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555 timer help please

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docscott999

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I am an amateur in electronics but I know the basics. I am trying to create a circuit with an on time of about 1s and an off time of about 8min. The circuit I have attached is supposed to allow this with the potentiometers. However with the potentiometers the on and off time are about equal, that is on/off of more than 4 min regardless of where I set the pot dials to.

Without the pots, I get an on time of about 4.6s and off time of about 1.2s. I know I need a duty cylce of less than 50% but putting a diode across R4 does absolutely nothing. I have read that the diode should allow my to get a duty cycle of <50%.

I don't know enough as to what to do next. Can anyone help me achieve the values I need of a time on of 1-2s and a time off of 8-10min?

R3 = 1K Ohm
R4 = 390 Ohm
C1 = 4,700 mF (electrolytic)

R1 = 100K Pot
R2 = 10K Pot
 

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The secret is to get rid of the inverter.
On time is about 1s (diddle R1) , off time ~470s (diddle R2).

Charge time constant is (R1 + R2)*680e-6, Discharge time constant is R1*680e-6.
 

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ok I see, well I have actually removed the relay and transistor from the circuit and I am testing the timing with the LED only. So even with out the inverter, I get an on time of 4.7s and off time of 1.2. What I need is an on time of 1s and an off time of 8 min. There has to be a way to get this.
 
If adjusting the pots has no effect on the timing then they aren't wired correctly.
With a 4700uF cap you need about 150k (R1 + R3 in series) to give 8mins 'off', and about 330 Ohms (R2 + R4 in series) to get 1s 'on'. The cap needs to have low leakage or the timings could be way out.
 
Alec, I put the values you listed into the tools calculator and got the results below.
On time (T1) is 489.639843 seconds.
Off time (T2) is 1.074843 seconds.
Total Period (T) is = 490.714686 seconds.
Duty cycle is 99.78096375945839%.
Frequency is 0.0020336053280459592 Hz.

As you can see the on time is 8 min and the off time is 1s, I need those reversed. This is the problem I seem to run into. I had read that puting a diode in parellel with R2 would help to get the duty cycle reversed but it had no effect. I essentually need the duty cycle reversed.
 
To invert the duty cycle, either use the other set of contacts on your relay, or wire the relay directly to the 555 timer output pin as shown in the diagram in post #2, without the transistor. If you must use the transistor for some reason, then the transistor can be wired differently. The 555 output can provide almost the same current as the transistor, though.
 
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If the 555 can provide enough current to drive the relay then the transistor is not needed. One end of the relay coil then connects directly to pin 3 of the 555 (as stated above) and the other end of the coil connects either to the 12V+ supply or to ground, depending on whether you want the relay to be on or off when the timer is running. For efficiency the relay should be on for 1 sec and off for 8mins. The diode across the relay coil should be pointing up (cathode towards 12V+), regardless of which way the coil is connected.
 
Thank you all very much. I got it now and it works just the way I needed it to. I didn't realize you could flip the duty cycle just by changing which side of the circuit the output is hooked up with.
 
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