Guest
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
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