Speed-limit Alert Circuit

Here’s a design circuit that can be used to alert the vehicle driver that he/she has reached the maximum fixed speed limit (i.e. in a motorway). It eliminates the necessity of looking at the tachometer and to be distracted from driving. There is a strict relation between engine's RPM and vehicle speed, so this device controls RPM, starting to beep and flashing a LED once per second, when maximum fixed speed is reached. Here’s the figure of the circuit;

In this circuit, IC1 forms a differential amplifier for the electromagnetic pulses generated by the engine sparking-plugs, picked-up by sensor coil L1. IC2A further amplifies the pulses and IC2B to IC2F inverters provide clean pulse squaring. The monostable multivibrator IC3A is used as a frequency discriminator, its pin 6 going firmly high when speed limit (settled by R11) is reached. IC3B, the transistors and associate components provide timings for the signaling part, formed by LED D5 and piezo sounder BZ1. D3 introduces a small amount of hysteresis.

D1 is necessary at set-up to monitor the sparking-plugs emission, thus allowing to find easily the best placement for the device on the dashboard or close to it. After the setting is done, D1 & R9 can be omitted or switched-off, with battery savings. During the preceding operation R8 must be adjusted for better results. The best setting of this trimmer is usually obtained when its value lies between 10 and 20K. You must do this first setting when the engine is on but the vehicle is stationary. The final simplest setting can be made with the help of a second person. Drive the vehicle and reach the speed needed. The helper must adjust the trimmer R11 until the device operates the beeper and D5.

Reducing vehicle's speed the beep must stop. L1 can be a 10mH small inductor usually sold in the form of a tiny rectangular plastic box. If you need an higher sensitivity you can build a special coil, winding 130 to 150 turns of 0.2 mm. enameled wire on a 5 cm. diameter former (e.g. a can). Extract the coil from the former and tape it with insulating tape making thus a stand-alone coil. Current drawing is about 10mA. If you intend to use the car 12V battery, you can connect the device to the lighter socket. In this case R20 must be 330R. Depending on the engine's cylinders number, R11 can be unable to set the device properly. In some cases you must use R11=200K and R12=100K or less. If you need to set-up the device on the bench, a sine or square wave variable generator is required. To calculate the frequency relation to RPM in a four strokes engine you can use the following formula:

Hz= (Number of cylinders * RPM) / 120. For a two strokes engine the formula is: Hz= (Number of cylinders * RPM) / 60. Thus, for a car with a four strokes engine and four cylinders the resulting frequency @ 3000 RPM is 100Hz. Temporarily disconnect C2 from IC1 pin 6. Connect the generator output across C2 and Ground. Set the generator frequency to e.g. 100Hz and trim R11 until you will hear the beeps and LED D5 will start flashing. Reducing the frequency to 99 or 98 Hz, beeping and flashing must stop.

Modular Burglar Alarm Circuit

In this figure is design circuit for modular burglar alarm circuit. This circuit features automatic Exit and Entry delays and a timed Bell Cut-off. It has provision for both normally-closed and normally-open contacts, and a 24-hour Personal Attack/Tamper zone. This circuit is connected permanently to the 12-volt supply and its operation is “enabled” by opening SW1. By using the expansion modules, you can add as many zones as you require; some or all of which may be the inertia (shock) sensor type.

All the green LEDs should be lighting before you open SW1. You then have up to about a minute to leave the building. As you do so, the Buzzer will sound. It should stop sounding when you shut the door behind you. This indicates that the Exit/Entry loop has been successfully restored within the time allowed. When you re-enter the building you have up to about a minute to move SW1 to the off position.

If SW1 is not switched off in time, the relay will energize and sound the main bell. It will ring for up to about 40 minutes. But it can be turned off at any time by SW1.However, it’s much easier to find a fault when the alarm is divided into zones and the control panel can remember which zone has caused the activation.

