IC2), TRIAC BT136 and a few discrete components. Timer IC1 is wired as an astable multi-vibrator whose reset pin 4 is controlled by the reed switch. The reed switch fitted in the door frame acts as the sensor. A magnet is fixed on the door panel close to the reed switch.

Fig.1: Simple Security Alarm Circuit Diagram : 

The reed switch consists of a pair of contacts on ferrous metal sealed in a glass envelope. The contacts may be normally-open (which close when a magnetic field is present) or normally closed type (which open when a magnetic field is applied). A normally open- type reed switch is used here.

When the door is closed, reed switch S2 is in open state. When the door is opened, the bar magnet moves away from reed switch S2. As a result, reset pin 4 of IC NE555 goes high. The high output at pin 3 of IC1 enables IC2. Pin 4 of IC2 is connected to the gate of TRIAC1. When the door is opened, bulb B1 flashes and the bell sounds (provided switch S4 is closed) indicating that the door has been opened. Flashing of the bulb and the alarm continue until the door is closed.

Assemble the circuit on a general purpose PCB and enclose in a suitable cabinet. Connect the call bell at the back side and the bulb at the front side of the cabinet. Install the unit on the door of the room as shown in Fig.2.

Fig.2: Reed Switch Fitting in Door :

The circuit is powered by mains supply.

Author :S.C. Dwivedi - Copyright : EFY

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Car Alarm Sound Booster

For car alarms, emphasis should be put on hearing the audible alert and identifying it as belonging to your ‘wheels’. Unfortunately, modern car alarm systems seem to have more or less the same alarm sound especially if they are from the same brand. Also, to comply with legal noise restrictions, the alarm sound is not always loud enough to be heard if the car is parked down the road.

The circuit shown here is designed to help boost the alarm sound by also activating the car’s horn(s) when the alarm goes off.Internally the car alarm system often provides a signal that activates the (optional) engine immobilizer and/or volume (ultrasound) sensors. This signal usually goes Low upon system triggering and high again when the alarm system is deactivated.

Car Alarm Sound Booster Circuit Diagram :

The alarm activation signal is fed to the circuit through D1. When in idle state, T1’s gate is High and consequently the FET conducts, keeping power FET T2 firmly switched of f. When the system gets an active low signal, T1 switches of f allowing timing capacitor C2 to charge via R2. About 15 seconds later, when the voltage across C2 is high enough, T2 starts to conduct and relay RE1 is energized. This, in turn, provides the required path for the ‘lights flashing’ signal to energize RE2 and feed battery power to the car’s horn(s).

When the alarm system is turned off the activation signal returns to High. T1 starts to con-duct and rapidly discharges C2 via R3. T2 is then cut off and RE1 is de-energized. Diode D2 suppresses back EMF from RE1.The circuit draws less than 2 mA when idling. When activated the circuit’s current consumption is virtually that of the RE1 coil.RE1 is any simple SPST or SPDT relay, capable of switching about 0.5 A (at 12 V). The coil rating is for 12 VDC and a current requirement as low as you can find. Fuse F1 should be a slow blow type and rated about twice RE1’s coil current.

The BS170 in position T2 can sink a continuous current of about 0.5 A. However, a value of 1.2 A pulsed is specified by Fairchild for their devices. To keep the FET’s d-s current due to C2 discharging within safe limits, R2 may be increased, C2 decreased and R3 increased, all proportionally. A factor of 2 will keep the FET out of harm’s way with maybe a slight change in the 15-second delay and the sensitivity of the circuit.C1 is used as a smoothing capacitor and F2 should be rated in accordance with the horn(s) maximum current draw.

Caution.The installation and use of this circuit may be subject to legal restrictions in your country, state or area.

Author : Hagay Ben-Elie - Copyright : Elektor

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Simple Security Monitor

A remote listening circuit. The area to be monitored is connected via a cable and allows remote audio listening.

Starting from the right hand side, the power supply. I have used 12V as a standard power supply voltage, or a 12V car battery may be used. The circuit is in two halves, a remote microphone preamp, and an audio amplifier based around the National Semiconductor LM386 audio amplifier.

