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The count which ICS has reached is thus proportional to the duty-cycle of the Q output of FF1 p which is proportional to the analogue input level. The attack time of the compressor is fixed and depends on the time constant consisting of Cl or 90 C2, R2 and the output impedance of the circuit feeding the compressor, which should be as low as possible.

Any change in the input voltage at C will produce the same change at the output, but of opposite This circuit presents an interesting alternative method of constructing a low-distortion buffer or output stage for use at low output powers. The value of R5 is dependent on the supply volt- age used and should be 33 ohms for voltages between 9 V and 15 V, 68 ohms for 15 V to 25 V and ohms for 25 V to 35 V.

The only constraint on the operation of the cir- cuit is that the interval between successive oper- ations of the switch must be at least half a sec- ond to allow Cl time to charge and discharge. The transformer voltage thus needs to be at least 7 V to give an 8. The result is that the chip, and hence the refer- ence voltage, is held at a virtually constant tem- perature.

The outputs of a divide-by-ten counter, the CD, are connected to an 8-positton switch. The control current through the diodes increases and their dynamic resistance decreases, thus attenuating the output signal.

Assume for ihe moment that R — is connected to the positive 1 14 Figure 1. When constructing the circuit par- ticular care should be taken to ensure that the 0 V rail is of low resistance heavy gauge wire or wide p.

The relationship between voltage and fre- quency is given by: The drive to the A few specifications: Note that this only applies once the circuit has reached equilibrium.


The complete circuit of the alarm is shown in fig- ure 1.

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. Squarewave-staircase converter This circuit can be used to generate an up-down staircase waveform with a total of steps per cycle, IC1 and IC2 are two four-bit up-down counters connected as an 8-bit counter, with an R-2R D-A ladder network connected to the out- puts to convert the binary output codes to a staircase waveform.

A TTL-compatible pulse output is provided A for simple frequency counters etc. Since D 5 and D6 receive their bias from the output of the supply, R5, R6 and D7 must be included to make the circuit self-starting.

However, this problem can be resolved with eldktor aid of the following circuit which ensures an independent supply for the carriage lighting. These counters are set in the decimal down-count mode. Any one oF the three waveforms may then be selected by a three-position switch and fed to an output buffer amplifier.

The phase-shift is dependent on the relative values of R and C and on the input frequency. A voltage versus current curve for the zener may be plotted, from which the dynamic resistance may be found. As far as component values are concerned, care should be taken to ensure that the maximum cur- elektoe taken by the triac is at least twice; She rnaxi- mum current drawn by the lamp La.

Elektor – 301 Circuits.pdf

If this is not required, C2 and D5 can be omitted and R14 and R15 are replaced by a single 1 k resistor, J. Any desired initial setting can be programmed in BCD by connecting the preset inputs of ICl and IC2 to positive supply or supply common.

The waveform of photo 2 seen on an oscilloscope with poor h.

The solid-state voltmeter described in this article allows continuous monitoring of the battery voltage so that eletkor failure can be spotted at an early stage. N3The ‘clock’ frequency is deter- mined by the value of the capacitor, C, and by the setting of the 1 M potentiometer. The relative values of C2, C3 and R4 determine the lowest fre- quency at which the circuit will function.


However, since the IC is fairly expensive it is recommended that a permanent buffer stage be connected to the out- put.

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 elektkr of the alignment generator is taken from the secondary winding of the IF transformer.

Elektor – Circuits

The input signals are fed to a pair of electronic switches each comprising two elements of a CMOS analogue switch IC. For example, in a 5 V 2 A supply for TTL about 3,5 V would be dropped across the series transistor and 7 W would be dissipated in it at full load current. The output of Eleitor will thus swing down to 0 V, turning on the stereo decoder. This is achieved by feeding the zener via Ri from the output of the op-amp.

The working voltages of the capacitors should at least equal the peak value of the transformer voltage. There are two possi- bilities: A practical circuit using the MK is shown in figure 2, whilst table 1 shows the division ratios that may be obtained for the different settings of 51 to S4.

Elektor – 301 Circuits

When the input of N1 is taken low by bridging the lower pair of touch contacts with a finger, the epektor of N1 goes low. Should the output voltage of the elektoor tend to fall then the lower end of D 5 will fall below 0 V and transistor T3 will draw more current.

D4 which, via Cl and C2, is capacitively- coupled to the transformer. However, by suitable choice of reservoir capaci- tor C2 the ripple on the main unregulated supply can be made such that the voltage falls to about 0. Elsktor using the circuit the wiring capacitance of any jigs and fixtures used to hold the capacitor should be taken into account and subtracted from the reading.

Levels between these limits are undefined. To overcome this problem a ‘pulse stretcher 1 circuit may be used, see figure 2. The reverse characteristic of a zener diode is il- lustrated in figure 1.

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