MAINS Failure Alarm

MAINS failure alarms are often employed in situations where the removal of the mains supply from a piece of equipment can have adverse or even disastrous consequences.

Very often, the “failure” may simply be the result of the switch on the outlet socket being operated inadvertently and this may have no immediately noticeable effect on the equipment. The results of the mistake may only become evident a few hours or even days later, when it is too late.

The circuit described here overcomes all of these problems at a stroke and also does away with the need for mains plugs, or indeed any connections to the mains at all. It does so by monitoring the electric field which exists around a cable connected to the a.c. supply (whether it is carrying a current or not).

By placing it on or near to the cable of the appliance to be monitored, it will also sound the alarm if the fuse in the plug blows, the outlet is switched off or the plug disconnected. It will only fail to detect the situation where the equipment itself has been switched off via its own built-in switch. However, since many appliances such as freezers do not have on/off switches, this is not really a problem.

The circuit is extremely simple and inexpensive to build. Its simplicity and lack of any specialized components should make it attractive to many constructors who will probably already have most of the components to hand. The absence of any mains connections should make it an ideal project for a beginner.

As with many simple circuits, however, the advantages and possible uses take longer to describe than the operation of the circuit diagram which is shown in Fig.1.

The heart of the circuit is an oscillator built around two CMOS NAND gates (IC1c and IC1d) which drive a small piezo sounder, WD1. With the values specified for resistor R5 and capacitor C3, the circuit oscillates at around 200Hz, producing a fairly loud sound from WD1.

Since an intermittent sound is better at gaining attention than a louder continuous alarm, this oscillator is driven by another similar circuit built around IC1a and IC1b. Here the frequency determining  components, R3 and C2, have been chosen to give a lower frequency so that when the circuit is activated, the sounder produces a tone of 200Hz in bursts at around 2Hz (i.e. two bursts per  second).

Mains Field
Oscillator IC1a/b is normally disabled, however, because of the action of transistors TR1 and TR2. These are switched on in the presence of the alternating electric field which exists around all cables carrying a mains potential, manifesting itself in high gain amplifier systems as “mains hum”.

In this circuit, transistor TR1 is used as the high gain amplifier, providing base current for TR2. There is a combined current gain of around 10,000.

With a high value of collector load resistance (VR1 plus R1), transistor TR2 does not need to pass very much current before its collector voltage falls below the nominal logic-high level for CMOS gates (approximately half of the supply voltage).

When the input to TR1 is placed near enough to a cable carrying a mains voltage, the positive mains half-cycles are amplified and TR2’s collector voltage falls, so disabling the oscillator around IC1a/b.

Capacitor C1 prevents the collector voltage from rising again during the negative half cycles.
Should the mains field cease, TR1 and TR2 will remain off and C1 will charge to the circuit’s d.c. supply voltage via VR1 and R1, allowing IC1a/b to oscillate and the alarm to sound.

Sensitivity
The sensitivity of the circuit depends on the value of the collector load and is best determined experimentally, so it has been made variable using preset VR1. In most cases, unless the mains field is generally very high, it will probably not need to be reduced from its maximum 470k ohm value.

Resistor R1 has been included to prevent a direct short-circuit between the two power lines via TR2 should VR1 be set to zero resistance.

Because capacitor C1 takes time to charge via VR1 and R1, there will be a delay of about one second (depending on the setting of VR1) before the alarm sounds following a mains failure.

The circuit can be powered by a PP3 9V battery, or two AA cells in series providing 3V, depending on the user’s preference. With the lower supply voltage, however, the sound produced will not be as loud. Since the current consumption in the stand-by mode is around 45uA (at 9V) the battery should last many months.

Testing
For initial tests, keep the unit well away from any potential source of mains electrical field. Solder a short length (say 100cm) of single core insulated wire to the base connection to TR1 to act as an “aerial”. Set VR1 to its maximum resistance.

When the battery is fitted, the alarm should sound after capacitor C1 has had time to charge. If it does not sound, there may be sufficient local mains electrical radiation for the unit to pick it up undesirably. If so, reduce the resistance of VR1 to reduce the circuit’s sensitivity, although the need to do this is unlikely.

Now the oscillator should only stop when you grip the aerial insulation. Once the circuit is working correctly it should be mounted in a plastic box of a size that will accommodate it and the battery. The piezo sounder was purchased mounted in a plastic package suitable for fixing to the box but uncased elements are also available and if this type is used it should be glued to the inside of the box lid. It may be found necessary to drill a small hole in the lid to allow the sound to be heard, although the lid will generally act as a sounding board.

The sensitivity of the unit may be high enough for the aerial wire to be left inside the box and the box simply placed alongside the mains cable. If this is not the case, the wire should be brought out and wrapped around the appliance mains lead a few times. This will not only increase the apparent sensitivity of the circuit but will also ensure that the unit remains in close proximity to the cable.

Alternately, a Bulldog type paper clip attached to the box and wired to the base of TR1 could be used as an aerial and a means of securing the device to the cable.

Once complete and in proximity to the mains cable of the appliance, or clipped to it, final testing of the circuit is very simple. Unplugging the mains lead or switching off the power at the mains socket should cause the alarm to sound. Restoring the power should mute the alarm. It is probably better to use a small appliance such as a table lamp to do this initially, rather than your freezer!

Other Applications
As well as a mains failure alarm, the circuit will be found to be a useful addition to any electrician’s tool box. It may be used in identifying which circuit is connected to which fuse in the main fuse box or indeed to ensure that the cable which is about to be cut is not carrying a mains voltage.

The circuit can be attached to the cable and the fuses removed in turn until the alarm sounds. Most non-contact cable testers provide only a visual indication and cannot be attached to a cable.

The circuit is also useful in finding a break in a cable or mains circuit. One application which comes to mind is the annual quest to discover which lamp in the Christmas tree lamp chain has mysteriously become disconnected during its yearlong storage.

Another is checking the fuse in a mains plug without the need to undo the whole thing – only to find that the fuse is OK!