I thought I was smart enough

When you release the short you're discharging that monster cap through the protection diodes on the chip, which probably dumps a huge amount of charge into the substrate. All sorts of weird things can happen. Does it work properly if you short out the cap on pin 2?

Also 470k is a pretty large resistor to use with a bipolar 555. How about a CMOS part and a 1 nF capacitor instead?

Cheers

Phil Hobbs

Maybe, but often a differentiated signal is used to trigger a 555, because the if the trigger is held low, it doesn't allow the output to time out. You just want a pulse on the trigger.

Yes, it does

I started smaller, but when it didn't work I increased the value hoping to be able to pull the trigger low easier.

I'll get the exact part #, not sure what he has. I have had him try from .001uf, 0.01uf, 0.1uf, 1uf, 10 uf.

Thank, Mikek

Your R2(470K) C2(1uf and higher) is suspect. You want a trigger pulse that is a lot shorter than the desired output pulse. There are no values given for R1 & C1, which set the output pulse duration. Try a much smaller cap for C2. You want R2*C2

ly.

en.

As Phil pointed out a large cap may have discharged too much current into the 555, so it may be damaged.

Have the fellow start with a fresh 555 and replace C2 with a 1N400X diode and get rid of R3, and change R2 to perhaps 10k. Pin 2 needs to get down to 1/3 of Vcc to trip. the diode will protect pin 2.

If it MUST be a capacitor isolated input (why?) then put the cap first in series with the diode and make it perhaps 100ufd. You will need to put a pair of resistors joined at the junction of the cap and the diode

- perhaps 20K to Vcc and 10K to ground. I think R2 will still work at

sure.

The diode solution is simpler...

John :-#)#

We finally got it triggering properly. The output timing was 1.1 seconds. The last final addition was adding a resistor from pin 2 to ground, setting the dc voltage on Pin 2 just above the 1/3 Vcc threshold. He used a 4.7uf cap. Once it worked I didn't ask him to alter anything. It was a long slog with more than 40 email exchanges. The last thing I mentioned was replacing the LDR with an infrared sensor and emitter. Mikek

To the one that ask whether the cap was needed, yes, if Pin 2 is held low, the output follows that instead of it's timing sequence. So we make it a pulse. There is an additional problem, If the input sensor has a slow change, it may not make it through the cap to Pin 2. That's a future fix.

Did you miss R3 at 5.6K in series with the cap??? That's hardly a HUGE amou nt of charge. It won't damage anything.