The circuit consists of several distinct blocks. The first are the input shapers, made from IC1A and IC1B and their associated components. These take whatever signal is fed into the module and convert them to signals appropriate for driving the rest of the circuitry. With the values given, the sensitivity is set at around 1.4V, allowing triggering from signals with a +/- 10 volt swing, or with a 0V to +10 volt swing, both of which are common in modulars. The output waveforms of some modules will never fall below the 1.4V level, preventing triggering. This can be solved by increasing the value of the 10k resistor between pin 3 of IC1 and ground to 22k, or higher if needed.
IC1A is part of the circuit used to trigger the burst event. Coupled with IC1F and its associated components, it forms a "gate to trigger converter", generating a narrow positive going pulse when the "Trigger" input goes above the 1.4 volt threshold. This pulse is buffered and sent to an output jack for external use if needed. It also sends a pulse to the reset pin the 4017 via a simple AND gate. (More on this later).
IC1B is used to process the "External Clock" input. The frequency of the clock signal determines the speed of the output pulses. It can be either an external clock derived from an LFO, sequencer or similar, or from the internal clock circuit, which is normalized to the input jack.
Unlike any external clock signal, the internal clock is synchronized so that it generates a series of even length pulses when the burst generator is triggered. It has two ranges, selected by switching in or out a 330nF capacitor. The 2 meg pot specified for speed is not critical, and be anything from 1M to 5M, though obviously this will affect the range.
The output of IC1B is fed to the clock input of the 4017 decade counter, and also to an AND gate consisting of a 100k resistor and 1N4148 diode. The output of this AND gate goes to a pulse generator made from IC2E, IC2D and associated components. This pulse generator functions very similarly to the gate to trigger converter mentioned above, converting each cycle of the clock signal to a narrow pulse. This is buffered and sent to an external jack, and is the primary output of this module, namely a burst of pulses.
The 4017 decade counter forms the heart of the module. When reset, it counts up to the number selected by the switch connected to its outputs at the speed determined by the clock frequency. If we consider the rotary switch to be set to position "2" as shown in the schematic, the second clock pulse sent to the 4017 after it has been reset will present a logic HIGH to its Clock Inhibit pin (13) via the diode OR gate. This will stop the counter at that point, and any further clock pulses will be ignored. This inhibit signal is also inverted by IC2B and sent to the AND gate preventing the clock signal from reaching the second pulse generator. The inhibit signal also sends the event "End Out" output high. |