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    • #8871

        Hi all,

        In previous post, I mentioned the troubles with the potentiometer. I discovered it’s faulty behavior when building project 46 of the school instruction manual (Ubung system). This circuit is the modern version of the legacy set.

        Hmmm, this seems an impossible circuit to me !

        It’s very hard to set the potentiometer so, that the lamp shines dimly.

        The amplification of T1 & T2 is so high that it’s very hard to set.

        The lamp is either ON or OFF.

        When the potentiometer is properly set (somewhere between the 2 lamp states), the NTC is so sensitive that a slight change of temperature will be enough.

        Can someone try this circuit out to compare the results ? I’ve only used the parts included in the school (Ubung) set.

        Here’s the diagram :


        NTC V1

        • This topic was modified 3 years, 11 months ago by LectronFan. Reason: Changed NTC V1 picture for adding NTC value
      • #8874

          Hi all,

          This is the original circuit from the legacy school (Ubung) manual. The main differences are the NTC (15k instead of 6,8k) and the use of Ge PNP transistors.

          And guess what ! This circuit performs much better. The meter can be easily tuned to the center position and it is very sensitive. Putting your hand (no touching !) above the NTC makes the meter deflect. Blowing some wind across it (to cool down) makes the meter deflect to the other direction.

          It’s a cloudy day today and sometimes the sun peeps through the clouds. This is even visible at the meter reading !

          A short circuit description :

          • The meter is set up in a Wheatstone bridge.

          The Wheatstone bridge consists of the resistor network as shown in this diagram :

          With the potentiometer, we can set the meter needle to the center position. Now we know that the left arm of the bridge (10 k resistor and C-E resistor of T2) is equal to the right arm (P1 and serial resistors R5, R6 and R7).

          Once this bridge is unbalanced (C-E path or P1 setting), the meter will deflect either to the left or either to the right.

          T1 and T2 form a DC amplifier. A very small change at the base of T1 will cause a very large change in the C-E path of T2.

          The NTC (in series with R2) together wit R1 form a voltage divider. Since the junction of this voltage divider is coupled to the Base of T1, a slight voltage change will be amplified and will cause a change to the Wien bridge.


          Here’s the circuit build with the legacy parts :

          NTC V2

        • #8875

            Hi all,

            Here I present a version which is a perfect combination of the 2 circuits above.

            I use the modern NTC module together with the PNP Ge transistors. Of course, you can use some legacy Lectron blocks as well, the NTC however is the new modern version of 6.8k

            For this, some changes of the circuit components were necessary.

            And it performs amazing ! Such a good sensitivity !

            When you form your hand around the NTC, the meter starts to deflect.

            When removing your hand and cooling down the NTC, it takes some time to recover to the original position.

            Try it out for yourself and be surprised !

            Here’s the diagram :

            Many greetings

            NTC V3

          • #8885

              Hi Frank,

              Do you want this built with a legacy set or contemporary set?


            • #8889

                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.
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