triggering a 555

Thread Starter


Joined Dec 13, 2005
hey, im currently working on my A level project and i decided to make an infra red sensor. This is made up of 5 diodes, one emmiting, one receiving and if anything passes through, it sets the alarm off. In theory anyway.

The bit im stuck on is that ive made the sound device, two 555 timers that give a pulsed output but they work all the time as soon as plugged in. In other circuits ive always rigged up the trigger bit with a button to set them off but it wont work here as once it has been set off, i need them to keep going. even if the person moves out of the infra red line.

The trigger pin of the chips are looped back to pin 6, the threshold which also doesnt help. Anyone out there have any ideas as to how i can get it to trigger, and stay triggered?


Joined Oct 11, 2004
Wire the first 555 as a bistable, ie. trigger to pin 2 and a resistor from pin 6 to 0V. Once triggered, the output wil remain high until reset by taking pin 6 high.

Wire the second 555 as your oscillator. Take its Reset pin to 1st 555 output. Oscillator will then be enabled when 555 is triggered and stay on until reset.