Can you guess the purpose of the component shown above? Four wires goes into the glass bulb. From the tip of each wire, very thin wires connect to a tiny little (approx. 0.5mm) grey cylinder shaped thing. I don’t remember where I got it, or what it was used for.
I found out by measuring resistance between all combinations of wires, and some experiments, that it is an NTC-resistor thermally coupled to a heating resistor. NTC and heater are galvanically isolated, the heater is 100 ohm, and the NTC is 3.3 kohm at room temperature. So it turns out that it is a so-called thermocouple, mainly used for “True RMS” measuring in analog instruments, before microprocessors took over. Sometimes a thermo-element generating a tiny voltage was used instead of the the NTC.
The responce from heater to NTC is rather slow when turning up the power, and much slower when turning power down. I thought of a way to utilize this strange component in my never ending analog synth project, and found that maybe it could do the job of gain control in a Wien-bridge oscillator. It turned out to be really good at this, especially at very low frequencies. Since I actually needed a low-frequency “clean” sine wave for my ringmodulator, I built the module you see below:
If you wonder what the cog-wheels are for, it is a simple way of creating the tandem potentiometer for the Wien bridge. This tandem-potentiometer is always the most critical part of this type oscillator, and making it this way you can select a pair that matches really good. I printed the cog-wheels on my Prusa.
The sine wave is really fine and stable. Here is a 3 Hz signal:
The circuit is very simple, as you can see here: WeinBridgeOsc