Remote controlled appliance switch circuit

Description.
Here is a versatile remote controlled appliance switch that can ON or OFF any appliance connected to it using a TV remote.
 IR remote sensor IC TSOP 1738 is used for recieving the signal. Normally when no signal is falling on IC3 the output of it will be high.This makes Q1 OFF.When a signal of 38 KHz from the TV remote falls on the IC3 its output goes low.This makes Q1 conduct and a negative pulse is obtained at pin 2 of IC 1  NE 555.Due to this IC1 wired as a monostable multivibrator produces a 4 Sec  long high signal at its out put.This high out put is the clock for IC 2 which is wired as a Flipflop and of , its two outputs pin 3 goes low and pin 2 goes high.The high output at pin 2 is amplified to drive the relay .For the next signal  the outputs of IC2 toggles state. Result, we get a relay toggling on each press on the remote.Any appliance connected to this circuit can be switched ON or OFF.
Circuit Diagram with Parts List .

Notes.

• Before wiring the circuit make sure that the carrier frequency  of the TV remote you have is 38 KHz.For that wire the sensor part only ,point your remote to the TSOP1738 and press any switch.If  out put of TSOP1738  goes  low them ok, your remote is of 38Khz type.Nothing to worry almost all TV remote are of this type.
• You can use any switch because for any switch the code only changes,the carrier frequency remains same.We need this carrier frequency only.
• Assemble the circuit on a good quality PCB or common board.
• The appliance can be connected through NO or NC  and contacts of the relay

Transistor intercom circuit.

Description.

Here is a simple but effective intercom circuit that is based fully on transistors.The circuit is based on a three stage RC coupled amplifier. When the pushbutton S2 is pressed, the amplifier circuit wired around T1 & T2 becomes an astable multivibrator and starts producing the ringing signals. These ringing signals will be amplified by the transistor T3 to drive the speaker. When the push button S2 is released the circuit will behave as an ordinary amplifier and you can talk to the other side through it.

To construct a two way intercom, make two identical copies of the circuit given below and connect it according to the given connection diagram. The stand by current consumption of this circuit is around 20mA.

Circuit diagram with Parts list.

Connection diagram. 

 

Notes.

• Assemble the circuit on a good quality PCB.
• Use 9V PP3 battery for powering the circuit.
• The Mic M1 can be condenser micro phone.
•  Use push to ON type push button switch for S2.
• Use a slide switch for switch S1.S1 can be used to power the circuit

Night security light

Description.

Here is a simple circuit switches on a light around 2 hours after midnight, the time at which most of the robberies taking place.

This simple circuit is build around a CMOS IC 4060 to obtain the required timing. During day time the LDR has low resistance and keeps the pin 12 of the IC1 high, preventing the IC1 from oscillating. When it is dark the LDR resistance becomes high and the pin 12 of IC1 becomes low and the IC starts oscillating, which indicated by the flashing of LED D3.The values of the timing components R1, R2, C4 are so selected that the out put pin3 of IC1 goes high after 8 hours. That means the high output drives the triac to switch on the lamp around 2’O clock. At morning, the LDR resistance drops and the pin 12 of IC1 goes high and stops the oscillation, making the lamp OFF. The switch S1 can be used to manually ON the lamp. The capacitor C2 prevents false triggering.

Circuit diagram with Parts list.

Notes.

• Assemble the circuit on a good quality PCB or common board.
• The LDR can be general purpose LDR.
• The light sensitivity can be adjusted using the preset R6.
• The IC1 must be mounted on an IC holder

Plant moisture level monitor

Description.

Here is a simple circuit that will give a visual indication when the soil water level inside your flower pot goes low below a certain limit.

