Interesting circuit ideas which we have checked but not built and tested. Contributions from readers are welcome and will be paid for at standard rates.

Making the Flexitimer cycle on and off

The Flexitimer published in the March 1991 issue of "Electronics Australia" is still a popular project but it has the drawback that it is "once only" timer. Once it has timed out, it is effectively disabled.

We have had many requests asking how it can be made to cycle on and off. This can easily done, as shown with this modified circuit. This makes it cycle on and off with a 50% duty cycle, for as long as power is applied. So for example, if it is set for a period of 8192 seconds, it will be off for 8192s, on for 8192s and so on.

The modification involves cutting the track connection between pin 4 (reset) of IC1 to the collector of Q1 and tying it to pin 8 (+V).

SILICON CHIP.

Low battery indicator

This simple circuit lights LED1 when the battery voltage drops below the setting set by trimpot VR1. In effect, VR1 and associated resistors bias Q1 on which holds Q2 and the LED off. When the voltage drops below the set value, Q1 turns off, allowing Q2 to turn on and light the LED. The circuit is suitable for nominal battery voltages up to 12V.

I. Ross,
Springwood, Qld. ($30)

Simple 9-way cable identifier

Here is a simple way of identifying multiple cables (with the aid of a multimeter). The circuit consists of a series of resistors, selected so that they give readings that coincide with the 1-9 numerals on the 10V scale on a multimeter switched to the Ohms x 100 range.

In use, a common wire needs to be chosen and this is usually the shield wire. The resistors go to one end of the cables to be identified, while the multimeter is used at the other end to check the values and identify each lead.

Up to nine cables can be identified at a time. If a mistake is made in choosing the common lead, the readings will all be wide of the 1-9 numerals on the 10V scale, thus making the mis­take obvious.

J. Begg,
Heidelberg, Vic. ($20)

Clipping indicator for audio amplifiers

A clipping indicator is a useful accessory on any audio amplifier. It indicates when the amplifier has reached its limit and is clipping the peaks of the audio signal.

In practice, quite a lot of clipping can occur before you can hear it. So why is it necessary to know when an amplifier is clipping if you can't notice it? The answer is that clipping "squares up" the waveform and square waves contain lots of higher-frequency harmonics which can easily damage the tweeters in loudspeaker systems.

This circuit is a true clipping indicator as opposed to the level indicators that are commonly used in preamplifier stages. The problem with level indicators is that an amplifier's maximum output power is not constant. That's because the amplifier's supply rails are not regulated and so the maximum power available at any given instant varies, depending on the applied signal.

The circuit is quite simple and is based on two BD140 PNP transistors and zener diode ZD1. During normal operation, Q1 is turned on via ZD1 and R1. As a result, Q2 is held off (since its base is pulled high) and so LED1 is also off.

However, if the output signal subsequently rises to within 4.7V of the positive supply rail, Q1 turns off since it no longer has any forward bias on its base. As a result Q2's base is now pulled low via R2 and so Q2 turns on and lights LED1. (Note: the 0.6V drop across Q1's base/emitter is ignored here because ZD1 conducts before its rated voltage due to the very low current involved).

Why choose 4.7V below the power rail as the turn-on point? The reason is that, due to the drive limitations and the nature of emitter followers, they can be expected to have at least 4V across them when they saturate (ie, clip).

ZD1 can be increased to a 5V or 6.2V type if the circuit is to be used with a monster amplifier.

The value of R3 should be customised according to the amplifier's supply rail, so that LED1 operates with the correct brightness. To do that, first measure the amplifier's positive supply voltage, then use Ohms Law (R = V/I) to calculate the value of R3 for a current of about 20mA.

As it stands, this circuit can only be used to monitor the positive-going half-cycles of the audio waveform. If you want to monitor the negative half-cycles as well, you will have to build a second circuit with the following changes: (1) reverse both LED1 and ZD1; and (2) use BD139 (NPN) transistors for Q1 & Q2.

Note that, in both cases, you should use the earth inside the amplifier, as the speaker negative may not be earth (such as in a bridged output).

Philip Chugg,
Launceston, Tas.

8V DC supply with overvoltage protection

This 8V DC power supply was designed for use with an expensive piece of electronic equipment. It features full over-voltage protection as a precaution against regulator failure, either in the supply itself or inside the equipment it is powering.

The circuit uses a conventional full-wave rectifier, followed by a 3-terminal voltage regulator (REG1) with appropriate filtering. When power is applied and switch S1 is in the "Run" position, REG1's output is fed to the load via a 500mA fuse and Schottky diode D3. This also lights LED2 (yellow) and LED3 (green), which respectively indicate the presence of the unregulated and regulated voltages.

D3 is there to protect the circuit against external voltage sources (eg, charged capacitors).

A "crowbar" circuit comprising ZD1 and SCR1 provides the over-voltage protection. It works like this: if a fault develops (eg, REG1 short circuit) which causes the output voltage to rise above 9.1V, ZD1 turns on and applies a voltage to the gate of SCR1. If the voltage then continues to rise, SCR1 turns on (at about 10V) and "blows" the fuse.

Zener diode ZD2 provides emergency over-voltage protection in case the "crowbar" circuit develops a fault.

Switch S1 is provided so the operator can occasionally test the "crowbar" function. When S1 is switched to the "Test" posi­tion, the load is disconnected by S1b and the unregulated supply voltage is applied by S1a to the "crowbar" circuit, thus causing it to trigger. When this happenS, LEDs 2 & 3 (green and yellow) extinguish and LED1 (red) lights to indicate that the SCR has triggered.

The SCR turns off again when S1 is switched back to the "Run" position.

L. Cox,
Forest Hill, Qld. (45)

Cheap switchmode DC-DC converter

This circuit is based on mobile phone chargers available from bargain stores such as "Silly Sollys" for about $4.99.

These chargers are based on the Motorola MC34063 switchmode IC. By changing the values of the feedback resistors (R1 & R2), the output voltage can be varied over a wide range.

Just modify R1 and R2 according to the formula:

Vout = 1.25 (1+R2/R1).

The values shown give an output of 3V.

Timo Mahoney,
Chillagoe, Qld. ($30)

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