pots and photoresistors to add to the CV inputs
  • I’d like to add 2 photoresistors to the Shruthi-1, and would like to put them in in sequence with two pots so I can control the maximum value.
    Now I’m wondering: what values should I use? I know some of you have already done that so maybe you have some values to suggest.

  • it depends on the resistances of your LDRs. they usually make a voltage divider with the pots and LDR. i’ve seen tons of info about that in threads on electro-music.com and Muff Wiggler. don’t know it off the head now how it is exactly done, but there were drawings explaining everything.

  • Yeah well, I thought to make a voltage divider you had to put two reistances in sequence but take the voltage between them, what I would do is just put two resistance in sequence and then attach it to the CV.
    I guess maybe I’m not fully understanding the whole CV control thing…

  • i just searched on EM for vactrol but couldn’t find the thread where all was explained.
    but you can look how they did it for instance in the resonant lopass gate clones, or any other vactrol project.

  • Thanks for the suggestion! Still, I’m not totally sure about one thing… do I need a vactrol?

  • a vactrol is the whole thing, a LDR plus an LED coupled and sealed in that plastic body.
    so actually with an LDR and an LED you do roll your own “vactrol” (vactrol is just the name of one producer)

  • yeah sorry I have been a bit vague in my questioning. I perfectly know what a vactrol is and have already built some. What I don’t understand is: why do I need a vactrol to interact with the CV of the shruthi.
    What I was planning to do was a lot simpler: take the 5V from the shruthi, pass them through a LDR and a pot in sequence and pipe that into the CV connector on the board. So I can assign the CV input in the matrix to some parameter and then control that with the LDR, adjusting the range with the pot.
    I don’t want any external CV input, I want just a way to control some parameters with an LDR instead of the good old knob.
    So now the question is: is my reasoning correct, is this how you do it?
    Though I have another doubt… don’t I need to connect to ground somewhere?

  • it’s basically the same as in using a vactrol, just that you use ambient light to control the resistance instead of a ldr.
    it depends on the light and dark values of your ldr and the voltage you have, and the voltage range you like.
    resistances in series add their values (Rges=R1+R2+Rn). if you form a divider with a pot and a ldr in series i think you need an additional resistor following them to ground, to avoid a short if the pot and ldr both are close to 0 ohm.
    you don’t need an additional resistor if you use one pot alone to make a voltage, it’s already a divider between 5V and 0V. but in this situation here there are only two pins of the pot used, so it can go all the way down to zero.

    if you have the pot turned all the way down it’s only the ldr giving the voltage.
    if your pot is all up the ldr will have no effect.
    that’s how i think it is done usually, and the values depend on what you have as voltages.

    but i don’t remember exactly how they explained it in that thread. it’s been a while since i’ve seen it. so i’m not sure if i missed something, maybe completely