The U1C and associated components are wired as an oscillator producing a 2KHz square wave. This square wave is given to one gate input of U1D via a variable potential divider former by R1 and R2.When the resistance across the probes A and B are low that is when soil moisture level is high, the C2 will divert the square wave to ground. The output of U1D will be high. The U1 A inverts this high state to low and so the IC U1B is blocked from producing oscillations. The LED will remain OFF. When there is no moisture across the probes, the C2 cannot bypass the 2KHz signal to the ground and it appears at the gate input of U1D.The output of U1D goes low, and it is inverted to high by U1A.The oscillator wired around U1B is activated and it starts oscillating. These oscillations are amplified by Q1 to drive the LED and LED starts pulsating as an indication of low moisture. Since square wave is used there won’t be any oxidation on the probes. The resistor R7 limits the current through LED and ensures a longer battery life.

Circuit diagram with Parts list.

Notes.

• Power the circuit from a 3V battery.
• Two metal wires 10 cm long and 5cm apart driven into the soil will do the job for probes.
• The probes are to be connected at the terminals A and B shown in circuit.
• Capacitors C1 and C2 must be polyester type.
• The IC U1 is a quad two input Schmitt NAND IC 4093.
• The sensitivity can be adjusted by varying the preset R2.
• Mount the IC on a holder.

Battery operated heater

Description.
Here is a simple heater circuit that can be operated from a 12V battery. The first part of the circuit is an astable multivibrator build around the two transistors Q1 and Q2 .The ON time of transistor Q2 is set to 0.5 S. The ON time of transistor Q1 can be varied by using the POT R2.The output pulses at the collector of Q2 is used to drive the Darlington power transistor Q3(TIP122).The transistor Q3 drives the heating elements L1 to L3.The net heat produced can be varied by  selecting the desired combination of heating elements at the output circuit sing switches S1 and S2.The net heat can be also varied by varying the duty cycle of the triggering pulse using POT R2.

Circuit diagram with Parts list.

Notes.

• The circuit can be assembled on a general purpose PCB.
• The transistor Q3 must be fitted with a heat sink.
• The elements L1 to L3 can be 10W heating coils.
• The switches S1 and S2 must be able to withstand at least 5A.
• The circuit can be powered from a 12V battery.
• The LED D1 gives a visual indication of the duty cycle of the circuit

Electronic mosquito repeller

Description.

Here is the circuit diagram of an ultrasonic mosquito repeller.The circuit is based on the theory that insects like mosquito can be repelled by using sound frequencies in the ultrasonic (above 20KHz) range.The circuit is nothing but a PLL IC CMOS 4047 wired as an oscillator working at 22KHz.A complementary symmetry amplifier consisting of four transistor is used to amplify the sound.The piezo buzzer converts the output of amplifier to ultrasonic sound that can be heard by the insects.

Circuit diagram with Parts list.

Notes.

• Assemble the circuit on a general purpose PCB.
• The circuit can be powered from 12V DC.
• The buzzer can be any general purpose piezo buzzer.
• The IC1 must be mounted on a holder

LED torch using MAX660

Description.
This is a simple LED torch circuit based on IC MAX660 from MAXIM semiconductors. The MAX 660 is a CMOS type monolithic type voltage converter IC. The IC can easily drive three extra bright white LEDs.The LEDs are connected in parallel to the output pin 8 of the IC. The circuit has good battery life. The switch S1 can be a push to ON switch.

Circuit diagram with Parts list.

Notes.

• Assemble the circuit on a general purpose PCB.
• The IC must be mounted on a holder.
• The circuit can be powered from two torch cells connected in series.
• The capacitors C1 and C2 must be Tantalum type.
• The diodes D1 to D3 must be of 1N4148

How to make a Rheostat

Components

1. The components needed for the connection are
2. Flashlight bulb and socket [1]
3. Dry cell lantern battery/D-cell battery [2]
4. Wire [About 15 to 17 inches and another one 2 inches]
5. Spring [1]
6. Wire Clippers [A pair]
7. A typical spring can be obtained from a widow roll up. You can even get to buy one at a cheap rate.

