timed gated circuit

Pommie said:

Is the "fixed time" pico seconds, hours, days etc?

Mike.

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rjenkinsgb said:

I'd start with an inverter, to take the single "master control" signal, whatever you want to call it, and create an inverse of that as well.

Then two identical output circuits with an instant-on and delayed-off function.

They could be eg. an OR gate with one input direct from the appropriate control signal and the other input via a resistor capacitor diode type delay from the first input.
(ie. capacitor on the second input, charged via a lowish value resistor and diode from the first input when the signal goes high, discharged via a higher value resistor when the first input goes low).

Two of those, one on each the signal in & out of the inverter.

At each input change, the new output becomes active instantly and the old output "holds on" for whatever time is set by the capacitor on the second OR gate input discharging.

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rjenkinsgb said:

Yep, that's using negative logic for the "or" function but should work just the same - plus the advantage of the schmitt trigger gate inputs.

I'd connect the RC circuit input just to the other input of the same gate, rather than the opposite output, but it will probably not make much difference.

Either way, add a resistor in series with the diode as otherwise the signal feeding that will be delayed by the capacitor charge (or discharge) current; eg. 4k7

You could also use rather higher value timing resistors and smaller caps, on the CMOS inputs. Something in the 100K to 1M should be fine. eg. 470K and 1uF would give the same time constant.
That minimises the charge time through the diode and its resistor while still allowing a long off delay if needed.

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