Zone Alarm 6 with Seven Segment Circuit

This circuit is a circuit diagram of an alarm system which has 6 independent zones, timed 1 entry / exit zone, a 7-segment LED display. Suitable for small office or home environment, can also be adapted to use a combination lock or keypad to set and reset the alarm. Each zone Z1 to 6 have their own indicators. Switch S1 is a single pole, double throw switch. One position is set, the other is reset / unset. This is the figure of the circuit;

Switch S2 allows a “manual” test to test all zones and screens. Zone 1 has been independent and out of time. Zone 1 is a timed zone which must be used as a point of entry and exit from the building. Straight 2-6 zone is the zone, which will trigger the alarm without delay. Some RF immunity provided for long wiring run by the input capacitors, C1 – C6. Key switches, S1 acts as the Set and Reset / unset switches. For the best security to this type of metal with a key switch. All IC’s except IC6 is a type of CMOS buffered output, is denoted by the suffix “B”. Unbuffered CMOS IC that has a suffix beginning “U” and will not work in this series. IC6 is a 5 volt regulator provides power to the main CMOS IC’s.

In operation S2 is the switch can be set to “run” position. When keyswitch S1 is restarted, this is unset (off) state of alarm. In this condition the capacitor C8 will discharge through D9, R1 aand Z1 and capacitor C7 will be discharged through D8, R17 and S1. Would not relay RLY1 energy and all the CMOS IC and the display will not have power. When S1 is activated to regulate all CMOS IC’s receive a 5 Volt power. C11 will be a while charging and a low input signal applied to one half of U7A a CMOS4001B, dual input OR gate. U5A output would also lower (make sure all windows and doors closed zones 2-5) and the output of high U7A. U7A output is then inverted by U7B and again in February through R18 to the input latch circuit U7A maintain. U7B low output and so Q1 and relay RLY1 inactive and no alarm will sound.

Also, when S1 is set, slowly C8 charges through R13. C8 and R13 form out timer and allow time to clear the building. The delay is approximately 1.1 x the value of C8 (at UF), or about 52 seconds with the values shown. During the delay out of the zone Z1 can switch opened and closed without triggering an alarm. After the exit time expires, the C8 will be filled and one half of the 2 input AND gate, U5A will be high. Each opening of zone 2 to 6 will cause the alarm to trigger and relay will RLY1 energy. If an intruder tried to break-ins through zone 4 for example, the output of U1D countries will change from low to high. When this happens, the signal transmitted by the high U2C triple input OR gate CMOS4075 and sent to the input D2 in the BCD to Decimal CMOS4511 display drivers. D2 is a binary code for the four and the LED display will illuminate the figure 4. High output from U2C also forwarded to U5A, another triple input OR gate. Output of U5A is now sent through S2 to input from U7A. U7A and bistable latch formed U7B, changes in circumstances that cause the output to change U7A low, the output from U7B to be high and feedback through R18 to the input of U7A again. The circuit is now locked in a high state. High output U7B do two things. First, in Q1 switches and alarm relays RLY1. Both high output applied to the blanking U7B input from CMOS4511B through S2 and also to enable the pins attached. View now will continue to show triggered zone numbers, even if the zone is opened or closed switch again. This is a similar process for all other zones immediately.

Part:
R1, R4, R6, R7, R10, R11 = 100k
R2, R3, R5, R8, R9, R12 = 270R
R13 = 1M
R14 = 4k7
R15, R18 = 470k
R16 = 100R
R17 = 1K
R19 = 10K
C1, C2, C3, C4, C5, C6, C9, C11 = 100nF
C7, C8 = 47u
C10  = 100u
D7, D8, D9 = 1N4148
Q1 = 2N3904
RLY1 = Relay 12V Coil 500R
Z1, Z2, Z3, Z4, Z5, Z6 = Contact NC
S1 = SPDT
S2 = Degree DPDT
U1 = 4050B
U2, U5 = 4075B
U3 = 4511B
U4 = 4081B
U6 = 7805
U7 = 4001B
BZ1 = Buzzer

Car Anti Theft Wireless Alarm Circuit

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

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 energized. When an intruder tries to drive the car and takes it a few meters 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. 

Automatic School Bell Circuit

This is a design circuit for alarm, but it can be used for school bell. This circuit is based on two IC. There are 555 timer IC and CD4017 IC. This is the figure of the circuit.

