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Variable stat on off timer

kimbear

Member
I need a repeating timer with variable on/off states (basically varying the time that it will be in an on state, and varying the time that it will be off) the timing of each state will be less than 5 seconds.
The only schematic that I have found is attached, and I would rather not have it start by pushing a button, and I guess stopping by killing its power. There has got to be a more elegant way to do this.
Whatcha think??
Thanks in advance
 

Attachments

alec_t

Well-Known Member
Most Helpful Member
Welcome to ETO!
If you replace S1 with a 330pF cap the circuit oscillation should self-start.
 

rjenkinsgb

Well-Known Member
Most Helpful Member
You can do it with a single 555.

Use this style of connection, where it is just functioning as a schmitt trigger:


Then replace R1 with two separate pots (variable resistors) each with a diode in series and a low low value resistor, which can be shared. The two circuits connect in parallel, with their diodes opposite ways around.

That means the capacitor charges through one pot, and discharges through the other, providing independent control of the two states. The fixed resistor sets the minimum time and prevents the capacitor acting as a near short on the output and taking excess current if a pot alone were turned to zero resistance.
 

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