Report: Simple Transistor Circuit

Topics: Transistor, Diode, Capacitor Pages: 6 (1474 words) Published: June 22, 2013







This project report could not have been prepared, if not for the help and encouragement from various people. Hence, for the same reason we would like to thank our guide and teacher CHRISTY A. KOSHY. It was for his support that we

Got proper guidelines for preparing this project.

Submitted to:-



We are the student of ME-Y declares that we have made our project on “BEEE” and it is authentic piece. We have not copied it from any source.








The operation is simple. Clap and the lamp turns on. Clap again and it turns off. The electret microphone picks up the sound of your claps, coughs, and the sound of that book knocked off the table. It produces a small electrical signal which is amplified by the succeeding transistor stage. Two transistors cross connected as a bistable multivibrator change state at each signal. One of these transistors drives a heavier transistor which controls a lamp. I built my prototype on a cardboard cover from an old notebook. Punched holes using dividers and placed the components down flat. It might look neater if you draw the circuit diagram on to the board before you begin. A photo is included below. The components are from my junk box and I found that it works even if you omit that 4.7 Megohm resistor. Your results may vary. The transistor types are not critical and any n-p-n silicon transistors should work.

Circuit Diagram

Sound operated switch using a simple transistor circuit


How it works

Input Transducer

The sound of your claps is picked up using an electret microphone. Some people call it by the name "condenser microphone" which usually refers to exhorbitantly priced things intended for the recording studio. If you could buy yours and still have your shirt on your back relax - it's an electret mike all right. Inside it is an electret film - which is the electrical analogue of a magnet - stretched so that it will vibrate in sympathy with any sound falling on it. These vibrations cause the electrical charge on a perforated plate nearby to change, and a field effect transistor converts these into corresponding changes in current. This microphone has a stage of amplification built in. The power for this built in amplifier is supplied by connecting a resistor to a positive source of voltage, and the changes in current get reflected as changes in voltage across this resistor according to the familiar relation V = I*R. A larger resistor will give you a larger voltage, but then, the current into the device gets reduced which brings down the gain. The value of 5600 ohms (usually abbreviated to 5.6K, and written down in schematics as 5K6) seems to work all right.


A transistor stage, biased near cut-off (that is, almost no current with no signal) amplifies the signal from the microphone. The output of the microphone is coupled to the base of the transistor using an electrolytic capacitor (note: using a better capacitor here will not work). The top of the electret microphone is at a few volts, the base conducts at around half a volt, so the leakage current of the capacitor (all electrolytic capacitors leak at least a little bit) will eventually cause the steady state condition in which the leakage of the capacitor goes into the base terminal of the transistor. So the collector will have Hfe times this leakage, which can usually be ignored. The first time the microphone output goes positive,...

References: MADHUR JAIN (E104063)
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