To ring this automatic school bell to start the first period, the peon needs to momentarily press switch S1. Thereafter, the bell sounds every 45 minutes to indicate the end of consecutive periods, except immediately after the fourth period, (IC2 and IC3) and AND gate CD4081 (IC4). Timer IC1 is wired as an astable multi vibrator, whose clock output pulses are fed to IC2. IC2 increases the time periods of IC1 (4.5 and 3 minutes) by ten times to provide a clock pulse to IC3 every 45 minutes or after 30 minutes, respectively. When the class periods are going on, the outputs of IC3 switch on transistors T1 and T2 via diodes D4 through D12.

Resistors R4 and R5 connected in series to the emitter of npn transistor T2 gate. When SCR1 is fired, it provides ground path to operate the circuit after resetting both decade counters IC2 and IC3. At the same time, LED1 glows to indicate that school bell is now active.

Automatic Intruder Alarm Using 4011 IC

This is a design circuit for a simple zone alarm circuit. This circuit has features automatic Exit and Entry delay and a timed Bell / Siren Cut-Off. This circuit is work with controlled by IC 4011. This circuit is easy to built and used. This is the figure of the circuit.

The operation of the circuit is begin when, first, check whether buildings are safe and green LED lights. Then move SW1 to the “set” position. Red LED will light up. You now have about 30 seconds to leave the building. When you go back and open the door – the Buzzer will sound. You then have about 30 seconds to move SW1 to the “off” position. If you fail to do so – the relay will energize and the Siren will sound. While at least one of the switches in the closed loop is usually kept open – the Siren will continue to be heard. However, about 15-minutes after the loop has been restored – the relay will de-energy – the Siren will Cut-off – and the alarm will reset. Of course, you can turn the Siren off at any time by moving SW1 to the “off” position. Because of manufacturing tolerances – the right length of the delay depends on the characteristics of the actual components you’ve used in the circuit. But by changing the values of R3, R6 & R9 you can adjust the Exit, Entry and Bell Cut-off times to suit your needs. Increasing the values increases the time – and vice versa.

Diesel and Horn Circuit for Train

This is a one module of diesel and horn train circuit. This circuit is work with built by 555 timers IC, and some op amp. This circuit is a complete system for the horn circuit. This is the figure of the circuit.


The main power supply to the system must be a regulated 12 volts DC with a minimum input from the train control AC or DC power supply of 13.5 VAC connected to pos 3 and 4 of the rectifier bridge. The ground bus of the regulated 12 volts supply must be connected to the system ground. The independent speed reference voltage is taken directly from the train speed control module or can be taken by connecting directly from the tracks to positions 5 and 6 of the rectifier bridge .The output of this bridge will always be a positive speed voltage signal whichever direction the train is going.

The 555 timer is really a poor replacement for the LM566 as a VCO (Voltage controlled oscillator) although it is linear in function a negative voltage range is needed to activate the timer and produce the RPM to relate to the actual engine speed thus the op-amp is used to invert the track positive voltage.

Baxandall Tone Control Circuits Using Two Transistor

This is design circuit of tone control circuit that is use very popular Baxandall configuration, a simple circuit configuration that provides boost and cut control in continuous manner. This circuit is very cheap to build, and it’s commonly implemented in commercial product. This circuit is built by two transistors. This is the figure of the schematic.


The transistors can be substituted by any general audio transistors with small current gain more than 100 (BC547, 2N3904, and many more). The supply voltage for this circuit is 9-15V DC. The components that is used is low cost and can buy in component electronics store.

Dual Channel Digital Volume Control Circuit

This is a design for digital volume control. This circuit could be used for replacing your manual volume control in a stereo amplifier. This circuit is built from 7555, 74193 and dual 4066 IC. This circuit is a dual channel volume control. This is the figure of the circuit.


IC1 timer 555 is configured as an un-stable flip-flop to provide low-frequency pulses to up/down clock input pins of pre-stable up/down counter 74LS193 (IC2) via push-to-on switches S1 and S2. To vary the pulse width of pulses from IC1, one may replace timing resistor R1 with a variable resistor. Operation of switch S1 (up) causes the binary output to increment while operation of S2 (down) causes the binary output to decrement. The active high outputs A, B, C and D of the counter are used for controlling two quad bi-polar analogue switches in each of the two CD4066 ICs (IC3 and IC4). Each of the output bits, when high, short a part of the resistor network comprising series resistors R6 through R9 for one channel and R10 through R13 for the other channel, and thereby control the output of the audio signals being fed to the inputs of stereo amplifier.

