# Short Circuits (quick-to-build DIY projects) (Hobby Electronics mag., Mar. 1979)

## PRECISION HALF-WAVE RECTIFIER

By putting a diode in the feedback loop of an op amp, a precision rectifier can be constructed. Normally a diode will need to be for ward biased by 0.6V before it will start conducting, so if you want to half wave rectify a low signal sine wave (say 0.1 V peak-to-peak) it is almost impossible to do it with just a diode.

However, by using an op amp it is possible to get this diode to drop to below 1 mV in most applications. Referring to the circuit imagine Vin goes positive: then the output of the op amp will swing negative to such an extent that D2 will be properly biased and will draw current through the feedback resistor Rfb. In fact the op amp adjusts its output such that the voltage at pin 2 is virtually at 0 V (virtual earth). Thus the output voltage:

Vout, =Vin x Rfb / Rin

.... which is just like a normal op amp. The diode D2 doesn't seem to have affected things and this is true for positive inputs even as low as a few millivolts.

Now when V_in goes negative, the output of the op amp swings positive, D1 conducts and maintains a virtual earth condition and D2 is reverse biased. So, now the output is just Rfb connected to effectively 0V.

What this means is that there is only an output voltage (negative) for positive going inputs: when the input goes negative, the output is zero. That is, the input signal has been precisely half wave rectified.

Now, if the RA. C network is connected to VA„,, the half wave rectifier can be turned into a negative envelope follower. When V,„„ goes negative C is charged via RA; V00, is unaffected whilst C is being charged. When VA„, returns towards 0V, C discharges through RA and Rfb. If C and RA are correctly selected then a contour of the envelope of the signal will be produced at VA„,. For an envelope attack time of 1 millisecond (1 kHz), with an envelope release time of 100 milliseconds, make Rin and Rfb 100k Rk and C 1uf.

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## 555 I.C. PROJECTS

THE 555 IC is one of the most useful devices to the electronics hobbyist, and three examples of its use in the astable (oscillator) mode are given here.

Electronic Doorbuzzer.

This design provides a novel door-buzzer signal which starts at a low pitch and gradually rises in frequency.

The normal method of oscillation for the 555 is for the timing capacitor (C2) to charge up two thirds of V+ via two timing resistors (R1 and R2). The IC is then triggered and C2 is discharged through R2 and an internal transistor of the 555. The IC resets when the charge voltage drops to one third of V +, with the discharge transistor switching off and C2 commencing to charge up to the trigger potential once again.

This particular circuit does not oscillate in precisely this basic way, since the network comprised of R3 and C3 is used to shunt the potential divider (within the IC) which sets the trigger voltage. When SW1 is initially closed, C3 will be discharged and the trigger voltage will be raised. This increases the charge and discharge times of C2, and reduces the frequency of operation. C3 is quickly charted through R3 though and after about one or two seconds the trigger voltage will have fallen to a level set by R3 and the integral potential divider. R3 pulls the trigger voltage below its normal level, reducing the charge and discharge times of C2 and causing an increase in the operating frequency. Thus, as C3 charges up, the output frequency is swept upwards, producing a novel and effective signal.

The main output at pin 3 of the 555 goes high during the charge period, and low during the discharge period, producing a rectangular waveform of low enough impedance to drive a speaker with up to a few hundred milliwatts of signal.

Continuity Tester.

A common failing of simple continuity tester circuits is that they will give an indication of continuity between the test prods when there may actually be a resistance of a few hundred ohms or more. This is often of no importance, but it can sometimes give misleading results.

This simple design can be adjusted so that it will not respond to resistances of more than a few ohms.

The circuit is basically just a standard 555 astable operating at a frequency of about 800 Hz and feeding a high impedance speaker.

However, reset terminal pin 4 is tied to the negative supply rail by R3, and this blocks the astable action. Pin 4 must be taken positive by about 0.5V or more in order to produce an audio output.

RV1 is adjusted so that with the rest prods shorted together there is only just sufficient voltage at pin 4 to enable oscillation to take place.

Therefore, with genuine continuity between the test prods the unit will produce an audio output, but with a resistance of more than about 7 or 8 ohms in circuit, the voltage at pin 4 will be inadequate due to the voltage drop across this resistance.

RV1 is fed from a stabilized supply provided by R4 and D1 so that minor variations in the supply voltage do not necessitate readjustment of RV1. Occasional readjustment of RV1 may still be needed if this is very critically adjusted for optimum discrimination.

Note that the circuit will consume power when the test products are not connected together (about 6 mA.), and so on /off switch SW1 is required.

Simple Timer.

This general purpose timer gives an audible alarm so predetermined time after the unit is switched on. With the specified values the time is variable from about 30 seconds to 5 minutes, but this can be altered to suit individual requirements.

When the unit is switched on using SW1, C1 begins to change via R1 and RV1. Initially the voltage at the inverting (-) input of IC1 will be higher than that appearing at the non-inverting (+) input, and so IC1 output will assume a very low voltage. As C1 charges up, the voltage fed to the inverting input gradually falls until it starts to go below the voltage at the non inverting input. IC1 output then begins to rise in voltage and due to coupling through R4 this increases the voltage at the non-inverting input This causes a further in crease in output voltage, and a regenerative action takes place which causes IC1 output to rapidly swing to almost the full positive supply potential The 555 is used to generate the alarm signal using a circuit which is basically the same as the continuity tester circuit described above.

