Frequency modulation in Lunetta world

[bleepbloopboy]

So, I'm a newbie in the Lunetta scene, but very interested in it as I managed to gather a good compilation of different CMOS chips.

Now, something that I'm interested in is if the 40106 or other CMOS based oscillators can be frequency modulated? Is this doable simply by inputing one oscillator into another, or is there something that would hinder this?

[Steveg]

Yeah ... sort of. They are not really suitable for that kind of modulation. The basic 40106 oscillator is a relaxation oscillator where a schmidt trigger inverter has a capacitor on its input and a resistor from the output charges and discharges the capacitor. If you have another oscillator that also connects to the capacitor via a higher value resistor it will speed up and slow down the oscillation rate giving a sort of FM.
There are various forms of hard sync using the second input of an oscillator built around a 4093 chip. You can find examples if you search this forum.

[ianbax]

Like Steveg replied - you can sorta - I've never worked out whether it's the fact that the source waves are square and are not really suitable for the type of sideband generating FM effects you get with sines but I've never achieved that classic FM sound with my lunetta set up.

But what I've done on one of my modules is set up a vactrol to modulate the pitch - should be loads of posts on the forum about these - this is an LED shining into an Light Dependent Resistor which you put across (in parallel) the resistor controlling the pitch of your oscillator.

LDRs can have a pretty big swing (think the one I had is like 2M fully dark), so you put it in parallel so the resistance only swings (roughly) between the values of the smallest resistor (a 100k pot say, controlling frequency)

The LED can be driven by a low frequency oscillator version of your 40106 and you get slow ramps up and down in the frequency like a siren. Bring it up to audio rates and you get a sort of crude FM. I've found the circuit can be improved by placing a capacitor in parallel with the LED (this explains it:
https://www.youtube.com/watch?v=zXu6o39ut44 to slow down the fade of the LED and smooth the effect (rather than rapid on and off). Because it's a capacitor there's no way to smoothly scale it (although some sort of variable resistor governing how quickly it drains might work) - I settled for a 'smoothing' on-off switch.

You can also put a variable resistor in series with the LDR to set the minimum resistance it can reach which acts as a sort of 'depth' control. Another useful electronzap video on that here https://www.youtube.com/watch?v=hYKe-y0WLpo

I guess you could get really sophisticated and make a dedicated attack-decay fade circuit of the LED. Or drive it with a different wave shape? On that - I haven't actually tried this circuit - it's been on my list of stuff to try - but it exploits the triangle wave at the oscillators input to the the FM'ing.

https://therepaircafe.wordpress.com/2017/12/31/square-fm/

Hope that's helpful!

[ianbax]

Also as a PS - I don't know if I was already getting too advanced given you've just started out. If you haven't got it, I'd strongly recommend the Nicolas Collins book Handmade Electronic Music - now in it's third edition but any copy is good (declaration of interest/idle boast: one of my devices is in the 2nd edition!) and in fact the original handout he used that turned into the book is freely available: https://www.nicolascollins.com/texts/originalhackingmanual.pdf

Whilst it's only really an introduction when it comes to lunetta world his basic coverage of the 40106 oscillator, it's extension to the 4093 and things like home made vactrols to effect pitch and amplitude really got the ball rolling for me.

Another link is the Elliot Williams logic noise series (again, apols if you're already on to this) https://hackaday.com/series_of_posts/logic-noise/ - that link is annoying as it starts with the end, if you get me.

Probably the most comprehensive step-by-step in CMOS / Lunettas I've seen.