Security Monitor Circuit Diagram :

The remote preamp uses an ECM microphone to monitor sound. A direct coupled 2 stage amplifier built around Q1 and Q2 amplify the weak microphone signal. Preset resistor R2 acts as a gain control, and C1 provides some high frequency roll off to the overall audio response. Q1 is run at a low collector current for a high signal to noise ratio, whilst Q2 collector is biased to around half the supply voltage for maximum dynamic range. The power supply for this preamp is fed via R10 and R6 from the 12V supply. C4 ensures that the preamp power supply is decoupled and no ac voltages are present on the power lines. The amplified audio output from Q2 collector is fed onto the supply lines via C6 a 220u capacitor. The output impedance of Q2 is low, hence the relatively high value of C6. C6 also has a second purpose of letting the output audio signals pass, whilst blocking the dc voltage of the power supply.

At the opposite end, C7 a 10u capacitor, brings home the amplified audio to the listening location. The signal is first further amplifier by a x10 voltage gain amplified using the TL071. C8, a 22p capacitor again rolls off some high frequency response above 100kHz. This is necessary as long wires may pick up a little radio interference. After amplification by the op-amp, the audio is finally passed to the LM386 audio amplifier. R14 acts as volume control. R13 and C12 prevent possible instability in the LM386 and are recommended by the manufacturer. Audio output is around 1 watt into an 8 ohm loudspeaker, distortion about 0.2%. If preferred headphones could be used, although I'd recommend a series resistor of the same value impedance as the headphones.

Notes:

You can use this in your garden and listen for any unusual sounds, or maybe just wildlife noises. If you have a car parked in a remote location, the microphone will also pick up any sounds od activity in this area. The cable may be visible or hidden, screened cable is not necessary and you can use bellwire or speaker cable if desired.

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Security Door Electronic Key

It is a relatively simple circuit of electronic lock of safety with code of 7 digits. It should is given attention in the time that will be stepped the keys, that shape code and it does not exist it delays. With the right step of keys and if code is right then is activated exit Q7 for roughly 4 seconds, driving the transistor Q2, which with the line can drive one relay, for the opening of door, or any other circuit.

Circuit diagram :

Security Door Electronic Key Circuit Diagram

With LED D we can have optical clue of activation. The code of circuit, as it has been given have been:1704570 but can change, if we change the connections between in the exits of IC1 and the switches.

Parts List :

• R1-7=4.7Kohm
• R8=15Kohm
• R9=1Mohm
• R10-13=10Kohm
• R11=100ohm
• R12=220Kohm
• R14=1.2Kohm
• C1-3=100nF 100V
• C2=4.7uF 25V
• D1-2=1N4148
• D3=RED LED 3mm
• IC1=4022
• Q1=BS170
• Q2=BD679
• S1-10=Push button or keyboard

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Best Automatic Car Alarm

Even the best car alarm is useless if you forget to set it upon leaving your car, whence this circuit. The relay has a make and a break contact: the  former is necessary to delay the switching in of the  alarm after you have got out of your car, and the  latter serves to switch on the car alarm proper. Immediately on re-entering your car, you must press the hidden switch, Si. This causes silicon-controlled rectifier Thi to conduct so that the relay is energized. At the same time, the green LED lights to indicate that the alarm is switched off. 

Circuit diagram :

 Best Automatic Car Alarm Circuit Diagram

As soon as the ignition is switched off, T, is off, T2  is on, and the buzzer sounds. At the same time,  monostable IC1 is triggered, which causes T3 to  conduct and the red LED to light. The silicon- controlled rectifier is then off, and D4 is reverse  biased, but the relay remains energized via its make  contact for a short time, preset by Pi As soon as this  time has lapsed, the relay returns to its quiescent  state, and the alarm is set via the break contact. The  delay time can be set to a maximum of about 1 minute.

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Current Monitor And Alarm Circuit

These circuits are intended for remote monitoring of the current consumption on the domestic mains line.

The circuit in Fig. I lights the signal lamp upon detecting a mains current consumption of more than 5 mA, and handles currents of several amperes with appropriate diodes fitted in the D, and D2 positions. Transistor Ti is switched on when the drop  across D,-D2 exceeds a certain level. Diodes from  the well-known I N400x series can be used for currents of up to I A, while lN540x types are rated for up to 3 A. Fuse F, should, of course, be dimensioned to suit the particular application.

A number of possible transistor types have been stated for use in the Ti position. Should you consider using a type not listed, be sure that it can cope with surges up to 700 V. As long as Ti does not con- duct, the gate of the triac is at mains potential via  C,, protective resistor R2 and diode Da, which  keeps C, charged. When Ti conducts, alternating current can flow through the capacitor, and the triac is triggered, so that Lai lights.

