MICRO AMPERE METER CIRCUIT USING uA 741

1. The micro ampere meter shown here is basically a DC millivolt meter.The circuit gives full scale deflection for 0.1V input.The current to be measured is passed through a known resistance R and the voltage drop across it is measured
2. Here the IC 1 uA 741 op-amp is wired as a non inverting amplifier.

   
   

NOTES:

• Assemble the circuit on a good quality PCB or common board.
• Use a +9V/-9V dual power supply for powering the circuit.
• The IC 1 ,uA 741 must be mounted on an IC base.
• The table given below shows the relationship between different values of R and the current that will give full scale deflection.

METAL DETECTOR CIRCUIT

1. This is the circuit diagram of a low cost metal detector using a single transistor circuit and an old pocket radio.This is nothing but colpitts oscillator working in the medium band frequency and a radio tuned to the same frequency.
2. First the radio and the circuit are placed close.Then the radio is tuned so that there is no sound from radio.
3. In this condition the radio and the circuit will be in same frequency and same frequencies beat off to produce no sound.
4.  When the metal detector circuit is placed near to a metal object the inductance of its coil changes , and so do the frequency of oscillations.Now the two frequency will be different , there will be no canceling and radio produces a hissing sound.The metal is detected.

NOTES: 

• To make L1 make 60 turns of 36SWG enameled Copper wire on a 1 cm PVC tube.

• Powering the circuit using a adapter rather than a battery induces noise. It is always good to power radio projects from battery.

15WATT CLASS-B AMPLIFIER

1. The circuit shown here is of a simple Class-B audio amplifier based on opamp TL082, transistors TIP41 and TIP42. LM833 is a dual opamp with high slew rate and low distortion particularly designed for audio applications.
2. This audio amplifier circuit can deliver 15 watt audio output into an 8 ohm speaker at +12/-12V DC dual supply.
3. Both opamps in the IC are used here. IC1a is wired as a buffer and capacitor C3 does the job of input DC decoupling.
4. Ic1b is wired in the inverting mode and it provides negative feedback.
5. Complementary power transistors TIP41 and TIP42 are wired in the Class B push pull scheme and they drives the loud speaker.
6. Diode D1 provides 0.7V bias voltage for the push pull pair and capacitor C2 protects the 0.7V bias voltage across D1 from heavy voltage swings at the IC1b’s output.
   
   
   
   
   
   
   NOTES:
   

   • The audio amplifier circuit must be assembled on a good quality PCB.
   • Use a holder for mounting IC1.
   • Use a +12/-12V dual supply for powering the amplifier.
   • Potentiometer R2 can be used as a volume control.
   • Raising the power supply voltage will increase the output power. Anyway note the following points.
   • TIP42 and 41 can handle only up to 6A.
   • Maximum supply voltage IC1 can handle is +16/-16 V DC.

   
   

BRIDGE AMPLIFIER USING TDA4935

1. TDA4935 is 2x15W high quality audio amplifier IC from Siemens.
2. The IC can be used in stereo mode or bridge mode.
3. In stereo mode it can deliver 15W per channel and in bridge mode it can deliver 30W into a 8 ohm load at 30V supply.
4. TDA4935 requires very few external components and has a wide power supply voltage range.
5. The IC operates in class B mode and has built-in circuitry for over temperature protection and overload protection.

NOTES:

• The circuit must be assembled on a good quality PCB.
• TDA 4935 must be fitted with a proper heat sink.
• The supply voltage can be anything between 8 to 30V DC.
• Capacitors C1, C2, C8 are polyester capacitors.
• Capacitors C3, C4 and C6 are ceramic capacitors while C5 and C9 are electrolytic.

POCKET HEADPHONE AMPLIFIER

1. Here I present a very simple and powerful headphone amplifier using OPA134. In addition to the IC OPA134, the circuit uses only few passive components and can easily generate a lot of sound from even the most inefficient headphones and there will be no compromise for the quality.
2. OPA134 is low noise, low distortion operational amplifier from BURR-BROWN semiconductors and it is exclusively meant for audio applications. The FET based input stage provides the IC with high input impedance and it makes the circuit very flexible in terms of the audio source.
3. You can plug almost all types of sound sources like, mp3 players, iPods, mobile phones etc to the circuit.
   In the circuit IC OPA134 is wired as a non-inverting amplifier. The +/-4.5V split power supply required for the IC is obtained from a 9V PP3 battery using the circuit comprising of components D1. R6, R7.R8, C2 and C3. D1 is an LED which indicates power ON.
4. Switch S1 can be used as a ON/OFF switch .Resistor R2 and capacitor C1 forms a high pass filter with corner frequency around 15KHz.
5. POT R1 can be used as a volume controller. The load resistor R5 will stabilize the virtual ground and prevents any noise or distortion in the output, but the output will be DC coupled.

