I have a 555 pulse generator controlling a relay going a little faster than 1hz. It is hooked up to start up when it receives positive voltage from the switched accessory circuit of a vehicle. That much works fine, but I would like to limit the number of pulses it puts out to around 15 or so, and then stop until the next time the vehicle is switched back on. Using another 555 chip, I thought about making a "one-shot" pulse generator to control the first one, but it seems that this circuit requires a negative pulse to initiate it. That doesn't quite work for the automated status that I require. Any thoughts?
Electronic – 555 timer circuitry
555control
Related Solutions
If you have reduced Rt,Ct and the pulse is still too long, consider that the trigger input signal may also be too long. (this circuit is designed to stretch an input pulse, not shorten it!) In the circuit you linked to, that is controlled by C1 and R4 (though their instructions suggest it is controlled by C1 and R3)
Reduce C1 to 10nf and try again. Reducing C1 too far will make the triggering unreliable.
You can have one 555 timer be the enable for a second 555 timer. One in mono-stable mode and the other oscillating at whatever frequency your pulses need to be at. Then just make the first 555's single pulse long enough to contain how ever many pulses of the second you want.
simulate this circuit – Schematic created using CircuitLab
Note the pin numbers are not in the same spots for the two packages. This was drawn pretty quickly, so also check for errors :)
Best Answer
The cleanest solution would be to add a counter (there are still available 4-bit counters that can count 16 pulses). Simply have the counter clock off the existing 555 timer pulses. An R/C time circuit can provide a simple reset to assure that the counter resets to 0 each time the power first comes on. When the counter reaches full count use the output to reset the 555 chip so it does not produce any more pulses until the power cycle reset.