The circuit in Fig. 2 is a current triggered alarm. Rectifier bridge D4-D7 can only provide the coil voltage for Re, when the current through Di-D2 exceeds a certain level, because then series capacitor C, passes the alternating mains current. Capacitor C, may need to be dimensioned otherwise than shown to suit the sensitivity of the relay coil. This is readily effected by connecting capacitors in parallel until the coil voltage is high enough for the relay to operate reliably.

Finally, an important point: Many points in these circuits are at mains potential and therefore extremely dangerous to touch.

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Schematic Security System Switcher

An audio signal can be used as a form of input to control any security system. For example, an automatic security camera can be configured to respond to a knock on the door. The circuit described here allows the security system to automatic in on state. It uses a transducer to detect intruders and a 5V regulated DC power supply provides power to the circuit.

As shown in Fig. 1, a condenser microphone is connected to the input of small signal Pre- amplifier built around transistor T1. Biasing resistor R1 determines to a large extent the microphone sensitivity. A microphone usually has an internal FET which requires a bias voltage to operate. The sound picked up by the microphone is amplified and fed to input pin 2 of IC1 (LMC555) wired in monostable configuration.

Circuit diagram :

Fig. 1: Schematic Security system switcher Circuit Diagram

IC2 (CD4538B) is a dual, precision monostable multivibrator with independent trigger and reset controls. The output of IC1 is connected to the first trigger input pin 4 of IC2(A) through switch S1. If an intruder opens or breaks the door, IC1 is triggered by sound signals; the timer output pin 3 of IC1 goes high and enables first monostable multivibrator IC2(A). IC2(A) provides a time period of around 5 to 125 seconds, which is adjusted with preset VR1.

Another monostable multivibrator IC2(B) also provides a time period of around 25 to 600 seconds, which is adjusted with preset VR2. The output of IC2(B) is used to energise relay RL1. Indicator LED1 is provided to display the relay activity. Any AC/DC operated security gadget is activated or deactivated through a security switch. Thus, the security switch of the gadget is connected in the n/o contacts of the relay.You can also operate high power beacons, sirens or hooters in place of the security switch for any AC/DC operated security gadget.

Fig. 2: Proposed cabinet

Assemble the circuit on a general-purpose PCB and enclose it in a cabinet as shown in Fig. 2 along with 5V adaptor for powering the circuit. Connect the security switch according to the circuit diagram and use appropriate AC/DC power supply required to operate the security gadget.

Warning! All relevant electrical safety precautions should be taken when connecting mains power supply to the relay contacts. With the help of single pole double throw (SPDT) switch S1, internal or external trigger input (active high signal) can be selected.

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Car Anti-Theft Wireless Alarm

This FM radio-controlled anti-theft alarm can be used with any vehicle having 6- to 12-volt DC supply system. The mini VHF, FM transmitter is fitted in the vehicle at night when it is parked in the car porch or car park. The receiver unit with CXA1019, a single IC-based FM radio module, which is freely available in the market at reasonable rate, is kept inside.

Receiver is tuned to the transmitter's frequency. When the transmitter is on and the signals are being received by FM radio receiver, no hissing noise is available at the output of receiver. Thus transistor T2 (BC548) does not conduct. This results in the relay driver transistor T3 getting its forward base bias via 10k resistor R5 and the relay gets energised.

When an intruder tries to drive the car and takes it a few metres away from the car porch, the radio link between the car (transmitter) and alarm (receiver) is broken. As a result FM radio module gene-rates hissing noise. Hissing AC signals are coupled to relay switching circuit via audio transformer. These AC signals are rectified and filtered by diode D1 and capacitor C8, and the resulting positive DC voltage provides a forward bias to transistor T2.

Thus transistor T2 conducts, and it pulls the base of relay driver transistor T3 to ground level. The relay thus gets de-activated and the alarm connected via N/C contacts of relay is switched on. If, by chance, the intruder finds out about the wireless alarm and disconnects the transmitter from battery, still remote alarm remains activated because in the absence of signal, the receiver continues to produce hissing noise at its output. So the burglar alarm is fool-proof and highly reliable. (Ed: You may have some problem catching the thief, though, if he decides to run away with your vehicle_in spite of the alarm!)

