555 timer help

[[email protected]]

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.

2. 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

 

[Fude]

Oops, try again.

+ = 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 -------------------------+

 

[Bob Monsen]

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.

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

2. 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.

This may be caused by improper power supply bypassing. If so, you need a
0.1uF cap between vcc and gnd.

Any help is greatly appreciated.

-Frank

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.

 

[Fude]

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

-Frank