Simple passive mono/stereo to stereo audio mixer circuit

A while ago I needed a very simple pasive miker to join one stereo and two mono signals together before amplifying them...

There is some mixed information on how to properly connect a mono signal to a stereo input, but most of it uses active mixing.

Repair faulty HC-SR04 ultrasonic sensor (solve erratic no-readings problems)

Recently I got some of these cheap sensors from Chinese suppliers, for a workshop I had to teach.

A couple of students reported that their sensors were behaving in an erratic way, sometimes working, then stopping again, the working...

After replacing them with more expensive ones bought at a local store, I decided to try fixing the faulty ones...

DIY PCBs on the cheap: toner transfer with Box Packing Tape and a Hair Straightener / Iron

A few months ago I started to design an affordable 2WD Robot Kit to teach 20 students in a workshop in the AVM Master, at the Universidad Politécnica de Valencia, Spain.

That means also making about 100 tiny PCBs!