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.