Procedure

• Connect the two Dry cell lantern/D-cell batteries tail-to-tail, so that the positive polarity of one battery is connected to the negative polarity of the other.
• Using a wire cutter, cut the wire in equal lengths. One wire should be at least 8 centimetres long.
• Connect the wires onto the open ends of both the batteries.
• The end of one wire must be connected to the bulb socket with the bulb in it.
• Connect the second wire to one end of the long spring.
• Connect the free end of one wire to one terminal of the light socket.
• Connect the other free wire to one end of the spring.
• Take the two inch wire and connect it to the second terminal of bulb socket.
• Connect the other end of the two inch wire onto the other end of the spring.

How to make a Rheostat

How to make a Rheostat

What happens?

As soon as the circuit is in closed loop, the bulb begins to glow. Although the glow intensity is less, when you move the wire through the spring onto the other end where the wire is connected, the bulb starts to glow more brightly. When both the wires are nearby the glow will be in its maximum.

The spring is mainly made of steel wire. Steel wires are not very good conductors of electricity. Thus the resistance of the circuit also increases. If the spring length is long enough you will get to see different stages of the glow. Thus you will get to see the working of a rheostat

Water level Controller

Water level controller circuit.

Description.

A simple but very reliable and effective water level controller circuit diagram is shown here. The circuit uses 6 transistors, 1 NE555 timer IC, a relay and few passive components. The circuit is completely automatic which starts the pump motor when the water level in the over head tank goes below a preset level and switchess OFF the pump when the water level in the over head tank goes above the full level.

Probe D is positioned at the bottom level of the tank while probes A, B and C are placed at full, half and medium levels of the tank respectively. The level sensing part of the circuit is built around transistors Q1, Q2 and Q3. When water level is below the quarter level probes A, B and C are open and the transistor Q1, Q2 and Q3 remains OFF. When the water level rises and touches the probes the corresponding transistors gets biased and switches ON. Resistors R1, R2, R3 limit the bases current of corresponding transistors while resistors R4, R5, R6 limit their collector current. LEDs D1, D2 and D3 provide a visible indication of the current water level.

When the water level goes below medium, transistor Q2 gets switches OFF and its collector goes positive. Collector of Q2 is connected to the base of transistor Q6 and as result transistor Q6 gets switched ON. Transistor Q5 will be also ON because its base in connected to the collector of Q4 which is presently OFF. As a result when the water level goes below medium relay K1 gets energised and the pump is driven. The relay is wired in the latching mode so that even if the water level goes above medium level the pump remains ON so that the tank gets completely filled. For wiring the relay in latching mode one set  of N/O contacts is used. When relay is activated these contacs close which forms a short across collector and emitter of Q6. This makes the state of Q6 irrelevant to the opertion of the relay and the relay remains ON as long as the transistor Q5 is ON. The only way to make the relay OFF is by switching OFF Q5 and it is done automatically when the water level reaches the full level.

Collector of transistor Q1 is connected to the trigger pin (pin2) of IC1. When the water level reaches full level the transistor Q1 gets switched ON. As a result its collector goes to ground level which triggers the IC1 which is wired as a monostable. The output of IC1 goes high for about 1S. This makes the transistor Q4 ON for the same time and transistor Q5 whose base is connected to the collector of Q4 is switched OFF cutting the supply to the relay. This makes the motor OFF and it remains OFF until the water level again goes below the medium level.

Resistor R8 is a pull up resistor for the trigger pin of the NE555. Capacitor C3 couples the collector of Q1 to the trigger pin of NE55 and facilitates edge triggering whenever the transistor Q1 goes ON. A monostable circuit can be made edge triggered by connecting the trigger signal to the trigger input pin through a capacitor. The capacitor blocks DC and passes sudden changes. The circuit used here is termed as negative edge triggered because the monostable is triggered when ever the trigger input signal falls. R10 and R12 limits the collector current of Q4 and Q5 respectively while R9 and R11 limits their base current. R13 limits the base current of Q6 while D4 is a freewheeling diode which protects the switching transistors from voltage transients.

