Motivation

Signal to Trigger converter

When you have the basics for your synthesizer like VCO, VCF, VCA, ADSR, LFO,... and some controllers and you want more then using your keyboard to steer the synthesizer it is time for some modules to produce trigger and gate signals out of different sources. Here is one of them. A signal to trigger and gate converter. You can feed in a changing signal and every time the signal went through zero a trigger is generated dependent on the direction from where the zero point is crossed. You can add a threshold manually or CV controlled to move the zero point up or down as well. You can feed the signal in through input one ore two. When both inputs are used the signals are added together. When the signal crosses zero from positive to negative a trigger of about 1.2msec is generated at output OUT1. When the signal crosses zero from negative to positive a trigger of about 1.2msec is generated at output OUT3. The gate output (OUT2) stays high as long as the signal is in the positive range or above the range which is set by threshold.


Specs and features


Implementation

Schematic

Signal to trigger converter schematic

Description:

The incoming signals are summed up. Every time when the summed signal changes polarity (moving through zero) a trigger is generated. Moving from plus to minus generates a trigger at the negative trigger output, moving from minus to plus generates a trigger at the positive trigger output. Trigger length is about 1msec. The gate output stay high as long as the summed input signal stays positive. Trigger and gate voltage is +5V.


Signal to Trigger

Example: Summed signal to trigger

The input signals are summed up: Uin1 + Uin2 – Uin3 – Uthreshold. If the sum is above zero volts the output of the comparator IC1A went low. C7 is charged and differentiates the signal. R10 and R13 builds a voltage divider which feeds the comparator IC2C. D4 clamps the negative pulse to ground. This is needed to make sure that the next positive charging pulse starts from zero load in the capacitor. Otherwise the output trigger length will vary with input frequency. R10 and R13 build the unload path for C7. C7, R10 and R13 determines the output trigger length. The comparator IC2C went high when the positive input reaches 7.5V. The output voltage is divided down by R14 and R15. D5 blocks the negative voltage when the comparator switches back to the negative rail. R16 scales the remaining voltage to 5V. IC2D serves as an output buffer. R17 protects the output against shorting to ground. The other path when the summed input signal moves from positive to negative works accordingly.

IC1B scales the output voltage for the gate output to 5V. The output stays at +5V as long as the input comparator stay negative. This means the summed input signal is positive because the comparator inverts the signal.

Signal to Trigger Sine

Screenshot - Sine to Trigger

The uppermost line (Yellow) shows the input signal. The second line (Blue) shows the trigger when the input signal moves to the positive site. The third line shoes the gate signal which stays positive as long the input signal stays positive. On the fourth line you can see the trigger when the signal is going negative. Those pictures are taken without any threshold.

Signal to Trigger RampUp

Screenshot - Ramp to Trigger

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Calibration

  • None

Special parts

  • None


Signal to Trigger Converter PCB

Signal to trigger converter. Populated PCB


Signal to Trigger Converter site

Signal to trigger converter. Assembled module. Backview


Signal to Trigger Converter back

Signal to trigger converter. Assembled module. Frontview

2015 August

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