Vactrols are getting hard to find like purple unicorns, and if you find someone you have to sell your house to get the money for them. So I decided to build my own.
In my parts bin I’ve found a bag of unknown LDR’s, they have values between 50MOhm and 50Ohm. I’ve tested a lot of different LED colors, which will give the largest range of resistance between 0.1mA and 20mA, and the best where clear white LEDs in 5mm. They give a resistance range between 90kOhm and 90Ohm. After grinding the LEDs to get a flat surface they fit perfectly in a white 5mm heatshrink tube, covered by 2 layers of black heatshrink. The open ends have to be closed by black FIMO - which is a brand name of a heat hardening modeling clay.
The Vactrols work quite well in comparison to the different types of 5C1’s according to the datasheets, the only downside is the somewhat higher forward voltage of around 2.4V, the 5C1 starting at around 1.6V, but I think this can be fixed by adding a bias voltage. I build 3 identical parts, wich interestingly measured quiet equal, so they are interchangeable.
Did someone made similar expericences? Which color do you use?
Here are some photos: