Retrofitting an old amplifier with a remote control.
Goal was to have a tiny remote control sender that essentially is the size of a knob controlling an old amplifier that didn't have a remote control.
Remote control is made from an Attiny44, a FET, two infrared LEDs and a rotational encoder with monentary switch option. Operated with a CR2450 battery. The final housing is in a cylinder made of laser-cut round acyrilic slices.
The amplifier itself got its mechanical pot replaced by an electronic pot (Dallas/Maxim DS1882, dual log pot), and a 32 detent optical rotational encoder (which feels close to the original potentiometer in that amplifier). Also, it has a momentary switch, which is used for the mute function. Everything controlled by an Attiny48.
To get user-interface feedback, there is an LED ring with 30 charlieplexed LEDs around knob on the amplifier, moving the dot where the knob on the amplifier or the remote control sets the volume. Being outside the case, it also has the TSOP75338 receiver. Connected with the inside of the amplifier using a flat-flex 0.5mm pitch cable.
I am quite pleased with the result. Things worked out of the box even though the original potentiometer was 25k and this is more like 47k.
There are little things to do, such as covering the LED ring in a contrast-enhancing grey foil. And maybe a next hardware version will have the negative supply just generated from 5V with a MAX1720 instead from a linear regulator from the (luckily available) negative supply of the amplifier (right now, there are three supplies +6V, -6V, +5V; in the future, that would only be one).
Also future todo would be to have a bunch of tantalums to provide the energy for an IR burst, so that the remote control could be operated with a CR2032 even if its internal resistance increases over time.https://github.com/hzeller/DerKnopf