Hi Michael,

    I would love to build this circuit with the modern parts since these are more stable and perform better.

    It was kind of a challenge to make a good performing sensitive NTC circuit.

    And here it is !

    The trick was to set the correct biasing of both transistors.

    To do this, I’ve added 2 resistors in the Emitter path. These reduce the very steep amplification rate of the silicon transistors.

    That’s where differ Ge and Si transistors. The Ge transistors have a more “curved” Hfe slope while the Si transistors have a “steep” Hfe slope.

    This is also the reason why I have altered some of the amplifier stages in the oscillator circuits (remember the phase shift oscillator ?)

    I’ve also added a 10K potentiometer for the voltage divider. This sets the “perfect” working bias for T1.

    When finishing assembling the circuit, set P2 in the middle position. Then turn P1 until you notice a meter deflection.

    Now it’s a matter of setting both potentiometer until the meter is in the middle position.

    When you hold your hand 5 cm above the NTC, you’ll notice that the meter starts to deflect. Blowing at the NTC afterwards will make the meter needle return.

    Try different heat or cold sourced to see the effect (sun or shade, ice cube, …). No need to touch the NTC, this circuit is extremely sensitive !

    Here’s the diagram and picture (I like a real picture of a setup, the Lectron system is very appealing !)

    NTC V4


    • This reply was modified 3 years, 11 months ago by LectronFan.