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555 Start up time

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kinarfi

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I'm using a 555 timer and I want it to have an off time of about 60 seconds and an on time of about 180 seconds, not too hard to accomplish except it starts in the on state which lasts for 312 seconds as the timing capacitor does it's initial charge from 0 to 66%, off for 67 seconds as it discharges to 33% and the on for 217 seconds as it charges to 66% and continues to repeat at the off for 67 and on for 217 cycle.
I would like some suggestions about how to "jump start" the timing capacitor so that when the switch is turned on, the out put of the 555 goes low shortly after and then continue with the 67 / 217 cycle.
Thanks, Kinarfi
 

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cowboybob

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Assuming the push-button switch is a push-on/push-off type:

a simple solution might be to add another monentary "on" push-button (located right beside the "on/off" button) that applies, through a resistive voltage divider, 70% (or whatever value works to your liking) of Vcc directly to the THR (threshold) pin simultaneously with the "on/off" push-button, thus precharging the cap.

Not totally elegant, since an "off" press would also apply the precharge, but so what?
 
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MikeMl

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Most Helpful Member
I'm using a 555 timer and I want it to have an off time of about 60 seconds and an on time of about 180 seconds, not too hard to accomplish except it starts in the on state which lasts for 312 seconds as the timing capacitor does it's initial charge from 0 to 66%, off for 67 seconds as it discharges to 33% and the on for 217 seconds as it charges to 66% and continues to repeat at the off for 67 and on for 217 cycle.
I would like some suggestions about how to "jump start" the timing capacitor so that when the switch is turned on, the out put of the 555 goes low shortly after and then continue with the 67 / 217 cycle.
Thanks, Kinarfi

Several things:

This circuit starts out with the delay, and then goes into the cadence of the motor being on for ~200s and off for ~60s. I did this by inverting the RC timing circuit. Yes, the initial delay is longer, but since the motor isn't running yet, the user will never notice. The LED lights up to confirm that the circuit is active.

I redrew the circuit to make it easier (for me) to understand. Note that I grouped the transient suppression diodes with the coils they are suppressing to show that short leads are required. Also, Since the coil currents are ~1A, I would use rectifiers instead of small-signal diodes.

To simulate starting up with every node discharged, you don't need the pushbutton. The Transient Simulation directive .tran 1200 startup does that. Don't use the .UIC.
 

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kinarfi

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Thanks for the help!
Decided to abandon the 555 and use a dual opamp seems the easiest considering the housing I'm using.
Kinarfi
 

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cowboybob

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Mike,

VERY clever, indeed ;). I will remember that trick.

CBB
 

MikeMl

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Here's another approach. Note the C2 has to be 2*C1.
 

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kinarfi

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FYI The timer drives an air pump used to inflate some cells wrapped around my calves which helps to warm them and reduce my restless leg syndrome problem which I think is caused by poor blood circulation in my legs and feet. I'll try the dual op-amp next because I like the idea of having the pump turn on almost instantly.
Thanks for the suggestions,
Kinarfi
 

MikeMl

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Unless I missed it, the pump turns on when the 555 output is low. If so, the second circuit I posted turns the pump on almost instantly.
 

kinarfi

Well-Known Member
Unless I missed it, the pump turns on when the 555 output is low. If so, the second circuit I posted turns the pump on almost instantly.

You're correct, but what I want is instant on, ~ 60 on, ~ 180 off and I can use either output, hi or low, via pnp on the low side or npn on the high side. Also, on my diagrams, I have been showing current flow.
oh yea, good idea to group the components, just lazy or too busy to get to it.
 

MikeMl

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kinarfi

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Final, almost, solution, dual op-amp oscillator, pFET switch, still adjusting on and off time. but it appears to be working just fine. when I turn it on, it starts pumping immediately
 

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