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 Elliott Sound Products Project 166 

Push-on, Push-off Mains Switch

© 2016, Rod Elliott (ESP)

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Introduction

Push-on, push-off switches are common in many products. In some cases the action is purely mechanical, with a special alternating latching mechanism that holds the switch in the 'on' position, and releases again next time the button is pressed. These switches used to be quite common, but were only ever available with a limited range of button styles. They are less common now, and it's likely that you won't be able to find a style or shape that suits your panel layout.

Momentary action switches are far more common, and come in a wide range of different styles. Some require only a gentle touch to operate, but of course they are not latching. This makes them unsuitable for switching on your equipment. You can use two switches, with one for 'on' and the other for 'off', but this is usually inconvenient and takes up more space on the front panel.

Electronic switching does have one down side, and that's the need for a continuous power supply. In some equipment that can be a problem, and of course a continuous supply means that some current is drawn from the mains all the time. If the supply is designed properly standby current will be very low, and the dissipated power can easily be under 1W with a suitable power supply.

This project will appeal to people who want to build an amplifier or preamp that has a 'modern' feel, using a single button to turn it on and off. As shown here, a mechanical switch is needed, so be aware that it is not a 'touch' switch. Most people won't be bothered by this though, since true touch switches seem to have fallen from favour.


Switching Circuit

The schematic of the switching circuit is shown below. It uses a single 555 timer and a few passive components. The mains will usually be switched using a relay, either electromechanical or 'solid state', and driven from the output of the switching circuit. See below for more information on relay driving. C3 is not needed if the circuit is close to the power supply (less than 50mm or so). Otherwise, C3 is necessary to ensure that there are no supply 'glitches' when the output changes state. It should be located as close to the IC as possible.


Figure 1 - Push-on, Push-off Switching Circuit

Figure 1 shows circuit of the switching system. When the pushbutton (Sw1) is operated, the output of the 555 timer will alternate between high (12V) and low (0V). The LED is used to indicate the state of the switch, and acts as a power-on indicator. C2 is much larger than is normally used with 555 timer circuits, and is used to force the 555 timer to a 'low' output voltage when power is first applied. This is the default and will be appropriate in most cases. If you prefer the 555 to output 12V when power is first applied, connect C2 between the 'Crtl' pin (pin 5) and the positive supply. The positive lead of C2 must go to +12V if wired this way.

R2 and R3 set the voltage on the 555's 'threshold' and 'trigger' pins to 1/2 the supply voltage - typically 6V. When the output is off (0V), the voltage across C1 will be close to zero, as it is discharged via R1. When Sw1 is closed (momentarily), the voltage at the threshold and trigger pins falls to zero for an instant, and causes the 555 timer's internal flip-flop to change state. The output goes high (12V), the external relay (or whatever is being controlled) turns on, and C1 now charges, reaching close to 12V in a couple of seconds.

The next time that Sw1 is pressed, the threshold and trigger inputs are forced to near +12V, causing the flip-flop to change state again. The output now falls to zero and the external relay turns off. Because of the rather long time constant of R1 and C1, switch contact bounce will not affect the circuit's operation, and spurious or random switching won't happen. If R1 or C1 are too low in value any contact bounce in Sw1 can easily leave the output in the wrong state, having gone high and low several times in a short period. The 1 second time constant used completely eliminates this problem.

If you think that you'll need to operate the switch faster than once a second (very unlikely I would expect), you can reduce the value of C1. Less than 1µF is not recommended.


Mains Switching

If you are not experienced with mains wiring, do not attempt the following circuits. In some countries it may be unlawful to work on mains powered equipment unless you are qualified to do so. Be aware that if someone is killed or injured as a result of faulty work you may have done, you may be held legally responsible, so make sure you understand the following ...

WARNING : The following description is for circuitry, some of which is not isolated from the mains. Extreme care is required to ensure that the final installation will be safe under all foreseeable circumstances (however unlikely they may seem). The mains and low voltage sections must be fully isolated from each other, observing required creepage and clearance distances. All mains circuitry operates at the full mains potential, and must be insulated accordingly. Do not work on the power supply while power is applied, as death or serious injury may result.

