20 sec timer
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CaptBubba

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Ok, this isn't about headphones in any way.

I have to make this device that will trigger a camera after 20 seconds, so basically just a timer isn't good enough, it has to be able to actuate something. We are allowed to use batteries, capacitors, resistors, inductors, and transistors, as far as electronics go (and wire and switches, of course).

I've come up with the following design which I think should work but I was hoping people could give me a bit of feedback before I go spend 3 hours in the lab swearing at it.

The inductor in the diagram will actually be a coil of wire that will function as an electromagnet, perhaps slamming a peice of metal into the shutter release or something. When the voltage at the gate rises above the MOSFET's minimum voltage, current rapidly begins to flow and triggers the shutter. Voltage can be up to 12V, but I was thinking of just using a 6V lantern battery (or maybe two D-cells).

 
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zzz

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1) starting condition is not defined for this circuit: capacitor has no way to discharge (by that I mean bringing + terminal to (-)) when switch is open, thus the circuit would either never work or work only the first time around.

2) you'll never know when mosfet is going to start conducting as threshold voltage is uhm a very unreliable parameter.

I'd recommend putting switch between cap and the resistor so that in the open position cap is connected to (-) and in the closed position cap is connected to the resistor, and replacing mosfet with npn bjt. there you at least know for sure when it will get open so that you can put values in the circuit. a darlington is probably going to be even better for your application.
 
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CaptBubba

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This is something that has to work like one or two times. Moving the switch is a good idea, I will do it.

The threshold voltage may be variable from device to device, but am I correct in thinking that the voltage for a certain decive at room temperature won't change? That's why the pot is there, so that the time can be adjusted. I plan to use a 2200uF capacitor, btw. All the parts have to be avalible at radioshack, so that kinda cutes down on the options.
 
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