I need to trigger a small motor with the leads that activate the alarm on a alarm clock. the voltage that is sent to the clocks speaker on the alarm being triggered is about 0.3 volts. The motor, which runs off an independent set of battries, runs on 3volts DC. What kind of generic transistor should i use for a switch to trigger my motor ?
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I need to trigger a small motor with the leads that activate the alarm on a alarm clock?
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Connect a Diode in series with a Resistor to the Base of a Transistor (like 2N3904), the Collector to one lead of the Motor, the other motor lead to +3 volts DC and the Emitter to the – of the 3 volts DC supply.
The Diode is to rectify the AC (sound) and the Resistor is to not-load the amplifier that feeds the speaker. The DC gain of a 2N3904 is about 50 (depending on the collector current), so if your motor would need 100 mA to run, you would need a base current of about 2 mA. If your signal source is 0.3 volts you cannot overcome the drop on the Diode and on the Base-Emitter junction of the transistor. Evidently, you need something else than what I suggested (like an AC stage with gain), but it gives you some ideas. Also, consider whether you want the motor to latch-on (continue running after the sound goes off). This would require a different circuit, or additional circuitry. One way to do this would be using a small relay with an extra set of contacts to keep the motor with power after the sound goes off. You would then need a switch (or push-button) to turn the motor off.
An AC stage with gain could be another 2N3904 with a Resistor in series with the Base and another on the Collector. Then connect that Collector to the Diode mentioned above. If your motor takes more than 500 mA, then a 2N2222 I think is like a 2N3904, but it handles a higher Collector current. Google it. http://en.wikipedia.org/wiki/2N2222
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Added: As somebody else suggested, in an answer to another question, if you use a Relay, then connect a Diode across the Relay’s Coil, with the Cathode to + Supply and the Anode to the Collector of the Transistor driving the Relay. This will prevent the Transistor from being damaged by the reverse-back-voltage generated by the Relay’s Coil, when the transistor switches to OFF.