vcd

Description: This module was originally developed as a timing accessory for a sequencer. When connected to a Voltage-Controlled Oscillator (VCO), it can produce interesting frequency trills reminiscent of old 8-bit computer games. Feeding the control voltage from a Psycho Low-Frequency Oscillator (LFO) yields intriguing effects. The module features two outputs: one providing a narrow "trigger" pulse and the other following the pulse width of the incoming signal. This means that any mark/space ratio sweep applied to a VCO connected to this module will also be reflected in the output signal. The input should be connected to a signal source such as a VCO, LFO, or sequencer master clock, while the control voltage input determines the number of incoming pulses per output pulse.

There are two pulse outputs labeled HO and LO on the PCB. HO (High Frequency out) produces a narrow trigger pulse output, whereas LO (Low Frequency out) generates a pulse output whose width varies with the pulse width of the incoming signal. The individual integrated circuits (ICs) used may limit the speed of the LO output due to propagation delays, with an estimated maximum frequency of around 10 kHz, although this can vary with different 40106 chips.

The circuitry surrounding IC1B processes the incoming frequency and squares the waveform for compatibility with the digital portions of the circuit. It drives the counter (4024) and sets the output flip-flop based on two sections of the 40106. This block is fed through a DC mixer stage, which includes both inverting and non-inverting inputs, as well as a DC offset for easy adjustment of the "sweet spot" for the incoming signal. The mixer also has an output, making the unit a useful utility module even when not dividing signals.

The output of the counter (4024) connects to a Digital-to-Analog (D/A) converter constructed with an R/2R resistor ladder. The incoming pulse train steps the counter (4024) until the output voltage of the D/A converter exceeds the voltage at pin 2 of IC1A, causing the output of IC1A to change state, producing a HIGH signal on the HO output, resetting the flip-flop, and triggering a delay that resets the counter to zero, which clears the HO output back to LOW.

IC2 is dedicated to processing the incoming control voltage. The first section of the circuit acts as a precision half-wave rectifier, blocking any control voltage that falls below 0 volts. The second section sets the minimum and maximum voltage presented to IC1A, as exceeding either end of the operating range can lead to silence or unpredictable oscillation, which is impractical.

There are two unmarked capacitors near the power connector that can either be ignored or fitted with decoupling capacitors, such as 100nF capacitors. Before assembly, it is crucial to inspect the board for etching faults, checking for shorts between tracks or open circuits due to over-etching. Additionally, the edges of the board should be sanded if necessary to remove any splinters or rough edges.

Assembly can proceed normally, starting with the resistors, followed by the IC sockets, and then the taller components. Care should be taken to avoid bending any pins when inserting ICs into sockets, ensuring that the notch on the chip aligns with the notch marked on the PCB overlay.

This module requires calibration. An audio signal should be fed into the Clock In input, and the output should be monitored at HO Pulse Out. With the control voltage set to 0 volts, the OFFSET trimmer should be adjusted until the input frequency matches the output frequency. With the control voltage set to the maximum (15 volts if wired according to the diagram), the SPAN trimmer should be adjusted to achieve the lowest possible frequency. If the output ceases, the adjustment should be reversed until a signal is restored. It is important to note that the first run of PCBs has a minor error related to the voltage follower IC4B.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 follo wing 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. 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. 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. This block is fed via a DC mixer stage which includes both inverting and non-inverting inputs, as well as a DC offset allowing for easy adjustment of the "sweet spot" for the incoming signal. The mixer also has an output making the unit a handy utility module even when you are not doing any dividing.

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. 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!) There are two unmarked capacitors near the power connector. These can be ignored, or if you prefer, have decoupling capacitors installed in them. Any 100n capacitors would do. 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 socket if used, then moving onto the taller components.

When inserting ICs into 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. This module requires setting up. Feed an audio signal into the Clock In input. Monitor the output at HO Pulse Out. 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. Important. The first run of PCBs has a minor error - the voltage follower IC4B (center