   
   

   
   

   
   

   
   

   NOTES:-
   

   • Assemble the circuit a good PCB.
   • The circuit can be powered using a 9V PP3 battery.
   • POT R1 can be used as a volume controller.

   
   

TEMPERATURE CONTROLLED LEDS

1. The circuit is nothing but two LEDs (D1 and D2), whose status are controlled by the temperature of the surroundings.
2. The famous IC LM35 is used as the temperature sensor here. Output of LM35 increases by 10mV per degree rise in temperature. Output of LM35 is connected to the non inverting input of the opamp CA3130. 
3. The inverting input of the same opamp can be given with the required reference voltage using POT R2. If the reference voltage is 0.8V, then the voltage at the non inverting input (output of LM35) becomes 0.8V when the temperature is 80 degree Celsius.
4. At this point the output of IC3 goes to positive saturation. This makes the transistor Q1 On and LED D1 glows.
5. Since the base of Q2 is connected to the collector of Q1, Q2 will be switched OFF and LED D2 remains OFF. When the temperature is below 80 degree Celsius the reverse happens.IC1 produces a stable 5V DC working voltage from the available9V DC supply. If you already have a 5V DC supply then you can use it directly.
   
   
   
   
   
   
   NOTES:-
   

   • The circuit can be assembled on a Vero board.
   • IC3 must be mounted on a holder.
   • The temperature trip point can be set by adjusting POTR2.
   • Type no of Q1 and Q2 are not very critical. Any general purpose NPN transistors will do it.

HUMIDITY TESTER

1. A variety of humidity tester circuits are available, but this  is a circuit which is as simple as possible.Using only a transistor ,LED and few resistors ,this circuit can be used to check the humidity level of materials like soil , paper etc. 
   
2. When the humidity in a substance increases the current conducted through  it also increases .
3.  This is the working principle.If there is required humidity, the current through R3 will be sufficient to produce a voltage drop across R3 which is sufficient enough(0.7V)  to switch on the transistor and LED glows.R1 is the current limiting resistor for LED.R1 protects the transistor from accidental shorting of the probes.

   
   

   
   

   
   

   NOTES:-
   

   • The probes can be anything like iron nail.pin paper clip etc.
   • To calibrate the circuit for particular density,insert the two probes to the required material and adjust R3 so that the LED glows.The LED will glow whenever the humidity of the testing sample becomes equal to this humidity level.
   • A 3V battery can be used to power the circuit.
   • Another general purpose NPN transistors like BC107,BC148,2N2222 etc can be also used for Q1.

   
   

USB LAMP CIRCUIT

1. Here is a simple USB powered lamp that can be used to light your desktop during power failures. The circuit operates from the 5 Volt available from the USB port.
2.  The 5V from the USB port is passed through current limiting resistor R2 and transistor Q1. The base of transistor Q1 is grounded via R1 which provides a constant bias voltage for Q1 together with D2.
3. The diode D1 prevents the reverse flow of current from battery.C1 is used as a noise filter.Two white LED’s are used here for the lamp, you can also use a 2 V torch bulb instead of LED’s. LED D3  indicates connection with USB port.
    

 

 

NOTES:-

• USB port is only able to provide up to 100 mA current.So don’t overload the circuit with more no of LED’s.
• Before wiring the circuit confirm the positive and ground leads of USB by a multimeter.
• Switch S1 can be used to turn on the lamp.

MULTI CHANNEL AUDIO MIXER USING LM 3900

1. A simple multi-channel audio mixer circuit using LM3900 quad amplifier is given below.
2. The circuit consists of 4 channel quad amplifier (LM3900). Two mic audio inputs and two direct line     inputs are available in this circuit.
3.  By adding the same circuit parallel with this, you can increase the number of inputs according to the applications. Each input is connected to the inverting terminal of LM3900.
4. The built in amplifier of each section amplifies every audio input separately and is fed to the output terminals. The output terminal from each channel is connected to a single output line with a resistance not greater than 680K and produces a mixed audio at the output with very low noise.
5.  This audio mixer circuit doesn’t use a low impedance input to mix ideal sources. Capacitors C1 to C4 are the decoupling capacitors for the corresponding channels. C5 is the output decoupling capacitor.
    
    
    

   The main features of this audio mixer circuit as compared to the other audio mixers are given below.