Circuit diagram.

 

Water level controller circuit diagram

Probe arrangement diagram

The probes can be arranged as shown in the diagram above. Insulated Aluminium wires can be used as the probes. The probes can be binded on a plastic rod and should be erected vertically inside the tank. The length of the probes wires and the supporting plastic rod must be chosen according to the depth of the tank. Since DC is used in the level sensing section electrolysis will occur in the probes and so the probes require small maintinances in 1 or 2 month intervals. Using AC in the sensing section will completely eliminates the chance of electrolysis and I am presently working on such a circuit. You can expect it soon.

Notes.

• Use 12V DC for powering the water level controller circuit.
• The relay I used was a 5V/220 ohm relay and that’s why the current limits resistor R12 was added in the circuit. If you use a 12V relay then the R12 can be shorted.
• Do not use a relay that consumes 500mA. Maximum collector current PN2222 can handle is 600mA.
• Use insulated single strand aluminium wires for probe and they can be arranged in the tank as per the probe arrangement diagram.
• Use a holder for mounting NE555.
• The circuit can be assembled on a Perf board.
• K1 must be a double pole relay.
• The load current, voltage ratings of the relay must be selected according to the ratings of the pump motor.
• The type number of the transistors used here are not very critical and you can do suitable replacements if any type number is not availble.
• Most of the components required for this project can be found inside your scrap box.

Power supply for this circuit.

12V DC power supply

A classic 12V regulated DC supply based on 7812 is shown above. A power ON indicator LED is also added in the circuit.Resistor R13 limits the LED current. A small aluminium heatsink can be fitted to the 7812 for better saftey.Small Al heatsinks for TO-220 package are readily available in the market

High voltage generator circuit

Description.
First of all let me remind you that this circuit is a very dangerous one. The output voltage of this circuit is in Kilo volts and it can seriously injure you or kill you. Try this circuit only if you have enough experience dealing with high voltages. I have no responsibility on any hazards caused by the circuit. Be very careful. This is a humble request.

The circuit given here has three sections namely oscillator, switching stage and a step up stage. The oscillator is build around a NE555 timer operating at 25 KHz. The output of the NE555 coupled to the base of the power transistor TIP3055 which is the switching device. The power transistor drives primary of the step up transformer at 25 KHz and as a result a high voltage will be induced across its secondary.
Circuit diagram.

Notes.

• A 12V lead acid battery can be used for powering the circuit.
• TIP3055 must be mounted on a heat sink.
• T1 can be an EHT (extra high tension) transformer used in television sets.
• For an EHT from 20inch TV, the output voltage will be 8 to 10 KV @12V supply voltage.
• This circuit is not an efficient one and is not suitable for any serious applications.
• Once again, be very careful with this circuit!

QRP antenna tuner circuit

Description.

Low power ( 3 to 30 MHz)  transmitters constructed by hams are generally called QRP’s. For such transmitters a well tuned antenna is a must.If the impedance is not properly matched there will be a little or no output.But if properly matched there will be great results.A circuit for matching the antenna properly with the transmitter id given below.

The output of the transmitter is given to the input of the tuner( connector BNC1). The output of the tuner(connector BNC2) must be connected to antenna.Then adjust the L1 and C1 to obtain the maximum transmission power.The transmission power can be checked using a SWR meter.

Circuit diagram with Parts list. 

Notes. 

• Assemble the circuit on a goos quality PCB or common board.
• If the matching is not satisfactory then change the values of L1,C1,C2&C3  to the next close value and tune again.
• Proper tuning requires some trial and error.
• The circuit can be enclosed in an aluminum casing for better performance

CW Practice oscillator

Description.

A circuit diagram that can be used for the generation of CW Morse code is shown here.This circuit can be very useful those who would like practice Ham Radio.The circuit is nothing but an astable multivibrator based on NE 555.The frequency of oscillations of the circuit depends on the components R1,R2 & C1.The circuit can be powered from a 9V PP3 battery.

