# Octave switch for simple CEM3340 Oscillator?

if not you can also add a jack for Sync in, FM in, a n other CV in, PWM in, PW pot … ???

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If you look at the circuit the CV and the tuning knob (which is really just another CV) both connect to pin 15 through 100k resistors. You can add more CVs the same way, a voltage souce to a 100k resistor to pin 15.

If you look at the schematic for the 1222 LMNC VCO it has this:

That’s just a rotary switch with 10k resistors between the positions, with some reference voltage at one end and ground at the other, and the center contact goes to the VCO chip. He has it buffered with an op amp but I think you can get away without that. Then there’s a 100k resistor and it goes to pin 15.

You can think of the rotary switch plus resistors as a quantized potentiometer if it helps. Then it’s just like the tuning knob.

Edit to add: In this case the reference voltage at the right is 4V, so the voltage at the positions between the resistors is 1V, 2V, and 3V. So each step on the rotary switch gives you a 1V difference, meaning a 1 octave change.

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Thanks guys. I actually have a few rotary switches laying around, and I’m guessing “Ref” is probably 5v? Maybe I can just slap down a 5v regulator off the power supply and wire a few 10K resistors from my rotary switch. Thanks!

Edit: just saw your edit about ref being 4v but yeah, the principle remains more or less the same. Should be pretty do-able.

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ref = 4v on schematic

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for a good octave it will be precise maybe a trimmer

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Of course you could use 5V with five resistors. Main thing is it has to be an accurate 5V and high precision resistors if being in tune is something you care about… a trimmer pot to adjust the voltage might be called for. The 1222 does it like this:

(except the 220R resistor probably ought to be at least 1k, see recent discussion in another thread.)

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we are very syncro

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You can try , but keep in mind that the LM4040AIZ is a precision voltage reference, not a regulator, and the 4V version provides 4.096 V ±4.1 mV (trimmed to 4.000). Your typical 78L05 regulator provides 5 V ±200 mV, and is also a lot more sensitive for temperature changes. So while a regulator kind of works, the octave switch won’t be very stable.

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Interesting. I was hoping to get this done without another trip to Mouser but I might have to add it to the next shopping list. I appreciate the input.

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Isn’t the precision pretty unimportant if we use the potentiometer voltage divider after that anyways? So it’s more about its stability (temperature and current wise?)?
I am just guessing here, so please correct me!

This is a kind-of-unrelated point, but it involves the other CV inputs - has anyone encountered the issue of your oscillator going out of tune when you try to add a vibrato LFO into an attenuated CV input? Like as soon as you plug in the cable, even if it’s at 0 volts, the internal CV mixer is now weighing 0 volts against the other CV signals and it goes flat. that is - unless there’s something else very wrong with my rig.

if i’d be using the +12v for the reference voltage, what could i use to reduce it to 4v?

Why not the way it’s done in the performance VCO?

(Except, again, with at least a 1k resistor instead of 220R)

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An LM4040AIZ, a resistor, and a trimmer. See the schematics above. A TL431 might work (requires a slightly different circuit), but it’s not quite as good.

Your 12 V supply rail is not a voltage reference.

EDIT: To clarify, it’s not a good reference, at least – it’s not designed to be that stable, it’'s not regulated close to the location where it’s used, and it has a lot of things plugged in to it, impacting the output.

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All the frequency controlling CVs are summed into a virtual ground, so unless what you plug in is at a negative voltage, it should not bring the frequency down.

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can i use a tl072x for the op amp and the sqare out expansion? or it has to be the tl072?
(im checking online and the tl072’s are sold out)

What’s a TL072x? Got a pointer to the product page? (the TL071/72/74 series is sometimes referred to as TL07x or TL07xx, but that’s something else)

(also, I assume you’re talking about a specific supplier, because the TL072 is about as jellybean as jellybeans can be, and e.g. from a quick check at mouser it looks like they have around 20,000 DIP ICs in stock…)

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i was precisely checking mouser, and the tl072 are sold out. next ic that came out was a tl072acps (i googled it and just came out tl072cp), 072qdrep, 072cd etc.
https://www.mouser.cl/Search/Refine?Keyword=tl072
https://www.mouser.cl/ProductDetail/Texas-Instruments/TL072ACPS?qs=sGAEpiMZZMve4%2FbfQkoj%2BBOlwMB6vrP%2FfgTcebvnOT0%3D

They’re all variants of the TL072 and the ones in PDIP packaging probably are all okay. Actually I have to admit I’ve never thought to concern myself with which version I’m using, aside from packaging (DIP vs SOIC). If I want DIP TL072s I get them from Tayda and looking closely at the listing I see they’re TL072CP. I suppose someday I might find myself with a circuit where it matters what VIOmax is but it hasn’t happened yet. I’m pretty sure I’ll never need to worry about whether my module will operate down to -55°C or not.

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