   1)      Wide range of supply voltage input (4volt to 32volt)

   2)      Dual voltage supplies also adoptable (+/- 2.2volt to +/-16volt)

   3)      Low input biasing current(30nA)

   4)      Providing very high open loop gain(70dB)

   5)      Output short circuit protection

   6)      Simple and compatible design

   7)      Low distortion

   8)      Good frequency response

   
   

   
   

   NOTES:- 

   • Assemble the circuit on a good quality PCB.
   • The circuit can be powered from anything between 5 to 30V DC.
   • The power supply must be well regulated and free from any sort of noise.
   • LM3900 must be mounted  on a holder.
   • VR1 to VR4 can be used for adjusting the volume level of the corresponding channels.
   • All fixed resistors are 1/4 watt carbon film type.
   • If the power supply circuit is far from the mixer circuit, then a 100uF/50V electrolytic capacitor must be connected from the positive supply rail to the ground.

   
   

   
   

   
   

   
   

    

   
   

    

SINGLE CHIP FM TRANSMITTER CIRCUIT

1. Here’s a single chip FM transmitter circuit using Maxim semiconductors IC MAX2606. The MAX2606 is a compact, high-performance intermediate frequency VCO specially designed for wireless communication circuits. They have monolithic construction with low-noise and a low-power operation in a compact 6-pin SOT23 packing .
2. Th1s low-noise IC feature an on-chip varicap diode and feedback capacitances that avoid the need for external tuning components, making the MAX2606 perfect for portable systems. Only an external inductor is needed to set the oscillation frequency.
3. integrated differential output buffer is also there for driving a mixer or prescaler.The MAX2606 can be operated from a single +2.8 V to +5.4V supply and consumes very less current .The chip can be operated from 45MHz to 650MHz .
4. In the circuit the nominal frequency is set to 100 Mhz by inductor L1, (390nH) .
5. The left and right channel audio signals from your source are added by R3 and R4, and attenuated by the POT R2. R2 can be used as a volume control .POT R1 can be used to select a channel of transmission between 88Mhz and 108Mhz.Use 80 cm long wire as the antenna.

NOTES:

• Assemble the circuit on a good quality PCB or common board.
• Use a battery for powering the circuit.It will reduce noise.
• An FM antenna from a old radio is a better option than the wire antenna.

TELEPHONE OPERATED REMOTE

1. The circuit given below is of a telephone operated DTMF remote. The circuit can be used to switch up to 9 devices using the keys 0 to 9 of the telephone. Digit 0 is used to switch the telephone system between remote switching mode and normal conversation mode.
2. IC KT3170 (DTMF to BCD decoder) is used to decode the DTMF signals transmitted over the telephone line to corresponding BCD format. IC 74154 ( 4 to 16 demultiplexer) and IC CD4023 (dual D flip flop) is used to switch the device according to the receive DTMF signal.
3. The operation of the circuit is as follows. After hearing the ringtone from the phone at receiver end, press the 0 button of the remote phone. 
4. The IC1 will decode this as 1010.The pin 11 of IC2 will go low and after inversion by the NOT gate in IC3 it will be high. This will toggle the flip flop IC5a and the transistor Q1 will be switched on. This will make the relay K1 ON. The two contacts C1 and C2 of the relay K1 will be closed.
5. C1 will form a 220 Ohm loop across the telephone line in order to disconnect the ringer from the telephone line (this condition is similar to taking the telephone receiver off hook).C2 will connect a 10KHz audio source to the telephone line in order to inform you that the system is now in the remote switch mode.
6. Now if you press 1 on the transmitter phone, the IC1 will decode it as 0001 and the pin 2 of IC2 will go low. After inversion by the corresponding NOT gate inside IC3, it will be high. This will toggle flip flop IC5b and transistor Q2 will be switched ON. The relay will be energized and the device connected through its contacts gets switched. 
7. Pressing the 1 again will toggle the state of device. In the same ways Keys 2 to 9 on the transmitter phone can be used to toggle the state of the device connected to the channels O2 to O9. After switching is over, press the O key on the transmitter phone in order to toggle the flip flop IC5a to de-energize the relay K1.The 200 Ohm loop will be disconnected from the line, the 10 KHz audio source will be removed and the telephone receiver will be ready to receive new calls.

NOTES:

• Assemble the circuit on a good quality PCB.
• Use 6V DC for powering the circuit.
• A simple NE555 based oscillator can be used as the 10 KHz audio source.
• All IC’s must be mounted on holders.
• The section drawn in red must be repeated eight times (not shown in circuit).
• In certain countries circuits like this cannot be connected to telephone line.I do not have any responsibility on the legal issues .