Door Knock Alarm Circuit

In the some building is need to using good security. For some example is using this design circuit. The door knock alarm is a simple design for the simple security for the house. This circuit is based by sensor piezoelectric wafer for detected the alarm. This is the figure of the circuit.


Operation of the circuit is the resistor R5 determines the knock sound sensitivity. The value shown should work in most cases. The box could be hard mounted to the door or suspended at about the middle of the door, by a string from the top of the door in such a way that the box rests against the door. It is suggested that the circuit be housed in a plastic box with a 9v battery holder.

Touch Door Alarm Circuit Using 555 Timer IC

This is a design simple alarm that can be used to provide a audible alarm when someone touches the door knob or handle of your 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.


The circuit attaches to the door knob at the end of the 1 Mega ohm resistor. This is operation of the circuit. Once the timer is triggered the LED will light and the UJT will output a tone to the speaker. The timers will time out in 5 seconds. The sensitivity of the trigger can be changed by changing the 1 Mega 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.

Motorcycle Alarm Circuit

This is a design alarm circuit that can use in motorcycle. This circuit is built by transistors. This is the figure of the circuit.


For the operation of the circuit, the circuit board and switches must be protected from the elements. Dampness or condensation will cause malfunction. Without its terminal blocks - the board is small. Ideally, you should try to find a siren with enough spare space inside to accommodate it. Fit a 1-amp in-line fuse as close as possible to the power source. The fuse is there to protect the wiring is not the circuit board. Instead of using a key-switch you can use a hidden switch, or you could use the normally closed contacts of a small relay. Wire the relay coil so that it's energized while the ignition is on. Then every time you turn the ignition off the alarm will set itself. When it's not sounding, the circuit uses virtually no current. This should make it useful in other circumstances.

Alarm Circuit Using Keypad

This is a design circuit for alarm circuit. This circuit is controlled by keypad. For the core of the circuit is using a single transistor with BC547 series. This is the figure of the circuit.


For operation, on a 12-key pad, look for 13 terminals. The matrix type with 7 terminals will NOT do. The Alarm is set by pressing a single key. Choose the key you want to use and wire it to 'E'. Choose the four keys you want to use to switch the alarm off, and connect them to 'A B C & D'. Your code can include the non-numeric symbols. With a 12-key pad, over 10 000 different codes are available. Wire the common to R1 and all the remaining keys to 'F'. When 'E' is pressed, current through D2 and R9 switches Q5 on. The relay energizes, and then holds itself on by providing base current for Q5 through R10. The 12-volt output is switched from the "off" to the "set" terminal, and the LED lights.

It locks itself high using R2 and it enables gate 2 by taking pin 5 high. The remaining gates operate in the same way, each locking itself on through a resistor and enabling its successor. If the correct code is entered, pin 10 will switch Q4 on and so connect the base of Q5 to ground. This removes the 'enable' from gate 1, and the code entry process fails. If 'C' or 'D' is pressed out of sequence, Q1 or Q2 will also take pin 1 low, with the same result. You can change the code by altering the keypad connections. If you need a more secure code use a bigger keypad with more 'wrong' keys wired to 'F'.

Super Sensitive Intruder Alarm Circuit Using 741

Here is the description of the circuit diagram of an ultra sensitive intruder alarm. The shadow of an intruder passing few meters nearby the circuit is enough to trigger the alarm. The circuit is work based on the 741IC as the core of the system. This is the figure of the circuit.


Here IC2 uA 741 is wired as a sensitive comparator, whose set point is set by R6 &R7. The voltage divide by LDR and R9 is given at non inverting pin of IC2. The LDR can be housed in a dark tube to increase sensitivity. At standby mode these two voltages are set equal by adjusting R9.Now the out put (pin6) of comparator will be high. Transistor Q1 will be off. The voltage at trigger pin of IC1 will be positive and there will be no alarm. When there is an intruder near the LDR the shadow causes its resistance to increase. Now the voltages at the inputs of comparator will be different and the out put of IC2 will be low. This makes Q1 on. This makes a negative going pulse to trigger the IC1 which is wired as a mono stable multi vibrator. The out put of IC1 will be amplified by Q2 (SL 100) to produce alarm. The sensitivity is very important here. If you cannot adjust the required sensitivity properly, use one LOW resistance (~1K ) POT in series with R9 for fine adjustment. To setup the alarm, power up the circuit and adjust R9 so that LED D1 goes off.

