04-04-2013, 04:01 PM
Sound operated switch using a simple transistor circuit
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INTRODUCTION
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.
Clap Switch II
A revised version
Based on the comments of the many people who built this circuit successfully, and the few who tried to and failed, the circuit was revised. Two changes, mainly: the filament lamp was replaced by a bunch of LEDs, and the output stage was coupled to the collector instead of the emitter of the bistable.
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.
Amplifier
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, however, (because somebody clapped) this change gets coupled to the base entirely due to the action of the capacitor. This causes the current through the transistor to increase, and this increase in current causes the voltage at the collector, which was sitting near the supply voltage, to fall to nearly zero. If you clapped loudly enough, of course.
This is not a high fidelity audio amplifier. Its function is to produce no output for small sounds and large output for (slightly) bigger sounds, so the customary biasing network can be omitted. The 4.7 Megohm resistor in the previous version was as good as an open circuit, and its omission does not affect the operation of the clap switch in any way. Provided, of course, that you use that 10 microfarad electrolytic capacitor.
Memory
Two cross connected transistors in a bistable multivibrator arrangement make up a circuit that remembers. You can set it to one of two possible states, and it will stay in that state until the end of time. When one transistor conducts, its collector is near ground, and a resistor from this collector feeds the base of the other. Since this resistor sees ground at the collector end the base at the other end receives no current, so that transistor is off. Since this transistor is off, its collector is near supply potential and a resistor connects from this to the base of the other transistor. Since this resistor sees voltage, it supplies the base with current, ensuring that the transistor remains on. Thus this state is stable. By symmetry, the other state is, too.
Changing state
On a clap, the state of the bistable changes. The output of the amplifier is converted to a sharp pulse by passing it through a (relatively) low valued capacitor, of 0.1 microfarads (100 nanofarads). This is connected through "steering" diodes to the base of the transistor which is conducting. This transistor stops conducting, and the other transistor was not conducting anyway. So at a clap, both transistors become off.
Then, those two capacitors across the base resistors come into action. The capacitor connecting to the base of the transistor which was ON has voltage across it. The capacitor connecting to the base of the transistor which was OFF has no voltage across it.