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non retriggerable timer

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sonalip

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i m using IC 4538 as a timer. in retriggerable mode, if once triggered and triggered again before time elapsed , then o/p remains high for time twice as per set. i want to use it as a non retriggerable. i want a circuit as when triggered once o/p will remain high for time t=RC, though it triggered again within time t , then it should ignore this trigger. but when i used the circuit given in datasheet on power on itself o/p is set to high for time t, goes low and again becomes high on next trigger. can any one tel me, where is my mistake? :confused:
 

ericgibbs

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i m using IC 4538 as a timer. in retriggerable mode, if once triggered and triggered again before time elapsed , then o/p remains high for time twice as per set. i want to use it as a non retriggerable. i want a circuit as when triggered once o/p will remain high for time t=RC, though it triggered again within time t , then it should ignore this trigger. but when i used the circuit given in datasheet on power on itself o/p is set to high for time t, goes low and again becomes high on next trigger. can any one tel me, where is my mistake? :confused:
hi,:)
To ensure that the 4538 is reset at power on, you need to add a simple Res/Cap to the CD pins 3 and 13.

A 10K resistor in series with 22n, connect the 10K to the +V rail and the lower end of the 22n to the 0V rail and the junction of the 10K/22n to the CD pins.

Do you also want the 4538 to be NON retriggerable in normal operation.?
 

sonalip

New Member
hi ericgibbs,
i connect the res 10k and cap 22 nF as u said , but it did make difference. i cheked the voltage at pin 3, it was constant 5 VDC. the voltage at input pin 4 is 0 volt still o/p is high. i think the i/p gets voltage internally from charging / discharging of any capacitor , dont know the exact reason and how to avoid it. not every time at power on , o/p becomes high. but can't predict when it will become high.
 
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