Fire Alarm Circuit Using 555 IC

This is a design for fire alarm circuit. This circuit is based on a LDR and lamp pair for sensing the fire. The alarm will works by sensing the smoke produced during fire. The circuit produces an audible alarm when the fire breaks out with smoke. This is the circuit of the circuit.


For operation the circuit will work when there is no smoke the light from the bulb will be directly falling on the LDR. The LDR resistance will be low and so the voltage across it (below .6V). The transistor will be OFF and nothing happens. When there is sufficient smoke to mask the light from falling on LDR, the LDR resistance increases and so do the voltage across it. Now the transistor will switch to ON. This gives power to the IC1 and it outputs 5V. This powers the tone generator IC UM66 (IC2) to play a music. This music will be amplified by IC3 (TDA 2002) to drive the speaker. The diode D1 and D2 in combination drops 1.4 V to give the rated voltage (3.5V) to UM66. UM 66 cannot withstand more than 4V.

The speaker that can be used for this circuit is a 8 ohm tweeter. The POT R4 can be used to adjust the sensitivity of the alarm circuit. The POT R3 can be used for varying the volume of the alarm. There are general purpose NPN transistor (like BC548, BC148, 2N222) can be used for Q1. The circuit can be powered from a 9V battery or a 9V DC power supply. Instead of bulb you can use a bright LED with a 1K resistor series to it.

Fire Alarm Circuit Based on Transistor

This is a design for circuit o f the fire alarm. This circuit is work based on 3 transistors. When there is a fire breakout in the room the temperature increases. This ultra compact and low cost fire alarm senses fire breakout based on this fact. This is the figure of the circuit.


The operation of the circuit will based on this explanation. The transistor BC177 (Q1) is used as the fire sensor here. When the temperature increases the leakage current of this transistor also increases. The circuit is designed so that when there is an increase in the leakage current of Q1, transistor Q2 will get biased. As a result when there is a fire breakout the transistor Q2 will be on. The emitter of Q2 (BC 108) is connected to the base of Q3 (AC 128).So when Q2 is ON Q3 will be also ON. The transistor Q3 drives the relay which is used to drive the load ie, light, bell, horn etc as an indication of the fire. The diode D1 is used as a free wheeling diode to protect it from back EMF generated when relay is switched. All capacitors are electrolytic and must be rated at least 10V.

The load can be connected through the C, NC, NO points of the relay according to your need. The calibration can be done using a soldering iron, and a thermo meter. Switch ON the power supply. Keep the tip of soldering iron near to the Q1.Same time also keep the thermometer close to it. When the temperature reaches your desired value adjust R1 so that relay gets ON. This is not a latching alarm, when the temperature in the vicinity of the sensor decreases below the set point the alarm stops. The circuit can be powered using a 9V battery or a 9V battery eliminator.

Door Handle Alarm Circuit Using Transistor

The automatic door handle alarm circuit gives a audible alarm and glows a LED when somebody touches the handle of the door. The circuit is latching type and continues to produce sound until it is switched off.


The transistor Q1 is wired as an astable multi vibrator whose output is used to bias transistor Q2 to conduction. As a result the transistor q3 and LED are in OFF state. When someone touches the handle, the capacitance of the human body damps the oscillations of Q1.The cuts the biasing of Q2 and it goes OFF.As a result the current flows to the base of Q3 , it conducts and LED glows. If the switch S1 is ON the transistor pairs Q4&Q4 which is wired in the latching mode is triggered and the Buzzer Z1 is activated. When the person removes his hand from door handle the LED goes OFF but the buzzer continues to beep. The only way to mute the buzzer is to open the switch S2. All capacitors must be rated 15V.

A 9V battery or 9V DC power supply can be used to power the circuit. To make L1 wind 25 turns of 0.4 mm enameled copper wire on resistor R2 and solder the ends of the wire to the resistor leads. This unit will stand for the R2 as well as L1 because both are parallel in the circuit. A readily available 10mH inductor can be used for L2. Connect the point A in the circuit to the door handle using a long wire.

