555 as VCO
The 555 is a pretty useful IC. Plenty of of websites have lots of schematics on its uses. For example this one. The application notes are here.
I wanted to play with pin 5. This is the control voltage pin. By configuring the 555 as a simple squarewave oscillator, the frequency can be manipulated by changing the voltage on pin 5.
I wanted a ramping signal to modulate the 555, but since I was home over the holidays, I only had one 555 and several 4558s. So I used the 4558 to create a square/triangle wave oscillator.
In this configuration, I took three samples.
The first one is just the 555 with pin 5 floating. The frequency is changed by manipulating the pot.
Sample 1: Simple squarewave 555.
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The next sample is with pin 5 connected to the squarewave output of the other oscillator
Sample 2: Squarewave LFO to VCO
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The final sample is with pin 5 connected to the triangle output
Sample 3: Triangle LFO to VCO
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Adding a second oscillator
Since I had all those 4558s sitting around and the other oscillator was so easy to build, I created another one.
I had previously made a vactrol using an LDR hot glue-gunned to a blue LED. Left alone as is, it worked well as a vactrol as well as a sensor to detect ambient light. Wrapped up in tape it worked more like a vactrol. This inserted into the oscillator as the timing resistor.
You may need to add pots in series with this as the frequency swing may shoot way into inaudible frequencies. The second circuit made a very distinct chirp.
Two samples were taken.
The first is the chirp
Sample 4: Chirp
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The second sample was the chirp summed up with the 555. I used a standard summing circuit to do this
Sample 5: Summing of chirp and 555