For some examples of mains switching, see Project 156, which describes a number of circuits designed for 12V triggered mains switches. Provided you use a relay that doesn't draw too much coil current (most will be fine), you can simply use the output of the 555 to drive the relay directly. Be aware that you must never omit the diode across the coil, and you need a diode in series with the output of the 555 IC as well. This is because the 555 has an active output stage, but the IC's internal circuitry can 'latch-up' if even a small negative voltage is applied to the output pin. See The 555 Timer article for more information about the IC and its uses and limitations.

It's far better to use a transistor to switch the relay. This ensures that the output of the 555 isn't loaded excessively, as loading will cause a voltage drop at the output which may cause the circuit to be unreliable. For the sake of a few cents and the addition of a resistor, transistor and diode, you can be certain that the 555 operates with the least possible external disturbance, and it will be free of any undesirable behaviour.


Figure 2 - Typical Mains Switching Circuit

The drawing above shows a simple example of a mains switch. This maintains isolation between the 12V supply and mains, so everything connected to the 12V supply can be handled safely. The power supply for the circuit can be a small transformer, rectifier and filter, or you can use the insides of an isolated switchmode plug-pack (aka 'wall wart') supply. There are many possibilities (including the one shown below), and you can have a look at the ESP article 'Small, Low Current Power Supplies' to see some other examples. The supply does not need to be regulated, but it should be reasonably free of ripple which again might cause unreliable operation.

If the switch is insulated to mains standards you can use a transformerless supply, but I never recommend them because they are inherently deadly. A transformer or switchmode supply is safe under most possible conditions, and failure between mains and low voltage circuitry is very rare. Provided the equipment is wired with a protective (safety) earth/ ground, even if there is a major failure, the users are still protected because the house mains safety switch will activate or the fuse will blow.


Power Supply

A suitable power supply is shown below. It's nothing fancy, and uses a small 9V transformer, four diodes and a capacitor. This is virtually identical to the supply shown for several other small ESP projects, and will provide close enough to 12V DC output. When loaded with a typical relay (having a coil resistance of around 270 ohms), the ripple voltage will be about 50mV RMS. You can use a small bridge rectifier IC or module, but I expect most hobbyists will have a stash of 1N4004 diodes already.


Figure 3 - Power Supply Circuit

There's nothing fancy about it, but if you use a transformer from an established supplier it will be safe and will usually last for the life of the equipment. The fuse is optional but recommended. It will typically be permanently wired (and insulated), since if it fails it probably means that your transformer has died. You can also use a switchmode supply (the innards of a plug-pack/ wall-wart for example) which will have a regulated output, but almost certainly won't last as long as the simple linear version shown.

Make sure that all mains wiring uses mains rated cable, and that joins are secure and insulated against accidental contact. If you are unsure about your ability to perform mains wiring safely and to the standards required in your country, then get someone to do it for you. Take careful notice of the red warning above - it's extremely important !


Conclusion

Using a push-on, push-off switch adds a small wow-factor, and it is a nice feature if you want to make your gear a little out of the ordinary. Of course you pay for it in extra parts and the need for a full-time power supply (and its attendant losses), but overall it does add something a bit unusual to a home-built project. If you choose a switch with a 'nice' feel, I expect you'll be very pleased with the result.

The circuit is very simple, and standby current is just that drawn by the 555 timer - a few milliamps at most, plus the small idle current of the transformer. The switching is completely immune from false triggering due to switch contact bounce, and it provides a 'lockout' of around 1 second before pressing the button again will do anything. This can be increased by using a larger cap for C1, but as it stands it's just about perfect.

The output of the 555 timer can also be used as a 12V trigger signal for other equipment you may have, and that increases the functionality of the circuit. Many subwoofer amplifiers have provision for a 12V trigger, as do many power amps. If you build your own, the Project 39 soft-start circuit PCB has provision for using a 12V trigger, and that can be incorporated into your project.


References
  1. Inspiration for this design was a number of similar circuits on the Net, but I have added more information and ensured that the circuit functions as described in a simulator and 'real life' on the test bench.

 

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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 Copyright © 2016. 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 the author.
Page Created and Copyright © - April 2016, Rod Elliott.