From: email@example.com (Tom Bruhns)
Subject: Re: I need a simple switch...
Date: 14 Dec 2002 22:49:40 -0800
NNTP-Posting-Date: 15 Dec 2002 06:49:40 GMT
I'm not sure how much help you need...the following probably will
work, but may work better with some tweaking of part values.
Imagine an N-channel mosfet that can turn the lamp on and off, lamp
from drain to +12V. Source grounded. Connect a resistor to the
drain, going off the a capacitor to ground. Let's try a 1 meg
resistor and a 4.7uF cap. This is going to end up meaning that you
have to wait maybe 10 seconds between turning the lamp on and turning
it back off. Switch from 4.7uF to the FET gate. Small cap from gate
to ground, maybe 0.01uF. Large R from gate to ground, maybe 10
megohms. One more transistor, a PNP. Emitter to +12. About 10k
base to +12. About 22k, or 47k, or 100k (doesn't matter a lot) from
base to FET drain. Collector through about 1 megohm to FET gate.
Switch pulses the FET gate to turn it ON if the lamp has been off long
enough to charge the 4.7uF cap high enough. Transistor keeps the FET
on. With lamp ON, the 4.7 discharges to a low voltage (fraction of a
volt) and pressing the switch turns the FET off, and also the
transistor, and the 10 meg keeps it off.
With a good enough (low enough gate leakage) FET and gate capacitor,
and if you turn it on and off often enough, you could drop the gate
resistor and the transistor and all its resistors. At 1pA, a 1uF gate
capacitor would only drift 1 volt per 1,000,000 seconds, which is
quite a few days. Then it would be the switch, two caps, a resistor,
the FET and the lamp. How low is the actual gate leakage current of
modern power mosfets?
"Vipero" wrote in message news:...
> I need to make a simple electronic switch that operates like this:
> When I press a momentary minibutton (no) switch a circuit closes, and turns
> on a small lamp, until I press the same momentary switch again, at which
> time the circuit opens again and the lamp goes off.
> (Press momentary on switch once and lamp goes on; press again and lamp goes
> Needs to work with 12v ac and dc source current.
> Any help appreciated,