Driving a 555 from another 555

[bitman]

I have a (to me) strange issue. I have a small circuit that needs to send out a set of small pulses from a push of a button. I have created two 555 timer circuits (using a LM556) where one is a momentary switch with about 1 sec hold time, and the other is approx 100Hz which is supposed to turn on ONLY when the output signal from the first timer is high. See the attached diagram.

First of, this setup works albeit it took quite a bit of experimentation with the Control Voltage.

Problem: When I don't use the switch, after a bit of time (5-10 seconds) I get a pulse about every second from the second timer - even though there's no signal coming out from the first timer (U1A).  I found that adding a small resister in parallel with the D1 LED would fix this (not sure why) but if there's another circuit that turns on an LED for instance, somehow that triggers a single pulse from this circuit too.

I'm sure I'm overlooking the obvious? I just cannot see it.

[malagas_on_fire]

Hi

Pull Resistor  33K to ground, connect pin 5 to reset of second 556 and its CV pin using a 10nF capacitor to ground.

[mikerj]

I agree with with the above, use the reset pin to gate the second 555, not the CV pin.

[Gyro]

Looking at your schematic, I don't see a supply decoupling capacitor. With LM555/556 it's important to have a 100nF capacitor between supply and ground, close to the package. Without this, false triggering is possible (particularly in a dual configuration) - the output stages take a large supply current spike when they switch.

While it isn't the problem in this instance, it's an important thing to remember.

[malagas_on_fire]

Hi

Pull Resistor  33K to ground, connect pin 5 to reset of second 556 and its CV pin using a 10nF capacitor to ground.

Here is a quick drawing of the purpose connection, plus the bypass cap sugested by Gyro . It was added a 220 Ohm in series between output of first 556 to second 556 reset to protect pin of overcurrent spikes.

[bitman]

Hello malagas_on_fire,
Just a quick followup - your design was pretty much where I started except for the pull DOWN resistor. That change made it work.  I've got some sources (books etc) that states what I did should work (and it actually did) so I'm now trying to figure out why yours is so much better   Anyway, thanks for the post - it was very helpful.