Homemade transistor from a photocell?

Homemade transistor from a photocell?

Like many of us, Nyle Steiner has long aspired to building a homemade transistor. While considering possible way to achieve this, he stumbled upon the interesting idea of building a FET transistor from a cadmium sulfide photocell.

Normally used to detect light, the photocell is pretty close to what one would need to make a transistor. The device consists of two pieces of metal that are separated by a very thin layer of a cadmium sulfide semiconductor. The semiconductor is normally an insulator, which means that no electricity can flow from one of the metal legs to the other. However, when light (photons) hit the surface of the semiconductor, they knock electrons free, and allow some current to flow across the semiconductor. To make a transistor, though, the device needs to react to electricity, not light. Nyle realized that the photocell could be used as a transistor if an insulated gate was added to the top of it. A bit of scotch tape and water later, and he claims to have a working transistor.

Of course, this isn’t a truly homemade transistor, it’s more of a DIY transistor conversion. The device appears to be functional, however it requires fairly high voltages to work, and only acts as a power amplifier instead of a voltage amplifier. Even so, it looks like an interesting way to experiment with transistors at home. I’d like to build one, and vary thickness of the insulator (scotch tape) to see what the results are. Anyone else have experience with this?


6 thoughts on “Homemade transistor from a photocell?

  1. ellisgl.myopenid.com says:

    This would be an awesome thing do to make a simple cpu….

  2. damoon says:

    Errr… all normal bijunction transistors (NPN, PNP) are current in/current out devices.

    That little load resistor (R2) is what converts output current to voltage. So assuming this works (which is pretty cool) it’s functioning as one would expect.

  3. ehrichweiss says:

    Have you tried other conductors besides water? A molded solder blob on the end of a wire might be easier and safer to use at high voltages.

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