Replacement of the classic bell button
The classic "bell button" (single return switch) can be and is even replaced, in many mini-automation assemblies, by an electronic switch with instant or timed return, operated by means of resistive sensors or of other translators.
The example below illustrates this "replacement" in the case of a common multivibrator circuit with transistors, used as a bell.
How does the circuit work?
The multivibrator consists of transistors T1 and T2, capacitors C1, C2, basic resistors, R2 and R3, and, respectively, collector, R1, R4. An additional transistor, T3, amplifies the generated audio signal for playback in a speaker, Diff.
As it is known, for a given supply voltage the frequency of such an oscillator is dictated, practically, by the values of C1, C2, R2 and R3. Note, however, that, in the present assembly, in series with the resistance R2 from the base of its T2 interleaved the emitter-collector circuit of the transistor T5 (between points A and B).
When T5 leads to saturation, its emitter-collector resistance is negligible compared to R2, so the operation of the multivibrator is practically not affected by the presence of T5.
But when T5 is blocked (infinite emitter-collector resistance), the oscillation of the multivibrator stops. Therefore, we will be able to control the ringing on and off by the corresponding polarization of the T5 base.
For the control "signal" to convey very low current intensities, between the base of T5 and the control circuit another DC amplification stage was inserted, respectively the transistor T4, with a very high beta factor.
Resistive sensor operation (touch) S
The actual control circuit consists of the voltage divider consisting of the resistive sensor S and the capacitor C3. The sensor can be made, for example, by corrosion of a textolite plate plated with copper foil, making two very close metal "zones" (so that they can be touched simultaneously with one finger), but perfectly isolated from each other.
The second element of the control divider is a capacitor, and not a resistor, in order to follow the timed maintenance of the bell.
List of required components (with equivalents)
R1 = 2k;
R2, R3 = 27k;
R4 = 1k;
R5 = 33 Ohm;
R6 = 10 MOhm;
R7 = 1k;
C1, C2 = 33nF;
C3 = 1-10nF;
T1, T2, T4 = BC 107 (or BC547, BC171);
T3 = BD 139 (or BD 135, BD 137);
T5 = 2N2219 (or 2N2222, BC547, BC171);
D1 = 1N4148 (or 1N4007);
DIF = 1-4W;
1 x resistive touch sensor (made in the laboratory);
K = normal switch (for power on / off circuit);
1 x 9V terminal contact (for power supply)
Fiz. Alexandru Marculescu
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