LMNC simple vco range

DIY STUFF
1

I know this might be somewhat of a dumb question, but is it possible to add a rotary switch to LMNC’s cem3340 vco like he has on 1222 performance oscillator? I already looked at the schematics and understand that they are very different, for example in his 1222 design, there is the reference trim pot that is connected to the rotary pot. I’m just wondering if adding this feature is technically possible on the cem3340 oscillator, or not really?

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2

Sure. The rotary switch is wired to output a multiple of 1V (1.00 V, 2.00 V etc), and that’s fed into the VCO via an extra 100k resistor, as an extra CV signal.

(pin 15 is a summing node, so you can feed in any number of CV voltages each via their own resistor. See the external and internal tuning CV connections here.)

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3

So in that case, would I just wire the rotary switch into pin 15 of the cem3340? Or would other I need to do additional wiring?

4

The standard is 1 Volt per Octave. So you set the rotary switch up as a series of voltage dividers making each step raise or lower the total voltage at the summing input by one volt (or one octave).

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5

I understand that part, and I am kind of using the 1222 schematic as a reference. I have 4 10k resistors wired from pins 1-5 on the switch, with a wire on pin 1 meant for ground, and another on pin 2 for the 1v. Thing is, would I connect the 1v wire to pin 15 on the cem3340 via a 100k resistor and connect the ground wire to ground, or would I need to do wire it differently and to what on the circuit?

6

Think of the rotary switch with its resistors as a discretized potentiometer. The “bottom” terminal, connected to ground, is like the counterclockwise terminal of a pot, the “top” terminal, in this case the one connected to the 4 V reference (you do have a 4 V reference connected to the rotary switch, right?) is the clockwise terminal. The center terminal of the rotary switch corresponds to the pot wiper, it has the selected voltage on it. That goes to a 100k resistor and from there to 3340 pin 15.

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7

Yeah, what he said. Cheers Rich.

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8

Also see:

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