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555 timer help

I'm a relative newbie with electronics, and I'm trying to build a small

555 based adjustable timing circuit.

I have a 555 set up in monostable mode which needs to be triggered by a positive pulse that may be longer than the desired output pulse. The

555 also needs to be immediately reset if the trigger pulse goes low.

My current circuit

  • = junction ^ = bypass T = npn bjt transistor

12v ----------*-------* | | 1k R1 10k R2 | | *--*-0.1uF-*----- pin 2 (trg) | C1 IN --*--1k--T pin 3 (out)--------- OUT | R3 | *------^----------*----- pin 4 (rst) | | | 10k R4 | |

0v --------*----------*

Current problems:

  1. With the RESET pin connected as in the diagram above, the timer output (OUT) does not pulse. It remains high as long as the input (IN) is high and goes low when IN is low.

  1. If the RESET pin is disconnected and left floating, the output will pulse when IN goes low to high, which is correct. However, it also pulses on the high to low edge if it occurs after the initial output pulse is complete.

Any help is greatly appreciated.

-Frank

Reply to
fudj.pakar
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  • = junction ^ = bypass T = transistor

12v ----------+------------+ | | 1k R1 10k R2 | | +--0.1uF--+----- pin 2 (trg) | C1 IN --+--1k--T pin 3 (out) ------ OUT

| R3 | +--------^-----------+------- pin 4 (rst) | | | 10k R4 | |

0v -------------------------+
Reply to
Fude

The chip is probably still in reset when the trigger pulse comes in.

This may be caused by improper power supply bypassing. If so, you need a

0.1uF cap between vcc and gnd.

If you have an AND gate laying around, you can use that to cut off the pulse. Instead of tying reset to IN, tie it to Vcc. Then, attach IN to one input of the AND gate, and the output of the 555 to the other. Now, when you bring up the input, the pulse should be seen by the chip, and the output will be gated by the input, just like you want.

There may also be a way to fool the reset, but it may not be portable across different version of 555.

--
"I\'m mad as hell, and I\'m probably going to just sit here and take it!"
Reply to
Bob Monsen

Thanks for the tips Bob. I'll give them a shot this weekend.

-Frank

Reply to
Fude

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