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Click on image to enlarge.
This circuit can be built “in air” with the help of some small pieces of plastic, or Thermo-shrinking sheath, to isolate the critical points.
The output current of the Theremino pin(maximum ten milli amps) is enough to drive LEDs, photos-couplers and solid state relays. But to drive mechanical relays or other users, as the engines continues, higher currents are required.
The maximum controllable current with this circuit is the maximum current supplied to the USB (almost 500 but with the USB 1 and 2, and almost 3 Amperes with USB 3).
Until the currents are small (50 or 100 mA), the protection diode on the transistor is enough. But in the case of large inductive loads (mechanical attraction reels, large relay or large motors), it would be better to add a 1N40xx diode in parallel with the relay (or motor), even in this case, with the collar upwards (towards the +5 Volts).
For low currents (up to 50 or 100 but small relays) you could use small transistors (BC237, Bc337, Bc547, Bc548 and others similar). With these transistors, you should add a protection diode on the relay coil with the clamp on the positive side. How can fit the protection diode 1N4148 or similar.An example with a small relay FEME
These small relay absorb only 50 but to 5 Volts. We tested in practice that any small NPN transistor can fit. So you can build a small circuit. To secure the connection cable and isolate components, you can use two drops of hot glue.
In these pictures the transistor is a BC548, the diode is a 1N4148 and the resistor is from 1 k (1/8 watt). Click on images to enlarge them.