[sdiy] Lowpower timer

[John Luciani]

On 5/3/06, Johannes Öberg <johannes.oberg at gmail.com> wrote:
> Is there some passive circuit I could use? I was thinking along the
> lines of an A/R envelope with a power MOSFET that would shut down
> almost completely. The A/R would be have a very short attack time and
> be activated by pressing a button, and would drive the MOSFET into
> conducting, the MOSFET would be in series with the rest of the circuit
> (before the regulator).

> Does this sound reasonable? Which A/R to use? Which MOSFET?

If the timing isn't critical that should work fine. Have the output of your A/R
circuit turn on and off an NPN transistor. Connect the collector of the NPN
to the gate off a P-channel MOSFET. The source of the MOSFET connects to
your power source and the drain to the circuit you want to turn on.

Make sure the minimum Vgs(on) voltage is appropriate for the power source and
and Vce(sat) of the NPN.

The A/R circuit can be two resistors and a cap. One resistor in series
with the cap
(the A) and the other in parrallel (the R). Keep Rseries low for fast attack and
Rparallel high for slow release.

(* jcl *)

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