LED BASED TRANSISTOR TESTER

1. Here is the circuit of a very simple transistor tester which used two LEDs for displaying the condition of a transistor. Both PNP as well as NPN transistors can be tested using this circuit. 
2. Quad 2 input CMOS NAND gate IC CD4011B is the heart of the circuit. Out of the four NAND gates inside the IC, only three are used here and they are used as NOT gates by shorting their input terminals.
3. Gates U1a, U1b, resistor R1 and capacitor C1 forms a square wave oscillator. The frequency of this oscillator can be adjusted by using R1. The output of the oscillator is inverted using the gate U1c. 
4. The inverted oscillator output is connected to the base of the transistor under test through the resistor R2 and the non inverted oscillator output is connected to the emitter of the transistor under test using the resistor R3.
5. The status of the LEDs D1 and D2 reveals the condition of the transistor under test. If red LED is ON, It indicates that the transistor under test is a good NPN. 
6. If green LED is ON, it indicates that the transistor under test is a good PNP. If both LEDs are ON, it indicates that the transistor under test is short.
7. If both LEDs are OFF, it indicates that either the transistor is bad or you may have connected it incorrectly.

NOTES:

• The circuit can be easily assembled on a Vero board.
• Use 5V DC for powering the circuit.
• All unused pins of the IC must be connected to ground.

TV ANTENNA BOOSTER

1. The circuit shown here is of a TV antenna booster based on the transistor BF180. The circuit operates in the UHF band and has a gain of 15dB.
2. Capacitors C2,C3 C4 ,C5 and inductors L3, L4 forms a UHF band pass filter. The input signal is fed to the emitter of Q1 through this filter.
3. Resistor R2 and R3 biases the transistor Q1 which is wired in the common base configuration.

NOTES:

• Assemble the circuit on a good quality PCB.
• For better performance, enclose the circuit in a metal box.
• The circuit can be powered from 12V DC.

SCORING GAME CIRCUIT

1. A simple scoring game circuit that can be used for all occasions when a dice is needed.The circuit is based on a NE555 timer,a 74LS192 counter,a74LS247 decoder and a & segment LED display.
2. The timer IC1 will produce the clock for the counter IC(IC2) whose frequency is determined by R1 and C2.When S2 is pressed the IC2 will count in up mode and when S3 is pressed the IC2 will count in down mode.The IC 3 will decode the count to display it on the seven segment LED display .Thats about the working of the circuit.The circuit is designed strictly sticking on to the basics of counters and is a good one for beginners.

     NOTES:

• To play the game switch the power ON and press S1 to reset the counter.
• Now press S2 or S3 and release .The IC2 will hold the last count .Now press S4 to see the score on display.That’s your score.Now the second person can try.
• Each time one tries, he should press the S1 to reset the count and then press S2 or S3 and then S4 to see the score.
• Circuit can be powered from a 9V radio cell or a 9V regulated DC power supply .

STEREO FM TRANSMITTR CIRCUIT USING BA1404

1. A high quality stereo FM transmitter circuit is shown here. The circuit is based on the IC BA1404 from ROHM Semiconductors.
2. BA1404 is a monolithic FM stereo modulator that has built in stereo modulator, FM modulator, RF amplifier circuitries. The FM modulator can be operated from 76 to 108MHz and power supply for the circuit can be anything between 1.25 to 3 volts.
3. In the circuit R7, C16, C14 and R6, C15, C13 forms the pre-emphasis network for the right and left channels respectively.
4. This is done for matching the frequency response of the FM transmitter with the FM receiver. Inductor L1 and capacitor C5 is used to set the oscillator frequency.
5. Network C9,C10, R4,R5 improves the channel separation. 38kHz crystal X1 is connected between pins 5 and 6 of the IC. 
6. Composite stereo signal is created by the stereo modulator circuit using the 38kHz quartz controlled frequency.
   
   
   
   
   
   
   NOTES:
   

   • Assemble the circuit on a good quality PCB.
   • Powering the circuit from a battery will reduce noise.
   • Use an 80 cm copper wire as antenna.
   • For L1 make 3 turns of 0.5mm dia enamelled copper wire on a 5mm dia ferrite core.

   
   

BUCK BOOST CONVERTER CIRCUIT

1. A very efficient buck boost converter circuit is shown here. The circuit is based on the LTC3440 buck boost regulator IC from Linear Technology.
2. The LTC3440 requires only one inductor and provides up to 96% efficiency. There is no need of Schottky diode for applications where output voltage is less than 4.3V and the IC can deliver up to 600mA output current.
3. In the circuit the IC is wired as a buck boost regulator providing 3.3V output from a 2.7 to 4.2V input.
4. R1 is the timing resistor which determines the oscillator frequency and it can be varied between 300 KHz to 2MHz by varying the value of R1. C1 is the input bypass capacitor for reducing noise and C3 is the output filter capacitor.
5. Resistors R4, R4 sets the output voltage and the network consisting of C2 and R2 is meant for frequency compensation.
   
