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circuit needed

J

Jamie

Jan 1, 1970
0
Could someone suggest a simple circuit that would light an led for about 2-4
seconds when power is applied and then turn off and stay off until power is
cut and applied again?

Thanks in advance JJ
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R

R.Spinks

Jan 1, 1970
0
Should not the resistor (R1) and capacitor (C2) be in opposite spots to
provide a negative going pulse on powerup?
 
T

Terry Pinnell

Jan 1, 1970
0
R.Spinks said:
Should not the resistor (R1) and capacitor (C2) be in opposite spots to
provide a negative going pulse on powerup?

No, that wouldn't work.

As shown the RC time constant is long relative to the typical ramp up
of the power supply. So pin 2 remains below required 1/3 threshold for
a short time, triggering the mono. Then it stays +ve so that it's
still high when mono ends, avoiding re-triggering.
 
J

John Fields

Jan 1, 1970
0
No, that wouldn't work.

As shown the RC time constant is long relative to the typical ramp up
of the power supply. So pin 2 remains below required 1/3 threshold for
a short time, triggering the mono. Then it stays +ve so that it's
still high when mono ends, avoiding re-triggering.
 
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