1001 Circuits Elektor: A Must-Have Resource for Electronics Enthusiasts

3 Elektor Electronics 1 Harlequin Avenue Great West Road BRENTFORD Middlesex TW8 9EW ENGLAND Telephone: Telex: All rights reserved. No part of this book may be reproduced or transmitted in any form, or by any means, including photocopying and recording, without the prior written permission of the publishers. Such written permission must also be obtained before any part of this book is stored in a retrieval system of any nature. The circuits and other diagrams contained in this book are for domestic use only. Patent protection may exist with respect to circuits, devices, components, etc. described in this book. The publishers do not accept responsibility for failing to identify such patent or other protection. This book is sold subject to the Standard Conditions of Sale of Net Books and may not be re -sold in the United Kingdom below the net price given by the publishers in their current price list. First printed 1988 Copyright 1988 Elektuur BV Printed in the Netherlands ISBN

4 WARNING! ELECTRICITY CAN BE DANGEROUS! The electronic projects in this book are, to the best of the author's knowledge and belief, both accurately described and safe. None the less, great care must always be taken when assembling electronic circuits which carry mains voltage, and neither the publishers nor the author can accept responsibility for any accidents which may occur. Because electricity is dangerous, its use, application and transmission are subject to rules, regulations and guidance. These are laid down in numerous laws, Electricity Generating Board regulations, British Standards, and IEE recommendations. Some of these may be obtained from your local electricity showroom, but most, if not all, should be available for reference in your local library.

1001 Circuits Elektor

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13 wise incurred by too heavily loading the IC, output. The rectified voltage across C5 is a direct measure of the output signal amplitude, and forward -biases the base of T,, which regulates the attenuation as discussed. The use of a diode with a low internal resistance, Di, and a buffer, T2, ensures fast charging and slow discharging of C5, and thus a short attack time and a long release time, respectively. As C5 is discharged via R7 and the base resistance of T1, the release time of the compressor is the product of the value of these three components. When the base bias is reduced, the base resistance of Ti increases, lenghtening the release time. This is a most welcome feature, especially with speech signals. The output of the opamp is fed to Ca -Pi -Rio, which provide DC insulation and level adjustment. Two compressors are readily combined to make a stereo version by feeding them from a common battery and connecting points X and points Y (never X to Y!). In this case, Ti and Di in both compressors must be matched types to ensure proper operation. Figure 2 shows two simple test circuits for selecting transistors and diodes with matching DC characteristics. The basic method is to start with noting the voltmeter reading for a particular device, and then find a matching type from an available lot by inserting devices until one is found that gives the preciously noted test voltage. In the diode test circuit, the LED lights to indicate the absence or reverse connection of a diode under test. A = V B = +4.5 V C = 6 ma D = 3.9 V All values are typical and within 1%. All voltages measured with respect to ground with a DMM (Zin = 1 MO). Provision has been made to use the circuit as a noise suppressor. Referring to Fig. 1, closing Si connects C8 across the regulator transistor to form a low-pass filter in conjunction with R1 and R2. The cut-off frequency of this LPF is a function of the current sent into the base of Ti. The overall effect thus obtained is an effective elimination of noise from quiet passages in the programme. For louder passages, the suppression of noise is not so important, as it is then virtually inaudible. Finally, when using this compressor, make sure that your amplifier has ample cooling provision, because it may well be continuously operated at the top of its power rating. For the same reason, check whether the loudspeakers can handle the available power. 5 CURRENT CORRECTED AF AMPLIFIER The majority of modern AF power amplifiers drive the loudspeaker(s) with a voltage that is simply a fixed factor greater than the input voltage. It is fairly evident, therefore, that the power delivered by such amplifiers is inversely proportional to the loudspeaker impedance, since the cone displacement of a loudspeaker is mainly a function of the current sent through the voice coil, whose impedance may vary considerably over the relevant frequency range. In multiway loudspeaker systems, this difficulty is overcome by appropriate dimensioning of the crossover filter, but a different approach is called for when there is but one loudspeaker. This amplifier is based on current feedback to ensure that the current sent through the voice coil remains in accordance with the input signal. The current through the voice coil and R7 develops a voltage across the resistor. A negative feedback loop _7=1W =MKT A=OV B=OV C=17V4 =-17V4 E=OV