Circuit diagram with Parts list.

Notes. 

• The POT R2 can be used for frequency adjustments.
• POT R3 can be used for volume adjustments.
• The switch S1 can  be a Morse code key

Touch Switch Circuit

Touch Switch Circuit using NE 555

Description

This is the circuit diagram of a small touch plate controller using IC NE 555 .This circuit is ideally useful for making touch operated doorbells, buzzers,toys etc which when touched on the touch plate operates the relay for a preset time and the turns off automatically.

This circuit is realized by utilizing the high input impedance of trigger pin of the 555 IC.When  the IC is triggered by the induced voltage of human body the output goes high for a time determined by R1 and C1.The transistor is used to drive the relay.The relay contacts can be used to drive the load like bell, motor , lights etc.

Notes.

To make the touch plate cut a 1 square cm thin metal sheet.

To setup the circuit connect to power supply and  adjust R1 while keeping touching on the touch plate.Stop at the point where relay activates.If relay is in the activated state initially then do the same until the relay is deactivated.

Touch Switch Circuit Diagram

UJT Organ circuit

Description.

Here is the circuit diagram of a simple yet interesting mini organ based on UJT 2N 4891( Q1).You can produce the desired sound by pressing each pushbuttons.You have also the option to set each push button to produce the desired tune you want.

The circuit is based on a UJT oscillator who’s frequency is determined by the Presets R6 to R13(one at a time) and capacitor C2.The transistor Q2 drives the speaker to produce reasonable sound output.For each pushbutton press the corresponding preset will be added to the circuit ,and the UJT oscillates in a frequency proportional to it.This frequency will be amplified by the transistor to produce the sound at the speakers.A 9V battery or 9V DC power supply can be used to power the circuit.

Circuit diagram with Parts list.

Notes.

• Power up the circuit and adjust the presets (R6 to R 13) to get the desired tune at the  output for the press of corresponding push button.
• Preset R1 (5K)  can be used as a volume control.
• You can also try different values for C2 get the desired tunes.
• All capacitors are of ceramic type

Sense of Time tester circuit

Description.
When S2 is ON ,the circuit here operates as an astable multi vibrator and the LED is lit for about o.1sec,flasing every 1.5 seconds.Since the human reaction time is more than this,you cannot catch it once it is seen o,by pressing S1.If your sense of time interval is good,and you press S1 with in that 0.1 sec,the discharging of C1 stops and then the lamp stays lit.you may change the ON and OFF periods by changing R1 and R2 or C1 to suit your convenience.
Circuit diagram with Parts list.

Notes.

• Switch S1 can be a pushbutton switch.
• Switch S2 can be a SPST switch.
• The circuit can be powered from a 9V battery or 9V DC power supply

Dancing light

Description.

Here is a simple dancing light circuit based on NE555 (IC1) & CD4017 (IC2) .The IC1 is wired as an astable multivibrator to provide the clock pulses for the CD4017.For each clock pulse receiving at the clock input (pin14) of IC CD4017, the outputs Q0 to Q9 (refer pin diagram of CD 401) becomes high one by one alternatively.The LEDs connected to these pins glow in the same fashion to give a dancing effect.The speed of the dancing LEDs depend on the frequency of the clock pulses generated by the IC1.

Circuit diagram with Parts list.

Notes.

• Assemble the circuit on a good quality PCB or common board.
• The ICs must be mounted on holders.
• The speed of the dancing LEDs can be adjusted by varying POT R2.
• The capacitor C1 must be rated 15V.
• Using different color LEDs could produce a better visual effect.

CD 4017 Pin configuration.

Puff to OFF LED circuit.

Descrption.