Copyright : EFY

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Simple Low-Cost Digital Code Lock

Many digital code lock circuits have been published in this magazine. In those circuits a set of switches (conforming to code) are pressed one by one within the specified time to open the lock. In some other circuits, custom-built ICs are used and positive and negative logic pulses are keyed in sequence as per the code by two switches to open the lock.

Circuit diagram :

Simple Low-Cost Digital Code Lock Circuit Diagram

A low-cost digital code lock circuit is presented in this article. Here the keying-in code is rather unique. Six switches are to be pressed to open the lock, but only two switches at a time. Thus a total of three sets of switches have to be pressed in a particular sequence. (Of these three sets, one set is repeated.) The salient features of this circuit are:

1. Use of 16 switches, which suggests that there is a microprocessor in-side.

2. Elimination of power amplifier transistor to energise the relay.

3. Low cost and small PCB size.

An essential property of this electronic code lock is that it works in monostable mode, i.e. once triggered, the output becomes high and remains so for a period of time, governed by the timing components, before returing to the quiescent low state. In this circuit, timer IC 555 with 8 pins is used. The IC is inexpensive and easily available. Its pin 2 is the triggering input pin which, when held below 1/3 of the sup-ply voltage, drives the output to high state. The threshold pin 6, when held higher than 2/3 of the supply voltage, drives the output to low state. By applying a low-going pulse to the reset pin 4, the output at pin 3 can be brought to the quiescent low level. Thus the reset pin 4 should be held high for normal operation of the IC.

Three sets of switches SA-SC, S1-S8 and S3-S4 are pressed, in that order, to open the lock. On pressing the switches SA and SC simultaneously, capacitor C3 charges through the potential divider comprising resistors R3 and R4, and on releasing these two switches, capacitor C3 starts discharging through resistor R4. Capacitor C3 and resistor R4 are so selected that it takes about five seconds to fully discharge C3.

Depressing switches S1 and S8 in unison, within five seconds of releasing the switches SA and SC, pulls pin 2 to ground and IC 555 is triggered. The capacitor C1 starts charging through resistor R1. As a result, the output (pin 3) goes high for five seconds (i.e. the charging time T of the capacitor C1 to the threshold voltage, which is calculated by the relation T=1.1 R1 x C1  seconds).

Within these five seconds, switches SA and SC are to be pressed momentarily once again, followed by the depression of last code-switch pair S3-S4.

These switches connect the relay to out-put pin 3 and the relay is energised. The contacts of the relay close and the solenoid pulls in the latch (forming part of a lock) and the lock opens. The remaining switches are connected between reset pin 4 and ground. If any one of these switches is pressed, the IC is re-set and the output goes to its quiescent low state. Possibilities of pressing these reset switches are more when a code breaker tries to open the lock.

LED D5 indicates the presence of power supply while resistor R5 is a cur-rent limiting resistor.

The given circuit can be recoded easily by rearranging connections to the switches as desired by the user.

 

Author : A. JEYABAL – Copyright : EFY

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Cheap Bicycle Alarm Schematics Circuit

The author wanted a very cheap and simple alarm for some of his possessions, such as his electrically assisted bicycle. This alarm is based on a cheap window alarm, which has a time-switch added to it with a 1-minute time-out. The output  pulse of the 555 replaces the reed switch in the window alarm. The 555 is triggered by a sensor mounted near the front  wheel, in combination with a magnet that is mounted on the spokes. This sensor and the magnet were taken from a cheap bicycle computer.

Circuit diagram :

Cheap Bicycle Alarm Circuit Diagram

The front wheel of the bicycle is kept unlocked, so that the reed  switch closes momentarily when the wheel turns. This  triggers the 555, which in turn activates the window alarm. The circuit around the 555 takes very little current and can  be powered by the batteries in the window alarm.  There  is just enough room  left inside the enclosure of the window  alarm to mount the time-switch inside it.

The result is a very cheap, compact device, with only a single cable going to the reed switch on the front wheel. And the noise this thing produces is just unbelievable! After about one minute the noise stops and the alarm goes back into standby mode. The bicycle alarm should be mounted in an inconspicuous place, such as underneath the saddle, inside a (large) front light, in the battery compartment, etc.

Hopefully the alarm scares any potential thief away, or at least it makes other members of the public aware that something isn't quite right.

Caution. The installation and use of this circuit may be subject to legal restrictions in your country, state or area.

