[sdiy] Hacking the expression pedal input on a JD-800

[spr at spridley.freeserve.co.uk]

> I've tried just connecting a CV source to the input, but that
> doesn't work. I'm not sure, but I'm guessing that this is
> some kind of current-loop interface; it expects to see a
> current flowing from the ring to the tip, so if you just hit the
> tip with a CV it doesn't draw current from the ring and
> therefore doesn't do anything. I'm thinking that I can use
> an optocoupler as, in effect, a voltage-controlled resistor
> to vary the current. I'm looking at the Fairchild H11F1.
> This is an optocoupler with a linear bilateral detector.


I've seen some synths (eg Siels) where the expression pedal
doesn't become active until the supply to it sees some load.
I think the idea is to avoid leaving a CV input "blowing in the
wind" when there's no pot connected.  

I don't know if that's how it's done in your example, but you
could try it by putting a resistor (around 10K) between the pedal 
5v and 0v and then stick a CV into the return connection.


Steve
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