LED RAMPING CIRCUIT

1. In this circuit the intensity of LED will vary in a ramping fashion. 
2. The circuit consists of three ICs: Two 555 timer ICs and one LM393 op-amp.
3. IC1 and IC2 are wired as oscillators to produce 10 KHz and 1 Hz frequencies respectively.
4. These two frequencies are given to the inputs of the op-amp LM393.LM393 is wired as a comparator and its output will be a PWM signal
5. This PWM signals controls the FET Q1 to drive the LED. The LED will rise from OFF state to full brightness slowly and then slowly fades to OFF state and this operation repeats.
6. The resistor R4 controls current through the LED.

   
   

NOTES:

• Assemble the circuit on a general purpose PCB.
• The ICs must be mounted on holders.
• The power supply for this circuit can be anything between 3 to 12V.
• I recommend 6V for this circuit.
• The effect can be varied by changing the values of C1, C2, R1 or R2.

SINGLE TRANSISTOR AUDIO MIXER

1. This very simple audio mixer circuit uses only one transistor.
2. The base emitter junction of the transistor is biased by the diodes D1 and D2.
3.  The signals to be mixed are directly coupled to the base of Q1.
4. Each input lines are current limited by using a 33K resistor. With the used component values the collector current is around 1mA.
    
    
    
    
   NOTES:
   

   • The circuit can be assembled on a Vero board.
   • The circuit can be powered from 15V DC.
   • Gain of the individual channels can be adjusted by adding a 10K POT in series to the lines.

ELECTRONIC MOSQUITO REPELLER

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

   
   
   
   
   
   
   
   
   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.

SIMPLE SOUND GENERATOR CIRCUIT

1. This is a real scream generator circuit suitable for any purpose like alarm or car horn.
2.  The circuit is based on two transistors Q1 and Q2.
3. When you press the switch S2 the siren starts up moving to a high frequency.
4. When the switch is released the tone slips down until you shift it up again by pushing the switch S2.

   NOTES:

• Adjustment of tone quality can be obtained by different values for C2.
• If the alarm oscillates before S2 is pressed.The transistor is leaky, replace it.
• S1 can be used as a power switch.

FAN SPEED CONTROLLER USING LM2941

1. 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.
2. 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. The maximum output current the IC can source is 1A.
3. The 12V DC supply is connected between the Vin (pin4) and ground (pin3) of the IC.
4. he 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. 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.
   
   
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
   
   

   
   

NIGHT SECURITY LIGHT

1. Here is a simple circuit switches on a light around 2 hours after midnight, the time at which most of the robberies taking place.
2. This simple circuit is build around a CMOS IC 4060 to obtain the required timing.
3.  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.
4. The values of the timing components R1, R2, C4 are so selected that the out put pin3 of IC1 goes high after 8 hours.
5.  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.
   
   

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.

MISSING PULSE DETECTOR CIRCUIT USING NE 555

1. An NE555 timer IC connected as shown here can detect a missing pulse or abnormally long period between two consecutive pulses in a train of pulses. Such circuits can be used to detect the intermittent firing of the spark plug of an automobile or to monitor the heart beat of a sick patient.
2. The signal from the pick up transducer is shaped to form a negative going pulse and is applied to pin 2 of the IC which is connected as a mono stable. 
3. As long as the spacing between the pulse is less than the timing interval,the timing cycle is continuously reset by the input pulses and the capacitor is discharged via T1.
4. A decrease in pulse frequency or a missing pulse permits completion of time interval which causes a change in the output level.

   
   

NOTES:

• Assemble the circuit on a good quality PCB or common board.
• The circuit can be powered  from a 9V battery or 9V DC power supply.
• The IC1 NE555 could be mounted on a holder.

SPEAKER TO MICROPHONE CONVERTER CIRCUIT

1. This circuit is a simple approach for converting a loud speaker into a microphone.
2. When the sound waves fall on the diaphragm of a speaker, there will be fluctuations in the coil and there will be a small proportional induced voltage. Usually this induced voltage is very low in magnitude and useless. Here in the circuit the low voltage is amplified using transistors to produce a reasonable output.
3.  The transistor Q1 is wired in common base mode and produces the required voltage gain. The transistor Q2 is wired as an emitter follower to produce enough current gain.
4. The voice quality of this circuit will not be as much as a conventional microphone but quite reasonable quality can be obtained. To set up the circuit, keep the preset R2 at around 10 Ohms and connect the battery. Now adjust R2 to obtain the optimum sound quality.
   
   

NOTES:

• Assemble the circuit on a general purpose PCB.
• Power the circuit from a 9 V PP3 battery.
• A 3 inch speaker can be used as K1.
• All capacitors must be rated at least 15V.
• An 8 Ohm speaker or head phone can be connected at the output to hear the picked sound.

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.