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With some microphone capsules having a higher output voltage, it may be necessary to reduce the gain of the preamp to prevent overloading. This could be due to the test probe not making good contact; or due to a circuit Fault. The negative supply functions in a similar manner. Capacitor C3, which is part of the trigger circuit of Tril, is now charged rapidly by capacitor C2 via resistor R3.

These counters are set in the decimal down-count mode. With the component values shown in the dia- gram, the frequency can be adjusted from ap- prox.

Using the in a low-voltage high-current supply, with an external series transistor oper- ating from the same supply rail as thein- variably results in excessive dissipation in the series transistor.

Readers in countries that use 60 Hz should note that Elektor circuits are designed for 50 Hz operation. Easy music For those who do not have the time or perhaps the patience to master a musical instrument, but would nonetheless like to make their own 3001, this simple circuit may provide the answer. The cell is charged by a constant current source T1 which is controlled by a voltage comparator based on a TTL Schmitt trigger Nl.

The VCO output from pin 4 may elektod used to drive CMOS circuits direct, however, for other applications a transis- tor buffer may be required. Furthermore, the duty-cycle of the output signal will be independent of the in- put frequency: Flasher bleeper Although extremely useful, the self cancelling devices fitted to car direction indicators are not infallible.

The top channel shows the Input signal of the circuit, whilst the bottom channel shows the output pulse. However, if a pushbutton is held down the output of N5 will remain high, charging C3 through R3. The four gates of the IC are used to form two astable multivibrators; the frequency of the elekfor N5, N6 is approx, I Hz, whilst that of the sec- ond N7, N8 is roughly 40 kHz. Additions to the basic circuit However, the possibilities do not end there. For this reason it should be well insulated.


The circuit actually consists of a sinewave generator followed by an amplifier which can deliver a current of approx. This prevents spot lamps and fog] amps from accidentally being left on whilst the car is unoccupied. This will usually be lower than the 9 volts required by the rest of the circuit, so the voltage is dropped by RS eoektor decoupled by C3. However, instruments, and particu- larly complex ones such as oscilloscopes, tend to drift with age, and if more than a year or so old can be highly inaccurate.

Oscillograph of the calibrator out- put waveform. It is then a simple matter to adjust PI until the maximum output voltage is obtained on the meter, where- upon the optimum collector current can be read off from the scale of P 1. Even when 3301 break- down voltage has been reached, the zener voltage varies slightly with current.

Elektor – Circuits

An additional bonus is that the alarm will also sound in the event of a lamp filament failure. As the gate voltage of the FETs must always be negative the output of this oscil- lator swings bet ween ” 2 and – 6 V. The frequency range of the circuit is approximately Hz to 30 kHz. Only when the oscillator receives a trig- ger pulse from N3 will it produce an output in the form of a decaying oscillation. The anode of D1 is simply connected to one of the low voltage secondaries of the oscilloscope mains transformer.

C4 N3 N4 discharges through R2 and the output of N2 goes high. Obviously the chip temperature must be maintained considerably higher than ambient for correct operation since the circuit cannot cool the chip to a temperature below ambient and PI is provided to adjust the chip tempera- ture.

No part of this publication may be reproduced or transmitted in any form or by any means, including photocopying and recording, without the prior written permission of the publishers. A constant current of – thus flows through R2. As was the case in the above circuit, there is a minimum permissible value for R fj. This simple signal injector should find many uses in trouble shooting and alignment appli- cations.

However, if the duty-cycle of a pulse train is very large or very small it will ap- pear that one LED is lit continuously. The control current through the diodes increases and their dynamic resistance decreases, thus attenuating the output signal.


Elektor – 301 Circuits

This is gated on and off by Tl, which is driven by a 50 Hz signal. While the cell is charging the terminal voltage remains at around 1. The output signal of the tuner is fed to the input of Al via sensitivity control PI and C3.

When the thief cuts this wire to remove the equipment then the alarm will sound. A voltage versus current curve for the zener may be plotted, from which the dynamic resistance may be found.

If desired the output of N4 may be used to drive a buzzer via a buffer transistor. However, the bottom end of R4 is returned to the output of IC1and as the voltage across Cl rises so does the output volt- age and hence the charging voltage applied to ous output pulses. It is also possible to vary the dock frequency by changing the value of C3 minimum p for 50 kHz.

If cur- rent is passed through any one of these resistors, a corresponding voltage drop will appear across this resistor. With modern transis- tor oscilloscopes this should be no problem, the supply can probably be derived from one of the low-voltage supplies in the oscilloscope either- direct or via a simple zener stabilizer.

However, it does vary slightly with supply voltage, so this should be stabilised. Unfortunately, some manufac- turers merely print an indecipherable code, which has to be looked up in the relevant data book in order to find the zener parameters.

The output waveform of IC1 thus consists of a series of positive pulses whose duration T2 is a constant and whose spacing Tl depends on the output voltage of IC2. The complete circuit of the voltmeter uses only this 1C and a handful of other components, since the 1C will drive the LEDs directly.

The ac- companying figure shows a circuit diagram of a simple and reliable FSK modulator which has the advantage of requiring no calibration.

Bosschaert Audible logic probe o Al With the component values shown, the resultant sound is similar to that of someone rapping their knuck- les on a wooden table. The main positive supply voltage is obtained from the transformer via bridge rectifier B1 and reservoir capacitor C3.