This is the extension to capture the state of an LED from any device (in my case, a washing machine). The circuit's main component is an LPT 80 A phototransistor. If the state changes from dark to illuminated, a short pulse is outputted.

R5 (100 KOhm) limits the current through the circuit. C4 (10 µF) couples the circuit to the switch. The capacitor is chosen in such a way that only a single short press is registered by the RODRET switch, even if the transistor is continuously illuminated.

C4 (1 µF) prevents the retriggering due to flicker. This flicker can be seen on many appliances if they run either with poorly rectified 50 Hz AC or if the display is time-multiplexed.

If the phototransistor is illuminated, a small current is constantly drawn from the battery.

The components are just soldered together in a small 3D-printed housing. It has a small hole for the phototransistor on the back.

Component values are negotiable ;)

The box is then just placed on top of the device's LED. Some double-sided tape can be used to hold it in place.