This is a simple circuit in which the glowing LED can be switched OFF just by a puff. A condenser mic (M1) is used to sense your puff. When the push button S1 is pressed, the transistors Q2 and Q3 wired as latching pair gets activated and drives the LED to glow. The LED remains in this condition. When you puff on the condenser mic, the sound pressure is converted into a voltage signal at its output. This voltage signal will be amplified by the transistor Q1.Since the collector of the Q1 is coupled to the emitter of the latching pair, the pair will stop conducting when ever there is a signal from the condenser mic due to puffing and the LED will go OFF.  The push button switch S1 has to be pressed again to switch the LED ON.

Circuit diagram with Parts list.

Notes.

• The circuit can be powered from a 3V battery.
• The M1 can be a general purpose condenser microphone.
• The switch S1 can be push button switch.
• The circuit can be assembled on a good quality PCB or common board.
• Instead of the LED, you can also try a low power 3V bulb

Electronic toss circuit

Description.

The circuit given here can be used for tossing head or tail. There are many games in which a tossing is required to start and this circuit can be used in all such instances.

The circuit uses two ICs NE 555 timer (IC1) and 74LS76 dual JK flip flop (IC2).The IC 1 is wired as an astable multi vibrator operating at 10Hz.The output of IC1 is inverted by using the transistor Q1.The collector of Q1 is connected to the pin 1 of IC2 via the push button switch S1.The IC2 is wired in toggle mode. When push button S1 is pressed the output pins 14 and 15 of IC2 starts toggling in state. The LEDs connected to these pins also toggles (Since the frequency of toggling is 10Hz, we feel both LEDs glowing).When push button S1 is released either one of the LED remains ON indicating the head or tail.

Circuit diagram with Parts list.

Notes.

• The circuit can be powered from 5 V DC.
• Switch S1 is a push button switch.
• The ICs must be mounted on holders.
• The circuit can be assembled on a general purpose PCB

Single chip metal detector circuit

Description.
This is a simple single chip metal detector circuit based on IC CS209A from the Cherry Semiconductors. A 100uH coil is used to sense the presence of metal. The IC CS209A has a built in oscillator circuit and the coil L1 forms a part of its external LC circuit which determines the frequency of oscillation. The inductance of the coil change in the presence of metals and the resultant change in oscillation is demodulated to create an alarm. The LED gives a visual indication too. This circuit can sense metals up to a distance of few inches.

Circuit diagram with Parts list.

Notes.

• Assemble the circuit on a general purpose PCB.
• The switch S1 can be a slide type ON/OFF switch.
• The IC must be mounted on a holder.
• The POT R1 can be used to adjust the sensitivity of the circuit.

Canary chirp generator

Description.
This is a simple electronic alarm circuit that imitates the chirping of a canary. The circuit is nothing but a Hartley oscillator with few more passive components added. As the capacitor C1 charges through the resistor R1 and the transistor Q1 is driven to cut off. This makes the oscillations to stop. As the capacitor discharges through the Resistor R1 and base emitter junction of the transistor the oscillation start again. Actually the R1 and C1 are the components that make the characteristic chirping sound.

Circuit diagram with Parts list.

Notes.

• Assemble the circuit on a general purpose PCB.
• The circuit can be powered from a 9V PP3 battery.
• The transformer T1 can be a audio output transformer like LT700.
• If LT700 is not available, try the audio output transformer used on you old transistor radio board.
• The speaker can be an 8 ohm tweeter.
• Switch S1 can be a push button switch.
• The chirping sound can be altered by changing the value of R1 and C1

Digital dice circuit

Description.

This is a simple and easy to construct digital dice circuit. The circuit is based on a single IC, CD4060B.The dice consists of six LEDs marked D1 to D6.The number of LEDs glowing indicates the numeral.

