Archive for August, 2012

Pruning the electronic forest

The version 2 relayboard is on the right; it uses the solid-state relays mentioned in my last post to control keys. It works. I’m making another one now; after that I will have enough to MIDI-ify the whole upper manual.

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(This is a nerdy update at least as much for my own memory as for anything else.)

To try to get around some of the issues I’ve raised in the last post, I’m going to try a new ‘relayboard’ using Opto-Isolators.  An opto-isolator is effectively an LED and a light sensor (a phototransistor) in a sealed, dark box.

It should be possible to wire one of these up like this (ish):

I’m looking at a Toshiba quad-opto-islator chip that will switch up to 55v at up to 150ish mA (datasheet) – and given that the existing diodes in this beast seem to be rated at 100mA, it probably won’t go toasty.  The voltage on the organ side is somewhere less than 23 volts, according to the service manual (schematic 5; page 18 of part 2; the left hand side – point BB is the pink common wire to all the keys on the upper keyboard)

So it should be fine to switch it through the transistor side of the opto-isolator.

Still need, though, to find the value of the resistor that protects the LED: according to the datasheet, the LED will eat 1.15 volts at up to 50 mA (0.05A).  The power supply is at 12v.  For safety’s sake (and to avoid warmth), we’ll stick to pushing 20mA through the LED, giving a resistor value of:

So a 620 ohm resistor should do the job.  Next step: prototyping this…

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