MODS: Digitech PDS-2000 under voltage control

[Strategy]

I've been very shy of mods through my 15-year DIY journey...until recently when I've been trying a lot. Tonight I put my Digitech PDS-2000, my favorite digital delay, under voltage control. Using a home-rolled vactrol wired in parallel with the Delay Time pot, I was able to add a CV input, controlling the delay time from an analog sequencer voltage. I can post my mod in the thread.

What I'm trying to do and hoping for forum advice is potentially control the HOLD button function with a CV or Gate (or Trigger) voltage +5V. The HOLD footswitch famous on these PDS series delays initiates infinite repeat. The footswitch is a tactile momentary switch under a plastic plate, which controls some logic at U3A (middle of first schematic page).

In all my gear modding I have not looked into how I would externally control logic based switching like this with a voltage pulse. Is it possible to safely feed a +5v pulse into the chip somewhere, like right after R90? Is this anything people here have tried?

The musical benefits could include deliberately timed / patterned delay repeat freezes for example. Some Eurorack modular sampling delays have a cv/gate controlled hold feature like this I think.

thanks to anyone for thoughts.
strategy

[ElectricDruid]

I would try putting a simple NPN transistor C to E across the momentary switch. Then you feed in your 5V pulse via a 10K to the transistor's base, and it switches the switch on, just like putting you foot on it. For extra safety add a 1N4148 diode from the emitter to the base to protect the tranny if you fed a negative voltage into the socket by accident.

[Strategy]

Thank you so much, electricdruid! with the input protection scheme which end of the diode is pointing towards the emitter? Looking forward to trying this!
Strategy

[Strategy]

Here are some mod tips for whoever tries to do this in the future. Big thanks to ElectricDruid for the advice that facilitated the amazing Hold gate input element!

CV Input
The CV input is an 1/8" jack. Jack tip goes to a 200 ohm resistor, into the + leg of an LED. I used bright white LED, but yellow may provide a nicer range. Jack sleeve (ground) goes to - leg of your LED. The LED is joined face to face with a 10k/1M LDR and enclosed in a tube of heatshrink wrap. I sort of wrap this assembly partly around the input jack's plastic shell; the delay pedal's battery compartment is roomy but not very deep. Two pretty long wires, woven together for easy handling, connect the two legs of the LDR (doesn't matter which) to pins 1 and 3 of the Delay Time pot, whose pads are easily accessible on the component side of the board. Take care you don't singe adjacent components or create shorts.



Gate Input
The Gate input is a 1/8" jack connected to a transistor set up as an voltage controlled switch, as ElectricDruid suggests per the instructions above in the thread: using an NPN transistor (I used 2n3904), the external voltage comes from jack tip through a 10k resistor to the transistor's Base. Emitter and Collector are connected to opposing poles of the momentary tactile switch, which lives on its own little pcb between the mainboard PCB and is mounted the enclosure. These are most easily accessed by unfastening all the potentiometers from the front panel and lightly pulling the mainboard PCB out of the enclosure just enough to reveal the enclosure mounted switch assembly beneath.

Mounting the mods
The battery compartments for these PDS pedals for me, are always empty, because they drain batteries wastefully. It is roomy but not very deep. The I kept the mod circuitry tightly connected to the panel mounted 1/8" jacks, but a tiny bit of perfboard is probably more stable if you are into that. There are several voids in the battery compartment that allow you to run wiring through. The wiring for the Gate jack wants to stay between the mainboard PCB and the enclosure, whereas the CV input wiring needs to go through a small gap between the pcb edge and the enclosure, so that the wire ends can meet the component side of the PCB. (If you take off the bottom plate of a PDS pedal, the component side of the board faces you.)




Controls
For the CV input, you could probably customize your input range by adding a trimmer or potentiometer to attenuate incoming  voltages to better suit the range of the Delay Time control. Since it's wired in parallel with the pot, the pot's position does this to some extent as well. Try a vactrol like VTL5C3 or other off the shelf optocoupler or, different LED colors/brightnesses, and different LDR values. By applying an external control voltage you can independently tweak the delay time for the woozy pitch bending effects. At the pedal's highest frequency delay rate you can get chorus/flanger effects using your external CV as the modulation source.

For the Gate input, you can send triggers from an analog sequencer, drum machine, or a square wave LFO (or other gate/trigger source) to virtually press the Hold footswitch, allowing for a synchronized or automated delay "freeze" effect.

here's a quick (sorry, very not pro) video showing the effect mods in action.

[ElectricDruid]

Thank you so much, electricdruid! with the input protection scheme which end of the diode is pointing towards the emitter? Looking forward to trying this!
Strategy

Glad it's working for you. The diode goes anode to emitter, cathode to base. So "pointing at" the base. It just provides a little bit of protection since the diode will conduct if you feed negative voltage in, which saves the poor transistor's junction from being heavily reverse-biased. The diode won't get fried because the 10K limits the current. So it's all pretty safe, and you could probably connect a modular synth LFO to it without harm.

[Strategy]

Thanks so much for your help! this will be an easy add. A little extra protection should save some future hassle.
This is such a great mod, really sparks some sound making ideas.

A few years ago I got an early 2000s (late 90s?) Manecolooper with pretty poor construction, I got it broken from Reverb and completely rehabilitated it. circuit was OK, but I had to replace all its switches, several pots, power jack. But the switching is similar to this Digitech. I am realizing that I could apply your transistor-as-switch trick to pretty much all of the foot controls for a fully modularized looper. My only hesitation is it's a real hassle to get into the enclosure to do things.