[sdiy] Fet to short pedal input?

[brianw]

A BJT will conduct current in only one direction. In that sense, it's not like a plain, shorted electrical contact, but instead more like a diode. Also, the Emitter of a BJT is probably connected to ground, and the Base would only turn on the BJT if it's sufficiently higher in voltage than the Emitter. If you just let the Emitter float, the BJT won't work reliably.

An FET will ideally conduct current in both directions, but this is only guaranteed for the rare 4-pin FET that has Source, Gate, Drain, and Body/Substrate on separate pins. The latter pin is usually connected to Source on 3-pin FET. If the voltages on the (Source and Drain) conductors vary too much relative to the Gate-to-Body voltage, then the FET might shut off.

However, just because a mechanical contact conducts in both directions, you don't necessarily have to design for all of that. The ARP probably has a (CMOS?) logic circuit that merely responds to High (5V) and Low (GND) voltages. Check the schematic for your synth, and that will inform you about what will work.

By the way, why are you choosing 'velocity' to trigger this? I would expect a Velocity voltage to vary greatly with the played velocity, while I assume the Portamento Pedal is either Off or On. You would instead want some sort of Gate (for Note On/Off). Tom basically asked this same question already.

Brian


On Nov 27, 2025, at 3:50 PM, el macaco wrote:
> Thanks guys,
> 
> I believe the oedal just closes a circuit from what I remember from the schematics, (gated) might have a voltage.  I thought fet because it gates and connects the two I/I pins, and i didn’t know bjts would work the same way.
> 
> I have a bunch of 2N5458's around, but if a 3904 will do it that's much better!
> 
> I have optocouplers for midi jacks, but bjt or fet seems more direct.
> 
> I thought about relays but I didn't know enough to find a match, and I have a few fets around.
> 
> Thanks again!
> 
> From: Tom Wiltshire
> Sent: Thursday, November 27, 2025 4:41:35 PM
>  
> Why FET? Can't a simple BJT like 2N3904 do this?
> 
> Does this portamento input have a graded response or is it a simple on/off affair?
> 
> Tom
> 
> On 27 Nov 2025, at 20:34, el macaco wrote:
>> Hi all,
>> 
>> The portamento pedal input on arps has me thinking of using my midi to cv converter's velocity output (+5V) to activate a fet with the two pedal contacts on the source and drain.
>> 
>> Can it be this simple?  The only power coming from the velocity 5v?
>> 
>> Thanks,
>> 
>> Ed