# I need a basic swtich driven by an audio signal

So I have to drive a coil with about 9 to 12 volts. I have a trigger signal which is basically a sine wave coming out of a cell phone's headphone jack, which I believe should be around 1/2 a volt.

The question is can't I just drive this with a Transistor used as a swtich with a diod attached to the latch pin to cut out the reverse voltage from 1/2 the sine wave to get pulsed DC at high Voltage?

If so, anyone know some transistors I can use? Obviously they need to be rated for the voltage/amperage I'd be working at.

Second. How badly is the pulsed DC going to kill me with the coil due to impedance/back EMF?

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I have apparently forgotten everything about circuit design.

I'm having trouble coming up with this circuit. I realized yesterday that I have two output pins from the headphone jack of the phone (of course.) To what do I attach the other wire?

Here is the circuit as I have it so far but It cannot be correct.

So I've put a mic as the source. it's really going to be the headphone jack off my old cell phone.
My problem w/ the above schematic is, isn't the 2nd 1/2 of the sine wave now going to want to flow out MC1's B through ground into C1's pin2 and onward?

I believe TF suggested I put a resistor betweek the Cap/Diode pair there and the transistor base.

I believe a 2n222 will do the trick, but at  1/2 volt you might need to do some Darlington pairing.  I think your diode will cover your back EMF, but two diodes (one across the coils and one off the collector) might be even better, and you might want to put a 100k pot between Vcc and the transistor base just to be safe and to attenuate it.

Actually if I use a cap instead of a diode won't it convert my wave (AC) to DC? I assume I'd need a resistor or two in there as current limiters to keep from blowing either the cap or the transistor.

A cap in series with the signal will remove the DC component and only let the AC through. A cap between signal and ground will 'short' the AC signal, removing most of it, and letting any DC component pass through. Probably neither will get you exactly what you want.

I'd suggest putting a diode in series like you first suggested, and feed that diode into the positive terminal of a small cap. The positive terminal also connects to your transistor (via a base resistor), and the negative terminal connects to ground.

When the sine wave is present, half the wave gets fed to the cap, charging it up. Eventually the cap voltage is high enough to drive the transistor hard enough to switch the coil. When the sine wave is removed, the cap discharges through the transistor's base, and after a short duration the cap is too low to keep the coil powered.

If you use a MOSFET instead of an NPN transistor you'll likely need to add a resistor across the cap terminals to allow it to self-discharge (bleed off) when the sine wave signal is not applied.

High gain transistor highly recommended, as Max suggested a Darlington would make a good choice.

Yeah, this is what I meant. Of course the current has to be going into the cap at the same time as it is going into the base pin on the transistor. Then when the current from the original source goes away it will flow back out from the cap into the base.

However until I read your description I had the cap wired COMPLETLY wrong. Once you described it, it sunk in.

Now I have a second problem about what to do with the source. I realized I have more than one wire and i'm not 100% sure where to put the other one. hehe, see my other comment.