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V-trigger to S-trigger stops working when sharing ground?
MUFF WIGGLER Forum Index -> Music Tech DIY  
Author V-trigger to S-trigger stops working when sharing ground?
Wirl
Hi all,

Many of you may are likely familiar with the following simple circuit: https://www.synthesizers.com/learn/gates/strigcircuit.jpg It takes the gate output from one piece of gear, and uses that signal to close a switch on another piece of gear (using a transistor).

Recently I've used this little circuit to trigger the "push" of the One-Key-Play button on my old Casio PT50 (allowing it to be sequenced in time). It works great, for the most part. More specifically it works while playing sound out of the Casio's speaker, and with headphones plugged in. The problem arises when I plug the Casio's audio output into a mixer which also receives audio output from the triggering device on another channel (in this case from a LepLoop). In other words, the LepLoop sends gate-out to the Casio and sends it's own audio to the mixer. In this setup the Casio is no longer triggered.

Some strange things to note:

- The actual gate pulse can be faintly heard on the Casio channel on the mixer. I'm guessing this means that the gate pulse found an easier path to ground through the Casio rather than returning to the LepLoop through the gate cable?

- I can also get the S-trigger to stop working by simply touching the grounds of the audio out cables from the LepLoop and Casio together (mixer aside).

- Checking the transistor emitter and collector with a multimeter, I get beeps in rhythm with the gate (but notes are not triggered). I think this means that the switch is still closing when the devices share ground, although I'm not sure how accurate this test is given that the test leads may be affecting the circuit.

- Using a multimeter on the LepLoop, I tested between the ground from the gate out and audio out, and found that I get beeps when the gate fires a pulse, so this means they share ground.

I've tried modifying the circuit in a hundred ways using different arrangements and values of resistance and diodes with no luck. At this point I'm pretty stumped so any help would be greatly appreciated. If I can't figure this out I might be forced to explore solid state relays for this application. Any suggestions there would also be appreciated. Thanks!
Graham Hinton
Wirl wrote:
I've tried modifying the circuit in a hundred ways using different arrangements and values of resistance and diodes with no luck. At this point I'm pretty stumped so any help would be greatly appreciated. If I can't figure this out I might be forced to explore solid state relays for this application. Any suggestions there would also be appreciated.


There is nothing wrong with that circuit.
There is probably something wrong with the grounding of the other equipment. You are connecting to a point in the Casio that you are assuming is referenced to 0V and that is the most likely mistake.
Wirl
Thanks for the reply. I think you are right in that there is something I don't understand about the Casio. The two points which close the One-Key-Play button are part of a button matrix which connects to the main NEC chip on the board. What happens inside that is anyone's guess.

If anyone had any ideas on how to troubleshoot what is going on I'd be happy to try it, but for now I'm going to order some optocouplers to convert the LepLoop V-trigger to S-trigger while keeping the two circuits isolated. I might even pair that with an LTC6993 to extend the gate pulse width so the Casio's button matrix scanner still recognizes the button close even when it's clocked down.
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