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Hy. I want to construct a 555 timer that has a delay for 20 seconds and then it turns on for 1 second and it turns off for 20 second and repeats in a loop...
Nikica if you want a precise timer, a 555 relies on stable supply voltage, a trimpot and very expensive large capacitors with very low leakage., making this design impractical due to the fact that most big caps have a self leakage in this range of time and going to an ultracap with long leakage time constants gets more expensive.
The traditional solution for this is to use a CD4020 counter that runs at a higher frequency and has ripple binary counters /2, /4 /8.... than can be gated to any combination to give a binary decoded output using logic gates to synthesize the result.. ANother method is use a 4017 decade Johnson counter with a /2 counter in front to gate clock and 1 bit out to get 1/20 duty cycle and then vary the clock using a much easier 1 Hz clock to generate the pulse. THen of course there is a PIC, 1 chip MCU approach and many others.
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a low cost Alum polymer cap at 1000uF 6V has leakage of 630uA thus with a 1V charge decay time constant is I=C dV/dt or dt= C dV/I = 1e-3 *1/0.63e-3 or around 1.6 second , so that wont work.
The longest time constant due to leakage in caps are FILM but due to density are small in value or huge in price.
If you find a cap that has < 10uA of leakage in this range of uF, you could use a Schmitt trigger Astable oscillator using R+Diode in parallel with 20R to get the asymmetrical pulse.
Or better use a precision comparator with precision R ratios above to create quick ramp down and slow ramp up to create 1sec High and 20 second low repeating. A current source is more stable.
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