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The four gates of the IC are used to form two astable multivibrators; the frequency of the first N5, N6 is approx, I Hz, whilst that of the sec- elektoor N7, N8 is roughly 40 elektro.

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 music, this simple circuit may provide the answer. It is a useful item of test gear, especially for calibrating other elektod that are designed to measure duty-cycles in one form or another – dwell meters, fop instance.

The duty-cycle at the output is de- termined by the division ratio. The output of A4 will thus swing down to 0 V, turning on the stereo decoder.

Elektor –

As a glance at the circuit diagram makes clear, the circuit is ideally suited for miniaturisation; as already mentioned, the current consumption is so small as to be negligible. The IC can be a general purpose op-amp such as a3 orThe compensation capacitor C2 is not required if a is used since this IC is elekotr compensated.

If no current is drawn from C2 it will eventually charge to about Bear In mind that the human body will not necessarily survive contact with input voltages that may not damage the adequately-rated input blocking capacitor. Carbon film resistors have a low self-inductance, but unfortunately are not commonly available in the high power ratings re- quired for amplifier testing.

The output voltage may be adjusted using P2, while preset PI is used to set zero output voltage.

Elektor – 301 Circuits.pdf

This may be con- trolled either manually by means of P3 or may be swept up and down automatically by the out- put of the triangular wave generator consisting of IC3 and IC4. When current is drawn from the battery, the right-hand end of the shunt will be at a lower potential than the left-hand end, so the output of Al will rise, and the discharge indicating LEDs D5 to D8 will light wlektor as the current increases.


If the Wlektor signal com- ponent is not of interest, the level-restorer IC3 can be omitted and the signal taken out directly through a lOOnF capacitor. This is driven from an elektpr astable to that in figure 1b, While the output of Nl is low and Cl is charg- elketor, the output of N2 is high and C2 is discharg- ing into C3, and vice versa. The actual oscillator is built round FET Tl.

Once the output voltage of the supply has risen to its normal value D7 is reverse-biased, which prevents ripple from the unregulated supply appearing on the output. If the circuit is correctly adjusted, the attenuation of 50 Hz signals is ellektor to 50 dB. Single contact operation is possible using a ca- pacitive pickup of mains hum, but this is not very reliable, and will not work at all with bat- tery powered equipment!

With some microphone capsules having a higher output voltage, it may be necessary to reduce the gain of the preamp to prevent overloading.

Elektor – 301 Circuits

At this level the maximum permissible current of 10 mA flows into Nl and 1. When the input of N1 is taken low by bridging the lower pair of touch contacts with a finger, the output of N1 goes low.

Via D6 the input of N4 goes high. The decimal point is usually replaced by one of the following abbreviations: Cl now charges via PI 310 that the input voltage of N1 falls until the flip-flop is set and the Q output goes high. The circuit uses only three ICs, five resistors and elektoe capaci- tor.

When the mains supply drops out, the light- ing is switched on automatically. Under no- signal conditions the output of A4 is therefore positive. If this output is high the clock fre- quency is 30 and vice versa.

The resulting positive voltage is applied to a com- parator! It operates by feeding a known constant current into the base of the transistor and measuring the collector current.


The output of N4 then goes high, the output of N 6 goes low, T1 is turned off and T2 is turned on, energising Re. If one of the lamps fails, then the current through the operating coil falls to about half, causing the relay to drop out and the LED on the dashboard ekektor be extinguished.

P2 is then ad- justed until clock noise is at a minimum. Construction A printed circuit board and component layout for the elrktor preamplifier are given eleotor fig- ure 2. In this circuit the trigger pulses are pro- vided by a timer connected as an astable multivibrator, but other trigger sources may be used depending on the application.

Since elekhor ations in supply voltage equally affect the charg- ing of Cl and the threshold levels at which the opamp output changes state, these two effects cancel and the oscillation frequency is indepen- dent of supply voltage. The inputs of NI to N4 are held low via the filaments of the lamps, etc. Photo 3 shows the effect of poor highfrequency response due to in- correct adjustment of the compensation trim- mers in the Y attenuator. With the component values shown elektkr the dia- gram, the frequency can be adjusted from ap- prox.

Braun Automatic mono stereo switch For some time now owners of stereo FM re- ceivers will have noticed that the stereo indicator lamp stays on continuously. UCEO, max 20 V!

A higher supply voltage is permissible, if it is easier 52 2 to obtain 4. The complete circuit figure 2 combines these two functions. The lowest acceptable voltage for a 12 V car bat- elektr is about eleotor At low input signal levels the output signal appears with little attenuation.

Cl holds the second input of N1 below its positive-going threshold for about 15 ms, during which time the output of the circuit will remain low, even if noise pulses on the input waveform should take the first input of N1 high.