Voltage Controlled Divider
for music synthesizers.
This was originally developed a timing accessory for a sequencer I was working on, though when fed from a VCO, interesting frequency trills like those of the old 8-bit computer games can be produced. Feeding the control voltage from a Psycho LFO produces some interesting effects.
There are two outputs, one a narrow "trigger" pulse, the other following the pulse width of the incoming signal. This means that any mark/space ratio sweep being applied to a VCO connected to this module, will also be on the output signal of this module.
The division can be varied from divide by 1 to approximately divide by 30.
How to use this module:
Connect the input to a signal source such as a VCO, LFO or sequencer master clock. Use the control voltage input to determine the number of incoming pulses per output pulse.
There are two pulse outputs, marked HO and LO on the PCB. HO (High Frequency out) gives a narrow trigger pulse output. LO (Low Frequency out) gives a pulse output, the width of which varies with the pulse width of the incoming signal. Depending on the individual ICs you use, the propagation delays of the chips will limit the speed at which the LO output can function. I estimate it drops out above 10 kHz, though as mentioned, it does vary with individual 40106 chips.
Experiment with it.
A little on how it works:
The schematic of the Voltage Controlled Divider.
The Voltage Controlled Divider consists of several distinct blocks, each which is fairly simple in its operation.
The CMOS portions of the circuit have been run from +9 volts to avoid exceeding the op-amp common mode input range.
The circuitry around IC1B processes the incoming frequency, and squares up the waveform so it is suitable for the digital portions of the circuit. It drives the counter (4024) and sets the output flip-flop based around two portions of the 40106.
The output of the counter (4024) is connected to a D to A converter made using an R/2R resistor ladder. The incoming pulse train will step the counter (4024) until the voltage on the output of the D to A converter is greater than that on pin 2 of IC1A, at which point the output of IC1A changes state, putting a HIGH on the HO output, resetting the flip-flop, and triggering a short delay that resets the counter to zero. This clears the HO output back to LOW.
The flip-flop drives the LO output through a transistor buffer.
IC2 is dedicated to processing the incoming control voltage. The first part of the circuit is a precession half-wave rectifier, blocking any CV that goes below 0 volts. The second part is used to set the minimum and maximum of the voltage presented to IC1A, as going past either end of the operating range results in either silence or an unpredictable oscillation. (Divide by 0 is NOT very practical!)
The component overlay. Original version
The component overlay. Rev 1.1
Before you start assembly, check the board for etching faults. Look for any shorts between tracks, or open circuits due to over etching. Take this opportunity to sand the edges of the board if needed, removing any splinters or rough edges.
When you are happy with the printed circuit board, construction can proceed as normal, starting with the resistors first, followed by the IC sockets if used, then moving onto the taller components.
Take particular care with the orientation of the polarized components, the electrolytics, diodes, transistors and ICs.
When inserting the ICs in their sockets, take care not to accidentally bend any of the pins under the chip. Also, make sure the notch on the chip is aligned with the notch marked on the PCB overlay. Please note that the CMOS chips are static sensitive devices, so make sure you handle them correctly.
The board is designed to use any variant of 7809, including the TO-92 and TO-5 packages, though I have not tested it with the TO-05 cans. No heatsink is required, due to the tiny current drawn.
It is a good idea to insert a 1k resistor between the +15V supply and the normalized connection on the CV jack. This is because +15 will momentarily be fed out of this jack when you plug something into it, due to the mechanical nature of the contacts.
This module requires setting up. Feed an audio signal into the F In input. Monitor the output at HO. With the control voltage set at 0 volts, adjust the OFFSET trimmer until the input frequency equals the output frequency. With the control voltage set at the maximum (15 volts if wired as per the diagram), adjust the SPAN trimmer until you get the lowest frequency possible. If the output stops then back up until you get a signal again.
The first run of PCBs has a minor error - the offset trimmer has been taken to the positive instead of negative rails. The modification is fairly simple. One end of the 10k resistor needs to be isolated from the positive rail, and jumpered to the negative rail instead. Follow the diagram.
The modification required on the first version of the Voltage Controlled Divider PCB.
This is a guide only. Parts needed will vary with individual constructor's needs.
If anyone is interested in buying these boards, please check the PCBs for Sale page to see if I have any in stock.
Article, art & design copyright 2001 by Ken Stone