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.  :-)

Chris

P.S. Thanks to everyone who helped with my previous question about DC
motors; the thread is still in my mailbox waiting for me to have time to
do something about it!

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