   
   
   

   NOTES:
   

   • Assemble the circuit on a good quality PCB.
   • Input voltage can be between 2.7 to 4.2V.
   • L1 can be a toroid, potcore or shielded bobbin inductor.
   • C1 to C3 are ceramic capacitors.

   
   

AM RECEIVER USING ZM 414Z

1. This is the circuit diagram of the simplest single chip AM radio
2. The circuit is designed around the IC ZN414Z which is a ten transistor tuned radio frequency receiver .The IC has only three leads and is available in the TO92 package.
3. All necessary circuits required for an AM receiver (RF amplifier, detector and AGC are incorporated inside the IC).
4. Capacitor C4 decouples DC from the output of the IC and C1 bye-passes the noise.
5. Transistor Q1 and associated components forms a classic driver stage for the headphone.
6. Head phone is connected across the resistor R5 and R4 gives necessary biasing for the transistor Q1.
   
   
   
   NOTES:
   

   • Assemble the circuit on a good quality PCB.
   • The circuit can be powered from a 1.5V cell.
   • Do not give more than 1.8V to the circuit as it will destroy the IC.
   • For L1 make 80 turns of 0.3mm enameled copper wire on a 5cm long ferrite rod.
   • K1 can be a high impedance crystal earpiece.

AUTOMOBILE TURN SIGNAL CIRCUIT

 

1. This is a simple circuit that can be used as a sequencial signal light in automobiles.
2. The circuit is based on two ICs. A TS 555 CN CMOS timer IC and a CD4017 decade counter IC
3. The IC1 is wired as an astable multivibrator to trigger the counter IC. When triggered, the outputs of the IC 2 (pins 3, 2, 4 and 7) will go high and low in sequence and the speed of this sequencing will be proportional to the triggering frequency.
4. The transistors Q1 to Q4 drives the corresponding LEDs.
5. The switch S1 can be used to select the direction of turning and the LEDs arranged at the corresponding side of vehicle will start sequencing.
    NOTES:

   • The switch S1 can be the existing changeover switch of the vehicle it self.

   • The circuit can be powered of the 12V available from the vehicle itself.

   • The color of the LED depends on your choice .

   • The ICs must be mounted on IC holders.

   • Assemble the circuit on a good quality PCB .

   • Be careful with the wiring of this circuit because any wrong connection may put the electricals of your vehicle  in trouble.

   
   

   
   

   
   

   
   

   
   

   
   

FAN SPEED CONTROLLER USING LM2941

1. Many electronic circuits related to fan speed controlling have been published here and this one is just another approach. 
2. The circuit diagram shown here is of 12V DC fan speed controller using the IC LM2941CT which is a low drop out 1A voltage regulator. The IC has a dropout voltage as low as 0.5 and has also many useful features like power supply reverse protection, thermal protection, short circuit protection etc.
3. The maximum output current the IC can source is 1A.
4. The 12V DC supply is connected between the Vin (pin4) and ground (pin3) of the IC.
5.  The load, which is the fan, is connected across the Vout (pin5) and ground (pin3) of the IC. The network comprising of potentiometers R1, R2 and resistor determines adjust current (Iadj) of the IC.
6. By varying the Iadj using the POT R2 we can adjust the output voltage of the IC and hence the fan speed.

   NOTES:
   

   • The circuit can be powered from 12V DC.
   • The maximum possible load current is 1A.
   • A heat sink is recommended for the IC.
   • POT R1 can be used to adjust the minimum fan speed.
   • POT R2 can be used to adjust the fan speed.

   
   
   
   
   
   

   
   
    

TWEETER CROSSOVER CIRCUIT

1. A single coil speaker is not good in handling high and low frequency at the same time.
2. If we could filter out the low frequency and play it through a tweeter ,it will produce more sound quality than using a single speaker.
3.  This is what is realized here.
4.  Here the high frequencies are filtered out using a low pass filter made of C1,C2 and R1 and send to the tweeter.The low frequency is only fed to the woofer.

   
   

   
   
   

   NOTES:
   

   • The tweeter can be a 2 to 3 inch, 4 to 8 Ohm one .

   •  R1 is used to adjust match the the tweeters output level to that of woofer.

   • C1 and C2 are electrolytic capacitors.

   • R1 should be rated more than 2 Watts.