I'm try to help the guy with the LDR triggering a 555. I'm slowly walking him through it and now just triggering it with a wire momentarily to grd, to pull Pin 2 low, through a capacitor. But it won't trigger. Here's the circuit, what is needed to make the circuit trigger properly.
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Ignore the LDR for now I realize a slow change is a problem.
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
--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510
http://electrooptical.net
http://hobbs-eo.com
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.
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
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
10K, but it might need to go up to 47K - I'd have to breadboard it to be
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.
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