8 stages LFO analogue sequencer

Posted on Dec 25, 2012

The CD4060 is a CMOS 14-stage binary counter with an internal oscillator on chip. The oscillator is controlled by the resistors and capacitor tied to the timing pins, 9-11. This oscillator runs the binary counter, and the various outputs count at the oscillator speed. In particular, pins 1, 2, and 3 count at 1/16384 th of the oscillator frequency. If we set the main oscillator to run from 16384 Hz to 163840Hz, then outputs 1,2 and 3 count at from 1 Hz to 10 Hz.

8 stages LFO analogue sequencer
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The outputs at pins 1-3 are supplied to the selector inputs of the CD4051. The 4051 is a one-of-eight analog selector switch. This is like a single pole, eight throw switch that connects one of eight positions to a single output pin. As the counting inputs change, input A, B, C, ... H are selected in turn and connected to the output on pin 3. Once H is reached, the cycle starts over at A. The inputs to the selector chip can be any voltage within the range of the chip's positive supply pin (shown as a plus sign with a circle around it) to its most negative supply pin, shown as ground. We could connect eight potentiometers to those inputs, with the potentiometers strung between the + supply and ground. If we did that, we could adjust the voltage for each step independently, and the sequencer would select each of those in turn. The LFO output could be any of eight different values, depending only on the settings of the eight pots. There is a commercial device that does a similar operation, although I have no idea if it's implemented in the same way. This task would be almost impossible to do with any reasonable number of opamps. To control the NPN transistor, all we need to do is to note that the transistor really doesn't care what voltage is there, it only cares about the current into its base. We can simply make the Rm mixing resistors be large, maybe 100K and up, and the resistors themselves make an effective...

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