2 Keyboards 1 Sustain pedal

[Woland]

Hello all, I have found myself in a FFM situation and plans to resolve it by a double penetration operation and I need some advice. (Sorry for the dirty puns, it seams I just cant help myself today :P). So anyway what I want to do is to use one sustain pedal to act on two Control keyboards. My plan of action is to build a modified Y-spliter with diodes on the Vcc lines. I have attached a schematic of this cable (the weird arrow thingies are the diodes)

Both keyboards use the tip for Vcc and the sleeve as ground and they both put out a voltage of about 3.55- 3.63 V. Normally open/ normally closed wont be a problem either since both boards can switch this.

Now have any one of you out there tried a similar operation before, if so, what where the results? Have I missed something in my plans? how likely is it that I destroy the sustain input stages on any or both of the keyboards?

[acreil]

Yeah, you should be able to get off with that. It shouldn't blow anything either.

It might introduce a ground loop, though. Maybe you could add a small resistor to each of the ground wires.

[a zombie]

Would the diode voltage drop be an issue?

[Woland]

@acreil: That is true, but since this is on digital equipment, lets hop that the circuits are stable enough. But it is true that this might cause some weird errors and malfunctions.

@ a_zombie:
I am not sure, they might prevent the keyboard from detecting the difference between the two states of the sustain pedal, but I guess that there is now way they could increase the risk of doing permanent damage to the devices.

But on the other hand, I have been questioning if the diodes would protect anything at all. Sure, as long as the pedal is open, they will protect a keyboards Vcc output from a potential higher Vcc output of another device, but as soon as the circuit is closed the "GND" line of both boards would still rise to the level of the highest Vcc signal. And since both keyboards use the same high level voltage, I should not worry, right? :P

[a zombie]

It's definetly sketchy, It would be much safer to modifiy/build a sustain pedal that uses a dual pole switch, so you could keep the two boards isolated at all times.

You could also build a box with a DPDT relay in it, both keyboards sustain pedals would be connected to a seperate pole on the relay, and the relay's control voltage would be switched on and off by a sustain pedal.

Something like this...

[acreil]

Originally Posted by Woland

But on the other hand, I have been questioning if the diodes would protect anything at all. Sure, as long as the pedal is open, they will protect a keyboards Vcc output from a potential higher Vcc output of another device, but as soon as the circuit is closed the "GND" line of both boards would still rise to the level of the highest Vcc signal. And since both keyboards use the same high level voltage, I should not worry, right? :P

Well, the diodes prevent any current going from one keyboard to the other, which will happen inevitably when the power supply voltages aren't exactly the same. And you don't want anything bad to happen when one is switched off. In practice the jacks are usually wired in a way that protects against stupid users (you could check the schematic), but there's no need to invite trouble.

I don't think the voltage drop from the diode would be a big problem either.

A DPDT pedal would certainly be better, but it might be hard to build or modify one in a way that works well. And the relay would be fine, but it would need a power source. I wonder if you could build something with reed switches and a magnet... or maybe just stick two pedals together somehow.

[Woland]

Hello again all, and thanks for all the helpful replies.

Good thought about what happens when only one is shut on, I had completely missed that situation.

The relay solution is very clean and safe and I like that, but what I really don't like the idea of needing a separate power supply. But as I thought about it I came up with a alternate solution using optocouplers (see newly attached schematic)

Now I just need to measure the current driving capabilities of my sustain jacks and see if I can find a optocouler that can be driven by that current and does not cause a to large voltage drop.