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 Elliott Sound Products Project 40 

Load Sensing Automatic Switch

© December 1999, Rod Elliott (ESP)

mains WARNING:  This circuit requires experience with mains wiring.  Do not attempt construction unless experienced and capable.  Death or serious injury may result from incorrect wiring.

PLEASE NOTE:  This project is superseded by Project 79, which is a better alternative.  Note especially that the current transformer shown below will almost certainly need a shunt resistor - I have tested several more small transformers, and their resistance is generally much too high to be used as shown in this project description.  The new version has a sensitivity control, which may sound superfluous - see the new version to see why it may be needed.

How many individual items do you need to turn on to get your Hi-Fi operational?  Typically, this will include things such as preamp, power amp, sub-woofer amp, CD player, and perhaps an electronic crossover.

Have you ever thought to yourself "There has to be a better way."?  Well there is.  Designed to allow a low power device (such as a preamp) to control everything else, this load sensing switch is easy to build, and can control as many devices as you want.

Power amps are catered for with a separate relay (or relays), and the remaining low power equipment can operate from the other.  The circuit is able to sense a mains load as low as 50mA, so even the smallest preamp will trigger it reliably.  Using a light dependent resistor (LDR) as the sensor, with a little care this unit will exceed any electrical safety test, since there is no electrical connection between the sensor and mains wiring.


The circuit of the mains current detector is shown in Figure 1.  By using a few cheap diodes, a resistor, LED and LDR, a simple opto-isolated detector can be created.  This entire circuit dissipates very low power, and can safely be housed in a heatshrink wrapper to ensure that contact with live wiring is not possible.  This will also keep light away from the LDR.  These are cheap and easy to use.  I found that I could detect as little as 10mA of mains current with this circuit, and no distress was created at 0.5A.  The diodes will get very warm at higher currents.

Note that because of the 1A diodes used, this is the absolute maximum current of the switched load.  If a higher current is expected, you must use high current diodes to prevent failure.  I shall leave it to the reader and the local electronics supplier to select suitable devices.  Voltage rating is not important, as they are in series with the load.  Don't be tempted to use Schottky diodes to reduce the loss - you need the loss to turn on the LED.

This circuit introduces a small amount of distortion into the mains waveform, but the added distortion is quite low, and the difference is completely inaudible in a preamp or other audio equipment.  I measured the AC mains distortion with and without the diode string in circuit, and could not detect any difference - mind you, the distortion was 5.6% at the time - somewhat higher than I would have expected, but it remained unchanged with this circuit in or out.  Since all of this occurs on the household AC side of the equipment, it is of absolutely no consequence to the final sound quality (despite what you might have heard or read).

figure 1
Figure 1 - Current Detector and Opto-Isolator

It is very important that the LDR is properly insulated from the mains wiring.  The method of construction shown in Figure 1 is safe, and provides a huge voltage isolation between the two circuits.  After heatshrink tubing is sealed around the LED and LDR, the entire unit can then be sealed up using more heatshrink.  Make sure that no mains wiring is exposed - this means everything to the left of the LDR.  With care, this unit can be made quite small and will not require much internal insulation, since everything is at the same potential (namely live!).

An alternative current detector is shown in Figure 2.  This version introduces no distortion into the mains waveform, but still causes a small voltage difference.  This circuit is also rated at a maximum of 1A, although this could be increased if desired.  The requirements for the relay controller are basically the same, but the difference in detection methods means that the input is modified, using a transistor instead of the LDR.

figure 2
Figure 2 - Current Transformer Isolation

The current transformer is a standard transformer connected backwards in series with the mains.  The secondary low voltage winding is connected to AC1 and AC2.  The secondary voltage should be low - preferably 5V or less, but anything with a secondary resistance of 2 to 3 Ohms will be fine.  It should be rated for at least one amp, since this is the current it will have to handle.  The primary (110, 220 or 240V) will supply a low current signal proportional to the current being drawn by the controlling load.  We don't care about the current, just if it is there or not.  In my tests, I found that a typical 240V to 12V transformer and a 110V to 7V unit both worked fine with a 1k load across the original primary winding.  The voltage obtained is only small, but enough to turn on a transistor as shown in the circuit.

