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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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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.

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.

What about this.

I don't know how to make DipTrace angle the diodes. That's supposed to be a bridge rectifier there.

Using the bridge rectifier allows for more current to be delivered from the signal line to the transistor, but in this case you'll still end up with pulsed current through the coil, since the rectifier output will swing through 0V twice for each sine wave period.

You could always combine this with your previous circuit (adding the capacitor to the transistor's base) to keep the transistor switched on whenever the sine wave is being transmitted.

You might run into a problem with the diode's forward voltage drop however - compared to the other circuit, the sine wave signal needs to overcome the minimum forward voltage of two diodes instead of just one. Normally this wouldn't be an issue, but the audio signal is very small, so you'll have to find/build a bridge rectifier with a low enough forward voltage drop.

Are you going to use a mono (2 contact) or stereo (3 contact) headphone plug? The ring contact nearest the wire is the ground, the other ring and/or tip contacts are for the audio signal itself.

Are you saying he should look for Germanium diodes to make his bridge then? I have read they have only a .2 to .3v drop vs the .6 to .7v drop of a standard diode.

Yep, germanium diodes have the lowest forward voltage drop of anything widely available that I can think of. At low current the voltage drop will be even less =)

Thanks for the tips. You just answered several of the questions I was going to ask.

I figured I was going to need a cap in there to smooth it from pulsed DC into solid DC but I wanted to look it up first so I left it out.

I realized just after I created this diagram that I had just designed an AC to DC converter. Now that's a phrase I can search the googles for and see a number of reference designs. So I figured I would check out some to make sure where I would be putting the cap would be correct. I'll have a new schematic soon.

The tip on the germainium diodes is great also TY.

New questions.

A friend of mine is speculating that 2600hz might be too fast. That I would have more success at something like 60 specifically because it would give the cap more time to charge. What do you think?


For the hardware I am going to use a stereo jack. I'm actually planning on just cutting the speakers off an old crummy pair of earbuds. I suppose it might be nice to the device if I soldered a resistor across the leads of the one that I'm not using? I know some electronics don't like generating a charge that doesn't have anywhere to go.