Re: looking for a 555 timer circuit

On 31 Jan 2006 02:35:11 -0800, "Wayne" <waynelewis@xxxxxxxxx> wrote:

>MM74C02N is the part number on the chip.

---
OK.

If U3-1 is high, and U3-9 is toggling, then U3-2 should be either
low (close to 0V) or high (close to +5V), depending on where in the
three phase cycle U3 happens to be.

That 2.2V is the problem, and since it's on U2-2, U3-2 and U3-5,
that indicates that those three points are connected, like they
should be. In my experience, a voltage on a CMOS output about
halfway between Vcc and GND more often than not indicates a high
output shorted to a low output. If that's the case, then the
chip(s) involved in the short should feel warmer than normal, which
is anything other than cool to the touch.

What I'd do is to go over the wiring one more time, making sure I
didn't have an extra wire in there going somewhere it wasn't
supposed to, or a solder bridge creating a short, or maybe even a
burned or cut wire with the exposed conductor touching something it
wasn't supposed to.

If that doesn't help, then I'd disconnect the wire(s) going from
U3-2 to U2-2 and U3-5, connect U2-2 and U3-5 to ground, power up,
and hit the pause switch. U3-2 should wind up low, and if it comes
up high or at 2.2V then U3 is bad.

If it comes up low, then the trouble lies somewhere else and we'll
have to run some more tests to find out where it is.

Post back with what you find and we'll take it from there...

--
John Fields
Professional Circuit Designer
.

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