If you're expecting a 4-5 minute charge time for C2 (I assume
you're figuring on achieving this by having a very low duty cycle
coming out of U1B), and C2 is only a 1uF capacitor, both C2 and
diode D2 must be EXTREMELY low-leakage devices.

For example, suppose D2 were to leak just one nanoampere.  In 5
minutes (300 seconds), one nanoamp of leakage will drag the
voltage on C2 down by:

  V  =  I * T / C  = 1E-9 * 300 / 1E-6  = 300 millivolts.

More than a couple of nanoamps leakage, and the voltage on C2
will never get to where you want it.

If D2 is an ordinary silicon switching diode, its leakage could
easily be that much; also, if C2 is any kind of electrolytic
capacitor (tantalum or aluminum) then it, too, could leak that
much.

You'd probably need a polypropylene or polyester (mylar)
capacitor for C2 to get this to work, and also use a low-leakage
JFET as a diode (for an N-channel JFET, short the drain and
source together and use as the cathode; use the gate as the
anode).

Also, when using ordinary switching diodes with clear glass
bodies, note that they're photosensitive: impinging light will
make them leak like crazy.

One final note: wouldn't this be a perfect way to use up some of
those old PIC16F84's, or some other ancient, dinky PIC laying
around?  Why play around with this funky analog stuff.

Hope this helps a bit...

Dave D.

Chris wrote...

>I'm confused about a circuit I've been experimenting with.  The circuit
>is at http://www.nosreme.org/tmp/timer1b.png, using a 74HC14 hex Schmitt
>trigger inverter.
>
>U1A (buffered by U1B) produces a low duty cycle square wave, which
>slowly charges C2 through D2 and R3.  So far so good.  The voltage on C2
>starts to rise with the expected exponential curve, but then stops at
>about 2.6V (Vcc is 5V from a 7805), and I don't understand why.  I would
>expect it to rise close to 5V minus a diode drop.
>
>I've tried the following:
>
>1. Disconnecting U1C, in case input leakage was causing problems - no
>effect on C2.
>
>2. Changing C2 does change the time constant (I've tried 0.1uF and
>100uF) as expected, but doesn't change the final voltage.
>
>3. Reducing R3 does increase the final voltage on C2, but not by very
>much.  With R3 around 20-30k C2 gets up to about 3V.
>
>I'm stumped.  Can anyone enlighten me?
>
>The overall intention is a 4-5 minute timer (turn on, and a few minutes
>later it beeps), using the first IC I happened to pick up.  I already
>know it's a terrible circuit.  :-)

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