Fire Alarm Circuit Based on Transistor

This is a design for circuit o f the fire alarm. This circuit is work based on 3 transistors. When there is a fire breakout in the room the temperature increases. This ultra compact and low cost fire alarm senses fire breakout based on this fact. This is the figure of the circuit.


The operation of the circuit will based on this explanation. The transistor BC177 (Q1) is used as the fire sensor here. When the temperature increases the leakage current of this transistor also increases. The circuit is designed so that when there is an increase in the leakage current of Q1, transistor Q2 will get biased. As a result when there is a fire breakout the transistor Q2 will be on. The emitter of Q2 (BC 108) is connected to the base of Q3 (AC 128).So when Q2 is ON Q3 will be also ON. The transistor Q3 drives the relay which is used to drive the load ie, light, bell, horn etc as an indication of the fire. The diode D1 is used as a free wheeling diode to protect it from back EMF generated when relay is switched. All capacitors are electrolytic and must be rated at least 10V.

The load can be connected through the C, NC, NO points of the relay according to your need. The calibration can be done using a soldering iron, and a thermo meter. Switch ON the power supply. Keep the tip of soldering iron near to the Q1.Same time also keep the thermometer close to it. When the temperature reaches your desired value adjust R1 so that relay gets ON. This is not a latching alarm, when the temperature in the vicinity of the sensor decreases below the set point the alarm stops. The circuit can be powered using a 9V battery or a 9V battery eliminator.

Door Handle Alarm Circuit Using Transistor

The automatic door handle alarm circuit gives a audible alarm and glows a LED when somebody touches the handle of the door. The circuit is latching type and continues to produce sound until it is switched off.


The transistor Q1 is wired as an astable multi vibrator whose output is used to bias transistor Q2 to conduction. As a result the transistor q3 and LED are in OFF state. When someone touches the handle, the capacitance of the human body damps the oscillations of Q1.The cuts the biasing of Q2 and it goes OFF.As a result the current flows to the base of Q3 , it conducts and LED glows. If the switch S1 is ON the transistor pairs Q4&Q4 which is wired in the latching mode is triggered and the Buzzer Z1 is activated. When the person removes his hand from door handle the LED goes OFF but the buzzer continues to beep. The only way to mute the buzzer is to open the switch S2. All capacitors must be rated 15V.

A 9V battery or 9V DC power supply can be used to power the circuit. To make L1 wind 25 turns of 0.4 mm enameled copper wire on resistor R2 and solder the ends of the wire to the resistor leads. This unit will stand for the R2 as well as L1 because both are parallel in the circuit. A readily available 10mH inductor can be used for L2. Connect the point A in the circuit to the door handle using a long wire.

MULTI SWITCH DOORBELL Circuit Using CD4042B

This circuit is built by CD4042 IC. The input of the design is result musical doorbell. CD4042B IC is popular IC to built up the voice of the bell. There are four data input that it will shown by LED (D1-D4). It will “on” if the main switch (S6) is turn on. The circuit figure is shown in below;


The principle work is when switch S6 is pushed to ‘on’ condition, the circuit gets +9V and the four data inputs (D1 through D4) of IC are in low state because these are tied to ground via resistors R1 through R4. Polarity input in pin 6 of CD4042B IC is also pulled down by resistor R5. Clock input (pin 5) of the quad D-latch is wired in normally low mode and hence all the four outputs (Q0 through Q3) have the same states as their corresponding data inputs. As a result, LED1 through LED4 are in off condition. There are four switches fitted at four different doors/gates outside the home and a monitoring panel in the common room of the home. If any switch is pressed by a visitor pins 2 and 4 of IC go high.

Simultaneously, pin 3 to IC (Q0 output) go low and LED1 starts glowing to indicate that switch S1 is pressed. Output in pin 13 of the dual 4-input NOR gate (IC2, here wired as a single 4-input OR gate) is high to forward bias buzzer driver transistor T1 via resistor R10. The final result is a soft and pleasing musical bell, which lasts until reset switch S5 is pressed by the owner. For this latching arrangement, output pin 13 of IC2 from the NOR gate is fed back to the clock input of IC1. The circuit costs around Rs 100.