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The direct and phase-shifted signals are summed by IC2d. The electret microphone, which has become very popular in recent years, operates in a similar manner to a capacitor microphone, but is cheaper and less bulky. This is the principle of the automatic mono-stereo switch.

When the voltage on C2 equals that at the non- inverting input the output of IC1 goes low. To eliminate the error that would otherwise oc- cur due to the voltage drop in this resistor, ellektor voltage follower feedback is taken from behind R12, To enable the full 10 volt output to be ob- tained in spite of the drop in R12 that has to be compensated by the IC the meter used should have an internal resistance of at least 5 kohm.

Resistors already provided with taps, such as television H.

For tuning purposes this value can be varied slightly by means of PI. If the eelktor involved in the circuit are AC signals this is no problem, a capacitor can be used to isolate the DC levels while allowing AC signals to pass. The function selector switch of the amplifier disc, tuner, aux, etc, is represented by SI.

The out- puts of all three comparators will be negative, so LED D1 will be lit. The diodes prevent excessive drive voltages from reaching the input of the comparator IC1.

Elektor – 301 Circuits

The output of N4 is thus low, the output of N6 is high, T1 is turned on, T2 is turned off and relay Re. By summing only the high-frequency com- ponents of the signal it is possible to eliminate the annoying high-frequency noise without de- stroying the stereo elsktor since channel separ- ation is still maintained at middle and low fre- quencies.

Figures la and lb show the basic circuits for voltage and current measurement respectively. Although a circuit that draws 25 mA is not too well suited to battery supply, one may need or wish to do this. The proportion of phase-shifted signal and hence the depth of phasing can be adjusted by means of P4. This produces a train of short pulses from the squarewave input signal and these are fed to an audio amplifier comprising T1 and T2.


The top channel shows the Input signal of elekto circuit, whilst the bottom channel shows the output pulse. The value of Cl is determined by the maximum base current that the must supply to the series output transistor. This should be adjusted so that the circuit operates on any stereo signal which is sufficiently strong for noise-free stereo listening. Zero crossing detector This circuit will detect precisely the negative- going zero-crossing point of an AC waveform, but requires only a single supply voltage, unlike zero crossing detectors using op-amps.

Elektor – Circuits

When the voltage across Cl reaches the thres- hold value of NlT3 is turned on via NJ and N2 S and Cl is discharged, after which the whole cycle repeats itself. A portion eelktor the output signal from AI is rectified by D1 and the resulting peak positive voltage is stored on C4.

If the trans- mission is mono then no signal will be fed through the selective amplifier and the compara- tor output will remain high, switching off the stereo decoder. The accompanying graph shows the response of the compressor using both silicon and germanium diodes, and it is obvious which are better! To this end C4 and R4 are included; these components enable the preset inputs of these counters 310 when power is first applied.

The circuit is based on the wellknown 1C regulator. Photos, The three output waveforms pro- duced by the function generator.

When a transistor is plugged into the appropri- ate socket a current of 10 M A leektor thus flow into the base and a current of hpg times this will flow through the mi Hi ammeter. Cl will thus discharge at a slower rate, making the interval Tl longer and reducing V 2If Vi is greater than V 2 then the eldktor of IC2 will ramp negative i.

Input C is grounded, while input B is connected to a DC voltage equal to the difference between the input voltage of the second circuit and the output volt- age of the first.


The reference voltage output is fed to the two threshold inputs via presets PI and P2, which are used to calibrate the circuit. By pressing one or more switches in different combinations a total of seven different zener currents can be obtained.

The stereo -mono crossover frequency can be in- is Since the electrical characteristics of LS TTL devices differ from those of standard TTL ICs, the relationship between the oscillator frequency and the values of R and C will also be difficult, whilst an extra resistor is required for the circuit to function satisfactorily.

The frequency of all three signals is varied by altering the Integrator time constant, which changes the rate at which the integrator ramps, and hence the signal fre- quency. Cl will charge to logic high through R3 from the output of Nl, ready for the contact to be touched again. The output of IC1 will now swing down to zero and the voltage on the non-inverting input will fall to about 5 V due to the hysteresis introduced by R3.

Elektor – 301 Circuits.pdf

It is so compact that it can be built into virtually any oscilloscope, making regular calibration a simple procedure.

Automotive voltmeter Although vital for satisfactory operation of the vehicle, the car battery is often taken for granted and rarely receives adequate maintenance. The output voltage V 0 is equal to the difference between the voltages at the non-inverting and inverting inputs i.

If higher input voltages are to be used or less dissipation is desirable then the values of Rl, R2 and PI should be increased, keeping them in the same ratio.

If cur- rent is passed through any one of these resistors, a corresponding voltage drop will appear across this resistor. With no current through the shunt, PI is used to set the output voitage of A1 to nominally 6 J V, so that the circuit is just on the point of switching over from D4 to D5.

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