555 oneshot woes

[Iowa Tom]

I have rigged up a standard 555 monostable set to have a one second high output. I want it to be triggered by a low frequency square wave produced by another 555 in the same circuit. I am running the square wave to the oneshot through a small disc capacitor (uF unknown). The problem is the 555 oneshot stays on the whole time the output of the first 555 is low, for a while. Then after a few cycles of the square wave the 555 oneshot output goes low (the LED hooked up to GND shuts off).

I have pin 5 routed to GND through a small disc capacitor with the hopes of quelling any noise interference. Pin 4 is connected to VCC (+9V).

Any help with how to hook up a one shot that is indeed a "one" shot will be greatly appreciated. I don't have to use a 555 if another IC would work better.

What I need is a circuit that will trigger once on for 1 second no matter how long the input trigger signal is low. I do want the oneshot to trigger "on" again on the next low part of the low frequency cycle of the signal input however.

I am using the CMOS version of the 555 chip.

Thanks!

 

[jbeng]

Do you have a pull-up resistor on pin 2 of the chip you are trying to trigger?
JB

 

[k7elp60]

The 1/2 cycle time of the first 555 neads to be longer than 1 second. A rule of thumb is the time constant of the coupling capacitor between the two 555's and the pull-up resistor on pin 2 of the 555 one-shot is 1/10 the time of the first 555.