However, the reset terminal is, of course, controlled by the output of IC1 rather than by the test prods and potential divider circuit.

The charge rate of C1 and thus the length of the timing interval can be altered by changing the resistance of RV1. The time delay is approx. 1.4 CR (with C in uF. R in Meg., and the time in seconds), but due to the high tolerances of the timing components it is impossible to obtain highly predictable results.

R3 has therefore been included so that the trigger voltage of the circuit can be varied, and by trial and error R3 can be adjusted to give the appropriate timing range. When the unit is switched off.

SW1 discharges C1 through cur rent limiting resistor R7 so that the unit is ready to start a new timing run almost immediately.

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## LIE DETECTOR

It is well known that a person per spires under tension, what is less well known is that this effect is a gradual one and that a small amount of perspiration takes place, especially in the palms of the hands, even under slight pressure In the normal course of events this is rarely noticed but this effect can be shown electronically When a person is embarrassed or tells a lie there is a very small, but noticeable, increase in the sweat on the palms of the hands Perspiration is reasonably conductive. holding the probes of a test meter in the hands will show a resistance reading, albeit at a high level It will therefore be seen that by measuring the resistance across a person's hands that we shall be able to see an indication of whether they are telling the truth or not Let us say straight away that this test is far from perfect and it has little serious use but it does illustrate an interesting phenomenon and makes for a little experimenting The change in the body resistance is quite small when shown as a percentage--about 5 or 10 per cent and showing this change directly on a meter leaves some thing to be desired For this reason we make use first of a transistor to--amplify " the resistance and secondly we place this in a bridge circuit When this is in balance the meter will only read changes in the resistance.

When the probes are held, one in each hand, the body resistance, in conjunction with R2, provides the bias for the transistor The body resistance varies enormously from person to person as well as with their emotional state but a typical value could be taken as 100 k ohms R2 is included solely as a safety resistor and will prevent damage to the device if the probes are touched directly together The current passing through this transistor and through R1 will depend upon the value of the resistance between the collector and emitter As the current vanes, so will the voltage at the collector For setting up the circuit the probes should be held in the hands This will give a particular voltage and RV1 is adjusted so that the voltage at the slider is the same as that at the collector of the transistor As the voltages are the same, no current will be flowing through the meter coil and no reading will be registered If the body resistance now falls 01 will conduct more and the voltage at the collector also falls and a reading will be shown on the meter, the size of the deflection will indicate by how much the body resistance has fallen Although the probes are held in the hand, there is no danger as only a 9V battery is being used RV1 will have to be adjusted for each individual and even for each set of readings with the same person The effect is quite remarkable and also surprisingly rapid, within a very short while one or two seconds) the meter will show a deflection There may be a small amount of wandering of the needle but this will be small compared to normal readings As we have said, the results should not be taken too seriously but very definite readings are given when the person being tested is under stress.

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## SIMPLE AMPLIFIER

The term amplifier covers a very wide range from a one transistor preamp to an ultra sophisticated high power Hi-Fu system. There is no doubt that the latter is much more pleasant to listen to but for many applications high quality is of little importance and simplicity is required. There is little doubt that the circuit shown here is very simple. The output is in the order 250 m W--which is quite sufficient for most purposes and is comparable to that of the average transistor radio. The distortion level is rather high, being about 5%. The amplifier is also reasonably sensitive and will give full output with an input of about 50 mV. Input impedance is about 50 k. A simple tone control is included though as since this is an active control, rather than a passive one, the range is quite sufficient.

The slider from the volume control is connected to the base of Q1 via a DC blocking capacitor. Q1 is connected as a pretty conventional common emitter amplifier with R2 providing the base bias and R3 acting as the collector load. This stage is directly connected to the second transistor which is a PNP type. In this way the current passing through Q1 provides the bias for the second transistor Because of the values used, the output of the second transistor is connected directly to the speech coil of the loudspeaker. This is not normally good practice since the standing current in the output transistor continually biases the coil either slightly in or out from its usual operating point. However, if a large speaker is used, as it should be, this has very little effect and since we are not aiming at Hi-Fi, it does not matter.

The tone control- comprises C2 and RV2 which are connected between the collector and base of Ql.

At high resistance settings of RV2 this has little effect but on mini mum settings the 100 n feeds back the high frequencies out of phase, thus cancelling them.

For this circuit to work properly R3 must be selected with great care. The value shown here of 39 ohms is only a typical one and although it may be used for initial setting up to ensure the circuit is operating, the value should be found by experiment. If it is too low there will be severe distortion at the higher volume settings. If it is too high the current drain will be excessive even though the quality of reproduction will be good.

There are two ways of finding the value. Without a multimeter the value should be selected as being the lowest which is compatible with good quality. If a multimeter is available this should be wired in series with the supply voltage and R3 should be selected so that the quiescent current, this is the current flowing with no input signal, is reading 20 mA. It is very important that Q2 is fitted with a heatsink as it will get very hot and will probably run away without it.

The speaker impedance is not all that critical and in the prototype speakers with an impedance as low as 8 ohms and as high as 80 ohms all worked well although changing the speaker impedance will also necessitate a change in the value of R3.

(adapted from: Hobby Electronics magazine, Mar. 1979)

Also see: SW Aerials (Antennas)

A Good Joint is Hard to Find (Feb. 1982)

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