Timer used LM555 problem.


Thread Starter

David Brown

Dear listers,
I used a IC LM555 to make a short timer, precision
timer, time control range from 0 - 0.4 second.
It can be adjustable by changing the value of the resistance from leg No.2,6 to positive .The type of timer is one short( on then off)
The timer is working OK , but some time , it stop working at one cycle then after that, it continues to work as normal , this mistake is repeated some time after 3000 jobs , some time about 100 . I have used good regulator, filter , choke, but this makes it even worse.
Anyone on the list have experienced the same problem, please give me advice.

Johan Bengtsson

Is it oscillating or delaying (ie astable or monostable operation)

If it is delaying, could it be that the trig pulses are close to each other when the error occurs? Depending on how it is connected and what resistor values are used it could take some short time for it to "recover" before accepting a new pulse when it is connected for delay.

It sounds of your description that this is the case, it also sounds by your description that it is possible to turn down that delay time to 0 (restor value = 0). This could actually damage the
555 chip if the timer value is actually turned down to 0.

Other than that I support the theory of spikes, they could be reacing the 555 chip by the power source, they are quite sensitive about that. Connecting a pair of capacitors parallel to the power source, one about 10uF and one about 10nF could do the trick (the optimal values heavily depend on how fast the spikes are)

Is the circuit mounted on a PCB with a ground plane? if not this theory is much more the possible cause, if this is "home made" and you have the possibility to change to a board with a ground plane it could greatly reduce disturbances.

/Johan Bengtsson

P&L, Innovation in training
Box 252, S-281 23 H{ssleholm SWEDEN
Tel: +46 451 49 460, Fax: +46 451 89 833
E-mail: [email protected]
Internet: http://www.pol.se/

Curt Wuollet

Could you possibly be missing a trigger pulse or have an overlapping pulse. IIRC overlap will restart the cycle giving a long cycle. There really isn't much to go wrong unless you have intermittants someplace..



It's probably the trigger that is causing this problem. Care to elaborate on how the device is triggered?


Willy Smith
Numatics, Inc.
Costa Rica