This is definitely a beginner question and I don't really know how to describe it.
This week I built an up/down counter with a 4029 counter and a 4511 BCD to 7segment decoder/driver. I needed a clock so plugged the output of a 556 astable I had already built on a tiny breadboard and ran it off the same 9V battery as the counter circuit.
I found this example and it's almost exactly what I built
http://www.electro-tech-online.com/attachments/up-down-counter-gif.26692/ except I have a 556 and not a 555.
THEN I connected another breadboard with a VCO on it to the same 9V battery as the counter circuit and used the BCD outputs of the 4029 counter to modify the frequency of the simple little saw wave VCO
http://electro-music.com/forum/topic-49901.htmlYou get little two note sequences at whatever tempo your clock is running at and they're different depending which of the 4 outputs you connect it to, like I was expecting. That's all fine, it's like a little step sequencer synth people use all the time.
QUESTION: Why is it that even when there's nothing connected to the control voltage input of the VCO I
STILL get the frequency of the VCO going up and down a little bit
at the same speed as the 556 of course just sitting there connected to the same power source (9V battery) as the counter circuit?
I even tried it with just the little 556 breadboard running (not connected to the counter at all or anything for that matter) and it still happens as long as the VCO and the 556 circuit are sharing the same 9V battery so obviously it has something to do with the 556 multi-vibrator messing with the power of the VCO.
How would I avoid this situation so the saw wave VCO stays at a stable frequency even when the 556 is running?
This IS and ancient LM556CN from what looks like 1984. Would using a 74c14 Schmitt Trigger or a modern 7555 to generate the clock signal for the counter help?
I guess that's what I should try next