Flavio plugs into smart extension sockets

September’s Gadget master project features Italian designer Flavio Dellepiane and his design for power sockets that can be switched automatically by an embedded Control Socket.


Put yourself back in control, and more easily determine the power status of a set of related devices. Simply access the control device through its normal interface or switches and avoid struggling to access inconveniently placed extension sockets.

A range of applications is possible. For example, if an electric drill is connected to the Control Socket, the Switched Sockets will be powered each time the electric drill is running. In this case, a lamp could be connected to a Switched Socket and will illuminate when the drill is operating.

Or maybe, a desk lamp could be connected to the Control Socket and a PC, a monitor and a printer could be connected to the Switched Sockets. They will be running when the lamp is switched on, but switching it off means all the other appliances will be automatically switched off.

Check out Flavio’s detailed description of his project, which includes a circuit diagram, parts list build instructions, and details of the circuit operation.

Warning! When the device is connected to 230Vac mains avoid touching the circuit when the mains cord is plugged in!

Circuit diagram

PCB Layout

Parts Layout


  • R1, R2: 100R 0.5W Carbon Film Resistors
  • C1: 100nF 630V Polyester Capacitor
  • D1 to D6: 1N5408 1000V 3A Diodes (See Notes)
  • D7: TIC225M 600V 8A Sensitive Gate Triac (See Notes)
  • Three 2-way std vertical terminals 5.08mm pitch (Optional)
  • A commercial trailing socket to be modified or a self-made box with several
  • sockets.

Purpose of the device

This circuit consists of a Trailing Socket (also called Extension or Distribution Socket) or similar device where two, three or more sockets (depending on the box dimensions and on constructor’s needs) will be powered when current flows in the Control Socket.

For example: if an electric drill is connected to the Control Socket, the Switched Sockets will be powered each time the electric drill is running. In this case, a lamp could be connected to a Switched Socket and will illuminate when the drill is operating.

Another example: a desk lamp could be connected to the Control Socket and a PC, a Monitor and a Printer could be connected to the Switched Sockets and will be running after the lamp is switched on. Switching off the lamp, all the above mentioned appliances will be automatically switched off.
A further application is the control of a High Fidelity chain, plugging the Power Amplifier in the Control Socket and – for example – CD Player, Tape Recorder and Tuner in the Switched Sockets. Usually, trailing sockets are placed to the rear of the appliances, often in places not easily reachable, so, even if the socket has a switch, it is much easier to switch on and off the High Fidelity chain from the main amplifier itself.

The same consideration is valid for computer-monitor-printer chains etc. Nevertheless, in this case, the use of a table lamp plugged in the Control Socket is almost mandatory, as explained below.

In fact, this very sensitive circuit works fine when appliances having full breaking switches like lamps, drills, most power amplifiers, old radios, old TV sets, fans, and almost all electrical household appliances etc. are plugged in the Control Socket. This is because these devices have a switch that fully excludes the internal circuitry from the mains.

Unfortunately, in modern devices like computers, monitors, CD players, recent radios and TV sets (usually powered by means of internal, “switching” supplies), the power switch does not completely isolate the internal circuitry from the mains, as transient suppressors and other components remain on circuit. This causes a very small current to flow across the sensing circuitry, but sufficient to trigger the output Triac.

Therefore, the switched devices will remain always on, no matter if the control appliance is on or off. This could also happen when devices connected to the mains by means of plug-in power supply adapters are used as control appliances, due to their lack of a mains switch.

In spite of this restriction, the circuit can be still useful, due to the high number and variety of devices allowing impeccable performance when they are plugged in the Control Socket.

Circuit operation

Six back-to-back power diodes are connected in series to the Control Socket. The current drawn by the device plugged into this socket when in the on state, flowing through the diode chain, causes a voltage drop of about 2V. This voltage, limited by R1, drives the Gate of the Triac D7 which, in turn, will switch on the output sockets.

C1 and R2 form a so called “Snubber network”, helping to eliminate switching transients generated by inductive loads.