The heart of this circuit is 14 stage binary ripple counter IC CD4060B.The IC also has a built-in oscillator. The oscillator output (here 2 KHz) is used to clock the binary ripple counter. The counter increments by one in its natural count sequence each time it is clocked. The oscillator in initially inhibited as long as the pushbutton switch S2 is not pressed. The counter outputs will be in logic zero state and all the six LEDs will be ON.As the push button S2 is pressed, oscillator is enabled and the counter starts counting. The counter outputs (pin 4, 5 & 7) changes from 000 to 101 and then resets to 000 to repeat the sequence. After 101 the counter does not advances to 110 because of R3, D7 & D8.When the counter just advances from 101 to 110 the diodes D7 & D8 become reverse biased and makes the reset pin (pin 12) high to reset the counter.

The counter counts as long as the push button switch S2 is pressed. Also the micro buzzer will sound as long as the IC is counting. When the push button switch S2 is released, the counting is stopped and holds the existing state to represent the random number.

Circuit diagram with Parts list.

Notes.

• Switch S1 is the ON/OFF switch.
• Switch S2 can be a push button switch.
• Buzzer K1 is a piezo buzzer.
• The circuit can be powered from a 9V PP3 battery.
• The IC must be mounted on a holder

Whistle to beep circuit

Description.
This simple circuit produces a beeping sound that lasts for around 3 seconds whenever you make a whistle. The CMOS Hex inverter CD4049 is the heart of this circuit. Out of the six inverters in CD4049, U1a is wired as an audio amplifier which amplifies the signal picked up by the microphone M1.The U1b is wired as a band pass filter with center frequency around 2KHz.The filter is necessary in order to pass the frequency corresponding to whistling sound and suppress all other frequencies .If the filter is not there, the circuit could easily get false triggered.U1d is wired as a 3S delay monostable multivibrator.The output U1d drives the astable multivibrator formed by U1e and U1f.The astable multivibrator is operating around 4Hz.The combined effect is a intermittent beeping sound that lasts for around 3S.Transistor Q1 is used to drive the buzzer B1.

Circuit diagram with Parts list.

Notes.

• Assemble the circuit on a good quality PCB.
• The circuit can be powered from a 3V battery.
• IC U1 is a CMOS CD4049 Hex inverter.
• M1 can be an electret microphone.
• B1 can be a 3V piezo buzzer.
• Mount the IC on a holder.
• The duration of beeping can be adjusted by varying the components C4 and R9

Jet engine sound generator

Description.
This jet engine sound generator circuit is based on the sound generator IC HT2844P from Holtek Semiconductors. This particular IC can make four sounds namely low speed sound of jet engine, high speed sound of jet engine, missile sound and machine gun sound. Each of these sounds can be activated by connecting the pins 12, 13, 14and 15 to ground by using the respective push button switches. Resistor R3 can be used for manually increasing or decreasing the speed.LED D1 gives a visible indication of the sound.

Circuit diagram.

Notes.

• The circuit can be powered from a 3V battery.
• Do not give more than 3.3V to the IC.
• K1 can be a 200mW/8 Ohm speaker.
• IC1 must be mounted on a holder.

Voice modulator circuit

Description.
This is a very versatile voice modulator circuit using IC HT8950A from Holtek Semiconductors. The IC is capable of creating 7 upward or downward steps on the frequency of the input voice at a rate of 8Hz. There is also two special variation effects namely Vibrato mode and Robot mode. This circuit finds a lot of application in systems like telephone, speech processors, toys, mixers etc. A microphone is used to pick up the input voice. Push button switches S2 and S3 can be used for the upward and downward frequency stepping .Push button switch S1 can be used to activate Vibrato mode and push button switch S4 can be used to activate the Robot mode. IC HT82V733 (also from Holtek) is used to amplify the output of the voice modulator.LED D1 indicates the voice level.

Circuit diagram.

Notes.

• The circuit can be assembled on a Vero board.
• Do not give more than 4.5v to the circuit.
• Switches S1 to S4 can be miniature push button switches.
• S5 can be a miniature ON/OFF switch.
• K1 can be an 8 ohm speaker.
• IC1 and IC2 must be mounted on holders