Tuesday, 14 January 2014

How to build MonotrOndes Part 4

The Expression Controller  

Finally I will show you how that volume and filter cutoff controller works. 

Lever is spring loaded. I used potentiometer's shaft as an axle and used this kind of torsion spring for the resetting mechanism.

The Torsion Spring


The potentiometer serving as an axle for the expression lever. Sheet metal is used for the lever system's base.


The Lever and the Torsion Spring. There is a hole in the knob that keeps the spring in place. I used aluminium knob. Aluminium is easier to drill than harder metals.


 The Lever System. The paint is worn out because of heavy playing. :)

The whole synthesizer and expression system is shielded. I wanted to make my MonotrOndes an usable musical instrument so any unwanted noise is minized with shielding. As you can see from the pictures I used sheet metal for the lever system's base.
If you don't know anything about shielding and grounding I suggest you google about shielding and guitar stompboxes. DIY guitar FX websites are very good places to learn basic audio electronics.  

Here is a picture from time when I was building the expression system.

 The Other Side 

You can see that the pot's metal housing is removed. I extended axle with a piece of glued acrylic tubing. There is a large hole in the wood for the axle. Pieces of aluminium tape are used for shielding. 
All the electronics are inside this metal box that I found from my local shopping mall. Even better if you can find a box with a lid that isn't painted at all.

As you can see the lid is on the lever side. There is a drilled hole in the lid for the axle to come through.

White paint is scrached off where the aluminium tape and lid makes an electrical contact. That black thing glued to the acrylic tubing is a shutter. I made it from polystyrene. Thin polystyrene is a wonderful material for a maker. You can cut it with scissors! Though I must say it's not environmental friendly. You can also use cardboard.

The Optical Potentiometer System

I wanted to make a scratch-free volume controller. I got the idea for an optical system from optical volume pedals. My first design was using only one LDR (Light-dependent resistor). In my first youtube video the MonotrOndes was still equipped with that design. There was problems. Sound was leaking a little when the lever was in the rest position. I also had to amplify the sound with preamp. Then I got the idea of using two LDRs as a voltage divider. That idea made possible also to use lever system for voltage control. Here is an animated GIF that shows how the shutter works.

LDR-pair is controlled with a shutter mechanism. Shutter in the middle gives same amount of light to both LDRs. Shutter in the another extreme shuts another LDR from light entirely and fully lights another LDR. Smooth and crackle-free potentiometer!

There are 2 pairs of LDRs. First pair controls volume and second pair controls cutoff CV. Cutoff control is a voltage divider that changes VCC and then goes to cutoff terminal in the PCB.


 I used LEDs from broken solar garden lights. They were 3V LEDs. Perfect! :)

Here are the schematics

 The Volume Control Part of the Circuit. 

The Filter Cutoff Controller Part of the Circuit.

There are extra trimmers in the filter cutoff circuit. Filter CV should be trimmed to usable voltage values. I added extra potentiometer to metal chassis for adjusting cutoff control depth. That way I can change between violin and brass type sounds. 

That's it! Feel free to contact me if you wan't more information about building your own MonotrOndes. 


  1. Thanks for your hard work and inspiration- I'm thinking of giving this project a go, albeit in a slightly different configuration. I'll be back to share/ ask questions. Keep up the innovation!

    1. Thank you Chiz! It's awesome that you are making your own version. Good luck with your project!

  2. Very nice work and great illustrations - this can be used in so many ways! Thanks!

  3. Thank you very much for your kind words! :)

  4. Whoa, optical volume control. Very neat :) . Maybe a motor rotating perforated disc for optical tremolo? That would be sweet!