 

Author : Gerard Seuren – Copyright : Elektor

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Little Door Guard

If some intruder tries to open the door of your house, this circuit sounds an alarm to alert you against the attempted intrusion. The circuit (Fig. 1) uses readily available, low-cost components. For compactness, an alkaline 12V battery is used for powering the unit. Input DC supply is further regulated to a steady DC voltage of 5V by 3-pin regulator IC 7805 (IC2).

Fig. 1: Circuit of the door guard

Assemble the unit on a general-purpose PCB as shown in Fig. 4 and mount the same on the door as shown in Fig. 3. Now mount a piece of mirror on the door frame such that it is exactly aligned with the unit. Pin configurations of IC UM3561 and transistors 2N5777 and BC547 are shown in Fig. 2.

Fig. 2: Pin configurations of UM3561 and transistors 2N5777 and BC547

Initially, when the door is closed, the infrared (IR) beam transmitted by IR LED1 is reflected (by the mirror) back to phototransistor 2N5777 (T1). The IR beam falling on phototransistor T1 reverse biases npn transistor T2 and IC1 does not get positive supply at its pin 5. As a result, no tone is produced at its output pin 3 and the loudspeaker remains silent. Resistor R1 limits the operating current for the IR LED.

When the door isopened, the absence of IR rays at phototransistor T1 forward biases npn transistor T2, which provides supply to  positiveIC1. Now 3-sirensound generator IC UM3561 (IC1) gets power via resistor R5. The output of IC1 at pin 3 is amplified by Darlington-pair transistors T3 and T4 to produce the alert tone via the loudspeaker.

Fig. 3: Back view of the door assembly

Rotary switch S2 is used to select the three preprogrammed tones of IC1. IC1 produces fire engine, police and ambulance siren sounds when its pin 6 is connected to point F, P or A, respectively.

Fig. 4: Suggested enclosure with major components layout

 

Author : T.K. Hareendran - Copyright : EFY

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Simple Car Alarm Sound Booster

For car alarms, emphasis should be put on hearing the audible alert and identifying it as belonging to your 'wheels'. Unfortunately, modern car alarm systems seem to have more or less the same alarm sound especially if they are from the same brand. Also, to comply with legal noise restrictions, the alarm sound is not always loud enough to be heard if the car is parked down the road.

Circuit diagram :

Simple Car Alarm Sound Booster Circuit Diagram

The circuit shown here is designed to help boost the alarm sound by also activating the car's horn(s) when the alarm goes off. lnternally the car alarm system often provides a signal that activates the (optional) engine immobilizer and/or volume (ultrasound) sensors. This signal usually goes Low upon sys-tem triggering and high again when the alarm system is deactivated.

The alarm activation signal is fed to the circuit through Dl . When in idle state, T1 's gate is High and consequently the FET conducts,  keeping  power  FET T2 firmly switched off. When the  system gets an  active  low signal, T1  switches  off allowing  timing  capacitor C2 to charge  via  R2. About 15 seconds later, when the voltage across C2 is high enough, T2 starts to conduct and relay RE1 is energized. This, in turn, provides the required path for the 'lights flashing' signal to energize RE2 and feed battery power to the car's horn(s).

When the alarm system is turned off the activation signal returns to High. T1 starts to conduct and rapidly discharges C2 via R3. T2 is then cut off and REl is de-energized. Diode D2 suppresses back EMF from REl.

The circuit draws less than 2 mA when idling. When activated the circuit's current consumption is virtually that of the RE1 coil. RE1 is any simple SPST or SPDT relay, capable of switching  about 0.5 A (at 12 V). The coil rating is for 12 VDC and a current requirement as low as you can find. Fuse F1 should be a slow blow type and rated about twice RE1's coil current.

The B5.170 in position T2 can sink a continuous current of about 0.5  A. However, a value of 1.2 A pulsed is specified by Fairchild  for their devices. To keep the FET's d-s current due to C2 discharging within safe limits, R2 may be increased, C2 decreased and R3 increased, all proportionally. A factor of 2 will keep the FET out of harm's way with maybe a slight change in the 15-second delay and the sensitivity of the circuit. C1 is used as a smoothing capacitor and F2 should be rated in accordance with the horn(s) maximum current draw.

Caution. The installation and use of this circuit may be subject to legal restrictions in your country, state or area.

Author : Hagay Ben-Elie - Copyright : Elektor

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