I need a sinple circuit that will be on for 2 secs and off for 4 secs and then repeat indefinetely.I built one with 555 and pots, but it is too sensistive to humid condition I am using it for: the timing is not constant. May an opamp?. Many thanks in advance, Bob.
What timing values did you use with the 555? What capacitor type are you using? Are you using the CMOS or junction transistor version of the 555?
Humidity affects the timing components, almost independently of what chip is watching their action, so you have to solve the humidity problem some other way.
Using an opamp as an active guard driver might help. This involves copying the timing voltage with a follower amplifier and surrounding the timing node with this copy voltage, so that there is no voltage across the board from the timing node. It doesn't help with leakage across the bodies of the components, though. Potting in epoxy may be the answer.
-- John Popelish
Use a crystal oscillator and divide that by an approiate amount to derive a
1 Hz pulse. Use that pulse to clock a divide by 6 counter. The MSB from the counter will give you the cadence you desire.count 4 2 1 0 0 0 0 off 1 0 0 1 off 2 0 1 0 off 3 0 1 1 off 4 1 0 0 on 5 1 0 1 on
What level of precision do you want? With decent components (especially the timing capacitor), you can expect the 555 itself (over a wide temperature range) to give 1% accuracy for the standard type,
2% for the CMOS type. My 555 data sheet doesn't mention humidity, but then I wouldn't expect it to, as that's going to affect *most* components, not just the IC. I'd guess that the extra stray resistance between leads, pins and circuit tracks might be the most significant effect. An interesting experiment would be to run a simple astable (any sort) first i a dry kitchen and then suspended over your morning hot bath.Can't the circuit be positioned away from the worst of the humidity? Or compensated with a draught of fresh air?
BTW, don't forget that the very *first* period of a 555 astable is significantly longer (maybe 40%) than all subsequent ones.
-- Terry Pinnell Hobbyist, West Sussex, UK
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Humidity influences neither the 555 chip nor most other components directly. It is the leads, the PCB traces or the coaltrace of the pots. You can fight humidity by laquer like nail polisher and using hermeticaly sealed pots. The right way these days is using some logic. Microchip has eight pins PICs and even six pin chips that can do the job very easely without the need of external timing components. AVR has some tiny eight pins chips as well.
petrus bitbyter
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