The diodes shown are to ensure that the voltage stays low at all currents, and that the transformer has little series inductance to the load - do not omit these.

NOTE: As indicated above (in the Introduction), most small transformers will have excessive resistance in the secondary winding for direct connection as shown in Figure 2.  A resistor - typically 1 Ohm 10 Watt - should be wired in parallel with the transformer.

It is inevitable that some experimentation will be needed, since I cannot predict the transformer you will use.  Be extremely careful when testing, and remember that without the 1k resistor and diodes, very high voltages may be present on the transformer secondary winding.  The greater the load, the greater the voltage - there is no good reason that 500 or more volts could be present - depending on the load and the transformer.  This will really make your hair stand on end!

The remainder of the unit is a simple relay switching circuit, and uses a couple of transistors.  There is a slight delay built in, so that the controlled equipment will not actually turn on for a couple of seconds after the preamp.  When the controlling load (e.g. the preamp) is turned off, there is also a slight delay before the remaining equipment is switched off.  The controlled AC is switched by the relay, or more than one if the power needs are high (such as for a large power amplifier).

I suggest that relays with a contact rating of at least 10A be used.  Use a separate one for the power amplifier if it is rated at more than 100W.  Large amps draw a significantly higher current than normal at power on, and this may damage lesser relay contacts.

The power supply is a simple unregulated type, and this is quite adequate for the application.  The size of the filter capacitor depends on how many relays you need to control, as does the transformer secondary current.

figure 3
Figure 3 - Relay Control Circuit

Make sure that the power supply transformer is of good quality, since it will be on permanently.  If possible, get one with an integral thermal fuse, so that in the event of a failure in the circuit, there is no risk of fire.  A faulty transformer in a video recorder cause a friend's house to be completely destroyed by fire - this is a real possibility, so don't skimp on the transformer.

Finally, Figure 4 shows the wiring for the mains - incoming and outgoing.  Make sure that this is wired according to the electrical safety rules where you live.  Remember, if you build a piece of equipment that kills or injures someone, you are responsible, so make sure that this is done properly, using approved mains wire and ensuring that no exposed parts can become live.  All terminals internally should be properly insulated using heatshrink tubing to ensure that contact with any live wire is not possible.

note NOTE: To not be tempted to skimp on insulation for the neutral conductor, just because it is supposedly at earth potential.  If all wiring is correct, this is the case, but it is illegal in most countries to treat the neutral any differently from active, since it is always possible that they may become reversed.  Equipment will still work exactly the same, but your 'safe' wiring is now live.

As can be seen, the wiring is very simple, but as stated must be done in a professional manner for safety.  Under no circumstances should the active and neutral be interchanged on an outlet.  If you don't know which is which, then you should not be building this project.

figure 4
Figure 4 - Mains Wiring For The Controller

When complete, the unit can be housed in a plastic or metal case (if metal, the case MUST be earthed).  There are no adjustments to make once you have tested the controller, so it can be hidden away behind all your other gear.

Note that I have used the Australian convention for mains wiring.  If you are not sure of the conversion, you probably should not be attempting to build this.  I do not know the terminology used elsewhere, except that in the US 'Earth' is called 'Ground' - other than that I'm afraid you are on your own.


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Copyright Notice. This article, including but not limited to all text and diagrams, is the intellectual property of Rod Elliott, and is © 1999. Reproduction or re-publication by any means whatsoever, whether electronic, mechanical or electro- mechanical, is strictly prohibited under International Copyright laws. The author (Rod Elliott) grants the reader the right to use this information for personal use only, and further allows that one (1) copy may be made for reference while constructing the project. Commercial use is prohibited without express written authorisation from Rod Elliott.
Page Created and Copyright (c) 08 Dec 1999 - Updated 30 Apr 2001-Added C2 to controller circuit, and referred readers to P79.