14 is created by feeding this reference voltage to the inverting input of ICI. The overall amplification of the circuit depends on the ratio of the loudspeaker's impedance, ZL, to the value of R7. In the present case the amplification is 16 times (ZLJR7 = 8/.5 = 16). The connection of the opamp's output to ground is slightly unusual, but enables the base current for output transistors Ti -T2 to be drawn from the supply rails, rather than from the opamp. Capacitor C6 functions to set the roll -off frequency at about 9 khz. The quiescent current of the amplifier is of the order of 5 to 1 ma for class A operation, and is determined by R3 -R4 and Its -R6. The complementary power transistors should be closely matched types to avoid fairly large offset currents (and voltages) arising. Some redimensioning of either R3 or R4 may be required to achieve the correct balance for the power output stage. The emitter current of Ti and T2 is about 5 ma when the amplifier is fully driven. The harmonic distortion of this amplifier is less than.1% at P= 6.25 W and Ub = 18 V. Source: Texas Instruments Linear Applications. 6 DIGITAL AUDIO SELECTOR Switching audio signals digitally could be done with the aid of CMOS analogue switches or multiplexers. Simple as this may seem, there is, however, an inevitable loss in the quality of the sound due to the noisy nature of CMOS switches. Furthermore, the high on -resistance of these devices together with the large parasitic capacitances generally present in CMOS circuits causes a high susceptibility to crosstalk. The circuit given here is a novel way of selecting one out of ten audio signals digitally without any of the foregoing drawbacks. As shown in the circuit diagram, the ten input signals numbered 1-1 are applied to the bases of transistors T, -Tio via capacitors Ci -C, respect - CONTROL INPUT D2 NOM {815 A 4 B C IC _ C11122n C11 122n R13 RI IC = 74 LS 45, 74 LS 145, 74 HCT 45, 74 HCT 145 A= OV9 B= 1V6 C = V2 I)=14V6 E = 14V6 F = 13V6 = 14V2 H= 2V1 J= 2V1 R22 VD11 R12 RIO 11 1N 41481D12 R11 T11 BC 56 C 11,8pA Th ZuD1 16 IC2 IC1 C15 C12 1n 176V D1 1x 1 N 4148 (D, 1x % 11 BC 56 C C14 min 7n 6 IC 2 LF 356 IC 3 78 L 5 C V 13 ma C V 15V 7 ma e

18 disconnects the input from the signal source when operated, and so readily gives rise to clicks and contact noise. Different problems crop up when designing an electronic volume control. Of these, distortion is probably the hardest to master, but reasonable results are still obtainable, as will be shown here. Basically, there are two methods for making an electronic potentiometer. One is to create a tapped resistor ladder (which is not much different from a normal potentiometer), the other is to change the resistance of the two "track sections" such that the total resistance remains constant. The circuit proposed here is based on the second method, and features 16 steps in its basic form. The number of steps can be increased to, say, 64 by adding four switches and resistors. The electronic potentiometer is composed of two equal sections, which have a total resistance of 15 k52 each. The electronic switches in each section are controlled by binary counter 15. Since the switches in section ESI-ES4 and those in ES5-ES8 are controlled in complementary fashion, the total resistance of the potentiometer remains constant. Resistors Ri-R2 and R7 -Rs serve to keep the potential at the input and output at V so as to preclude clicks when the step switch, S2, is operated. Switch Si is the up/down selector. Gates N5 -N6 form a bistable to ensure that the counter is clocked with debounced step pulses. The number of steps can be increased by adding a counter and the required number of electronic switches, divided over the two "track sections". These switches are then connected in parallel with resistors whose values correspond to binary order , etc., as shown in the circuit diagram. Fortunately, precise binary ratios are not required here, since adequate results are obtainable with approximations of the theoretical resistance values, and as long as the actual resistors are kept equal in both sections. 9 DISCOMIXER This mixer is a typical example of the way modern components can, and do, simplify the realization of good quality audio circuits. In the given configuration it is eminently suitable for use as a discomixer, but the number of input channels can easily be enlarged. As can be seen in figure 1, in its basic form the mixer has four input channels. These could, for instance, serve as inputs for a microphone, stereo pickup, and cassette player or tape recorder. The power supply has been kept as simple as possible; if it proves difficult to obtain the XR4195 regulator IC, it may be replaced by a combination of a 78L15 and 79L15. The transformer is preferably of the PCB type to keep the mixer as compact as possible. The values of Ci and Ri are dependent on the type of microphone used. If this is a high -impedance type, the values should be 47 nf and 22 k52 respectively, whereas with low -impedance types, 1 µf and 68 Q are required. Unfortunately, miniature bipolar electrolytic capacitors (CI, CI', C9, and CO are not yet available everywhere, although they are almost indispensable in applications such as described here. Standard electrolytics may be used with maximum reverse voltages of 1 V, but their use introduces distortion and premature ageing (because of the reverse polarity). Provision has been made on the printed circuit board for up to four channels. Two or more PCBs may be connected together; the output and supply sections may then be cut off as required. Current consumption is about 1 ma per channel. Parts list Resistors: R.1*...Rs*,Rv*...R5'* see table Rs*,Rs*,Ra,Ra' = 47 k R7,RT = 22 k R9*,Rw* = 1 k Pia*,Pib* = 22 k stereo slide potentiometer, log, 58 mm long Capacitors: = see table Cs*,Cs' = 47 n Cs",C7",C1o,C11,C1a,C1s = 1 n Cs,Ca' = 1 p Cs,Cs' = 1 /1/25 V C12,C13,C14,C15 = 22 n C1e,C17 = 47 p/25 V C2o,C21 = 1 p/16 V C22,C23 = 1 p 18

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