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Hello!

I'm a long time—but still somewhat newbie—electronics hobbyist, who recently got back into it thanks to the Arduino and friends. Then I've programmed some bare AVR chips (they are cheaper than a ready-made Arduino) to control relays and things like that, and now I'm looking to build a small project around the Spark.

In my latest breadboarding I tried to chain two 555 timers to make a crude LFO, that would sweep an audible frequency up and down. I was pretty much stymied until I drew the schematic properly on here and I found out that I was wiring the transistor wrong!

By the way, is there any way to do it without putting a transistor between the two chips? I tried with a single resistor, but I think it was draining C2 through the Control pin of U2. Maybe I will try with different values.

Anyway, now that the physical circuit works, I'd like to get a plot of the frequency sweep.

Is there any way to do that? I looked at the Expressions help page, but I couldn't find an expression that would compute the (main) frequency at a certain node. That would be helpful, for example to quickly test various R and C value combinations, or to debug frequency-related issues.

Here's the circuit. If you run the Time Domain simulation, basically I'd like to get a third line, on a Hz scale, to show the frequency at OUT1. Something like F(OUT1), if there was a function F().

The other issue is that the actual circuit is a bit noisy. I'm powering it with a regulated and filtered 5V, with plenty of power, so it shouldn't be due to that. In the first picture below you can see the voltage at THRS1 (blue) and at CTRL2 (red). You can tell there's a high frequency component because the traces are quite thick.

In the second picture I've zoomed on the red trace, on a much smaller scale, to show the noise. Does anybody have any clue where that could be coming from? I've tried adding a few caps in parallel to the power source, to no avail.

Ok, I discovered a couple of things.

First, a 10k resistor is enough to couple the two ICs without any disruptions. It works on the breadboard too:

Second, I'm quite sure I was measuring my own instrument's noise above :-/ The 100MHz frequency band gave it away. Also, that's probably what the instrument's Bandwidth Limit option is for. So much to learn!

Still, my request for a frequency expression in CL stands. Is there such a thing?