How (not?) to make a Modulation Lever for the REMI

This page is a continuation of an earlier article ... See page 1 here.

Ideally, the Mod Lever should have a rocker or thumb-wheel action with spring return to centre position. Here's the DIY approach I took, not that I recommend it, because it was a lot of work. Alternatively, you could use a linear slider pot, or a mini rotary pot mounted sideways, or a joystick salvaged from a game console or a toy remote controller. The software will (hopefully) compensate for electro-optical and/or mechanical deficiencies such as "zero drift" (spring hysteresis), hyper-sensitivity and other undesirable behaviour.

mod lever pic 1
The spring -- sourced from a clothes peg -- has about the right stiffness for the Mod Lever application.
mod lever pic 2
Mod lever mechanical parts, disassembled. The frame was made from an offcut of 1.2mm Alum. angle, 20 x 16mm. The "armature" was made from 0.8mm Alum., with the spring crimped and glued in place with 2-part epoxy resin (Araldyte). The rocker is made of 12mm thick hardwood.
modlver pic 3
Rocker attached to armature with self-tapper.
modlvr 4
Assembled mechanism, without opto-electronic parts.
pic 5
Assembled mechanism, interior view.
pic 6
Assembled mechanism, exterior view.
pic 8
Assembled mechanism with opto board.
A vane on the armature moves between the LED and the photo-transistor, partially blocking the light. An LDR photo-cell might be better than the transistor to allow greater mechanical displacement, i.e. to reduce the sensitivity.
pic 8
The optics will be covered with matte black paper to block out ambient light. The output voltage swing from the photo-detector is about 2.5V with a 100k emitter resistor. This is just about ideal for input into the ADC on the PIC32. The op-amp (LM358) on the handset circuit board is wired as a non-inverting buffer with very high-Z input.
handset bottom view
Handset with Modulation Lever fitted.
(Still no side pieces -- I will get around to making the sides one day!)

handset internal layout
Handset internals modified.
Note that the breath pressure sensor has been relocated to make room for the modulation lever.
Putting the sensor above the T-joiner also helps to prevent moisture from getting inside the sensor.
Go to REMI page 1