Deedee

Hi guys...
I am total newbie at electronics and i am trying to make a relay driver circuit using a 555 timer. The requirements were to generate pulses with 50% duty cycle and time period of 1 minute, 2 minutes, 4 minutes and always ON. I am attaching my circuit diagram... will any of you kind souls tell me if i have made a mistake?
Btw the relay is a 12 V 4PDT relay and has a coil resistance of 100 ohms.

k7elp60

I am not sure why you are using Q1 and Q2 to switch in extra capacitors. It had been my experience that large values of capacitors on a normal 555 will not work effectively. That is to say the leakage current get's in the way and they will not charge to the 2/3 value of Vcc. Two ways around this problem are 1. Go to a cmos version so larger values of resistors and smaller values of capacitors can be used. 2. Use faster times with a counter following the the 555 to divide the time.
The other problem if you want 50% duty cycles you need to change the configuration of the 555 as it isnt 50% with the values of the resistors shown.

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Deedee

Thanks for your response!
When I place an oscillator probe at pin 6 (threshold pin) i see something strange. The output at pin 3 is supposed to trigger from High to low when the Voltage at pin 6 is 2/3 Vcc (8 V). however the output actually triggers when the voltage across capacitor C2 reaches 10 V. As a result I get a very high on time (70s) compared to off time ( 30s). Is this also due to leakage of capacitor?
I am switching in extra capacitors to get different time constants.

rjvh

why don't you use the 555 in combination with a 4017 and if you want the longer times you conect (with diodes of course)the outputs from the 4017 together and through a colector switchc onect it then to the transistor

that's much more acurate

k7elp60

I know you are switching in the extra capacitors to get different time constants. I calculated high time a low time with the normal 555 equations and your values are very close. Why the circuit is acting that way I am not sure. I think you mean oscilloscope probe instead of ocillator probe on pin 6.
It is possible it could be giving a problem as a lot of oscilloscopes have a 1M input impedance.
I'll do some research and get back to you.

__________________
The great thing about electronics is unlimited ways to do the job. The only limit is one\'s imagination. I generally think my way is best.
Show me a different way. I have an open mind.