At 10:30 PM 12/6/2003 +0000, you 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.  :-)

What kind of diode are you using for D1, D2? Should be something like a
1N4148. If you have an oscilloscope, what is the voltage swing at pin1/2?

Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com

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