[sdiy] Negative-going triggers in the Boss DR-110 drum machine - Why?

[mskala at northcoastsynthesis.com]

On Fri, 29 Sep 2023, Tom Wiltshire wrote:

> You all seem to agree that it's down to the feeble output power of the
> processors ports, which makes PNP transistors a better bet, and that
> necessitates the negative-going pulses.
>
> On a modern processor with much more powerful outputs we could
> presumably (I'll have to check the currents) drop the PNPs too and use
> positive triggers, saving even a few more parts.

Beware of voltages.  In the schematic that was posted, the microcontroller
and the circuit it's triggering are on different power supplies.  They
only differ by a diode drop (6V versus 5.4V), but a "modern processor"
would likely have 3.3V output and an analog circuit downstream might well
need to be on a higher power supply voltage than that even if the
microcontroller can source a fair bit of current at its own power voltage.

Putting the microcontroller port into open-drain mode and using
negative-going pulses would in many cases be the best way to interface it
to a circuit on a higher-voltage supply.

-- 
Matthew Skala
North Coast Synthesis Ltd.