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The value for RI depends on the amplitude of the square-wave input: For use as an amplifier load resistor some means must be found of reducing the inductance of a wirewound resistor.

The count which ICS has reached is thus proportional to the duty-cycle of elekktor Q output of FF1 p which is proportional to the analogue input level.

On the other hand, coal fires are difficult to light, slow in producing heat, and also extremely messy to dean out. This signal is essential for mono compatibility, as it is the only part of the steteo Figure 1, Block diagram of the automatic mono-stereo switch.

The I kHz squarewave generator also consists of an astable multivibrator, which utilises the two remaining NAND gates in the package. The current limit is continuously variable by means of P3.

The amplitude of the calibration signal is approximately equal to the supply voltage, which can be measured with a multimeter. It operates by feeding a known constant current into the base of the transistor and measuring the collector current.

CMOS ICs used as linear circuits draw more cur- rent than when used in the normal switching mode, and the supply voltage should not be greater than 6 V, otherwise the IC may overheat due to excessive power dissipation. Thus in order to reverse the polarity of the servo one simply arranges for the control pulses to be sub- tracted from a reference pulse of 3 ms. 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.

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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. At each zero-crossing of the sinewave the Schmitt trigger changes state, thus producing the original squarewave that is fed to the input of IC2. If one contemplates measur- ing the frequency of such high voltages the cir- cuit should be assembled in a well -insulated box! 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.

The amplifier must then be operated with the tape monitor switch de- pressed.

Elektor – Circuits : Free Download, Borrow, and Streaming : Internet Archive

Since most readers are unlikely to possess more than one multimeter this can be rather tedious. The result is compared with a fixed reference frequency, and some type of control system usually a PLL adjusts the frequency of the VCO so that the result of the division has the same frequency as the reference. Setting a number n on switches SI to S3 therefore gives an output frequency of n kHz. The preamplifier itself consists of a two stage amplifier T!

The frequency of the generator elektod then coincide with the desired value of kHz. The cell is charged by a constant current source T1 which is controlled by a voltage comparator based on a Leektor Schmitt trigger Nl.

At low supply voltages it may also be found that the frequency changes with supply voltage due to variations in the output characteristics of the op- amps.

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.

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Since the squarewave is sym- metrical it contains only odd harmonics of the fundamental frequency, the higher harmonics being fairly weak due to the relatively slow rise time of the CMOS devices employed. The oscilloscope photos show the result before and after a pulse has been processed by the cir- cuit. The voltage across R4 is approximately mV, so Cl charges through R2 at a current of around nA, which is times lower than could be achieved if R2 were connected direct to positive supply.

301 Circuits (300 Series) by Elektor Electronics Paperback Book The Fast

It does not apply during the initial phase when C2 is charging from zero. By changing over the position of this switch a transmission rate of Baud can be obtained, in which case CMOS FSK modulator 54 the frequencies used then become 2, and 4, Hz. The complete output waveform, shown in figure 2comprises a burst of pulses approx. The output voltage may be adjusted using P2, while preset PI is used to set zero output voltage. A portion of the input signal is rectified by D1 and D2 and used to charge capacitors Cl and C2.

The circuit will thus always reach equilibrium with Vj – V 2. The reference voltage from the zener is applied to the non-inverting input of a op-amp, and the output voltage is the zener voltage multiplied by the op-amp gain i.