Subject: Re: Timer circuit design w/o 555?
Date: Tue, 15 Oct 2002 12:55:56 -0700
References: <0001HW.B9D1935C02BEE5C71662EAD0@news.covad.net> <3DAC6503.email@example.com>
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On Tue, 15 Oct 2002 11:57:07 -0700, Uwe Zimmermann wrote
(in message <3DAC6503.firstname.lastname@example.org>):
> found this one, it uses an additional transistor to scale down the
> capacitor charging current, i.e. you could use a 100x smaller C than
> normally required, which brings you back to small size and lower
This looks like it might work for me. But I -- being a beginner in this --
don't understand which components control the timing.
The C (.47uF) is multiplied by the transistor's gain by about 75 which gives
Which R is used to calculate the time value? 200K, or 1K? I'm guessing the
If I change these values to give a value of 10 minutes, I'd choose a 100K
potentiomenter (to allow some adjustment of the time value) and a 100uF cap
(times 75 via the transistor) will give about 800 seconds (13 minutes, max).
My choices from the local supplier for a 100uF cap is a resin-dipped solid
tantalum (10 percent tolerance). This is a polarized cap.
When changing the RC components that determine the time value, should all
othes remain unchanged?
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