• The circuit is sufficiently small to be embedded into some types of commercial trailing sockets, or a box with a number of sockets can be made at will.
• The diode types suggested in the Parts List for D1 to D6 will allow an appliance of up to about 500W power to be plugged in the Control Socket. Use BY550-800 diodes for up to 800 – 1000W.
• For less power demanding appliances, 1N4007 diodes will allow up to
200W power.
• The Triac type suggested in the Parts List for D7 will allow a total power available to the Switched Sockets of more than 1000W. If you intend to drive loads of more than 500W total, please use a suitable
• Wanting to drive less powerful loads, you can use for D7 a TIC216M
(up to 800 – 1000W) or a TIC206M (up to 500 – 600W).

Build instructions

You can mount all the parts on a printed circuit board having the dimensions and layout as shown above in full size. Or you can use the suggested layout to accommodate the parts on a pre-drilled board of about the same size with holes spaced 2.54mm. In this case, the connections of the components can be made on the underside of the board using pieces of wire (e.g. resistors, diodes and capacitors leads) cut to the right length and carefully soldered to the appropriate pins.

In any case, you must at first insert and solder the two resistors after bending their leads at 90 degrees. There is no polarity to respect, but you must pay attention to the colour code of the main three coloured bands, indicating the resistor value.

R1 and R2 have brown-black-brown bands plus a golden band indicating the tolerance (5%).

Then you should insert the Triac D7 that has three pins but, due to the particular shape of its case, can be correctly inserted with no doubts, after bending the pins at 90 degrees.

Next step will be the insertion of the six diodes D1 to D6. They must be mounted vertically, bending the Anode lead of each diode at 180 degrees as shown in the Parts Layout drawing above. Pay attention to the polarity: the coloured band indicates the Cathode.

C1 insertion is easy: its value is clearly stamped on the case and it can be inserted in any way.

At this point, six pieces of electric wire of suitable length must be used to connect the PCB to the sockets and to the mains cord of the trailing socket.

Connection to the PCB can be made by inserting and soldering directly the wires into the six lined up holes of the PCB, or by means of a 6-way terminal to which the wires can be fastened by means of screws.

This terminal can be made up by stacking three 2-way, 5.08mm pitch, std vertical terminals and then inserted and soldered in place.

The PCB can be easily encased into the trailing socket (some types allow this) or room can be gained by taking apart the (possible) power switch.

Obviously, the internal connections of the trailing socket must be modified in order to single out one (or more) socket(s) to be used as the Control Socket.

Warning! The device is connected to 230Vac mains, so some parts in the circuit board are subjected to lethal potential! Avoid touching the circuit when the mains cord is plugged in!



  1. Do let us know if you find a way to complete this, Philip. That sounds like a useful app for gadget-builders! Keeping mess to a minimum is always a good priority, isn’t it 😉

  2. Hi there,
    This circuit looks ideal for my application!
    I want to switch a vaccum on when I am using my power tools, then off when I finish.
    Of course the vac is about 1500 – 2000 Watts so I will need to switch via a realy.
    Some commercial grade vac’s have a power out socket, which is ideal, but I already have a suitable machine.
    The inteli plug mentioned above is no use for my application as there is a 5 second delay before they start, then a 5 second run on time when switched off.

  3. Alan, sorry, the maths on my last post was a bit faulty. Using a 10 ohm resistor across the diodes, at 25ma the voltage across the resistor will be 0.025×10 = 250mV as stated, but the power will only be 0.25^2/10 (V^2/R) = 6.25mW.
    When the diodes conduct they will have 3 x 0.7 = 2.1 volts and this one I got right: 2.1^2/10 = 0.44W.
    I’d still recommend at least a 1W resistor for safety.
    Sorry for the error…long time since I was at school… 😉

  4. Excellent. Thanks for the detailed feedback, David.

  5. Hi
    I had a think about this one and the fact that ANY load on the main socket will switch on the trailing sockets.
    You could overcome this by putting a resistor across the series-parallel diodes, such that at small currents (say 25 MA) the resistor would only drop <1v and would effectively “Short-circuit” the diodes. As the current increased the voltage across the resistor would increase to the point where the diodes start conducting and the diodes would then “Short-circuit” the resistor.
    10 Ohms would drop 250 mV at 25 MA (0.25 W) and when the diodes start conducting it would take 2.1v ^2 / 10 = 0.45W approx. So 10 ohms 1 watt Resistor would be OK. A 20 ohm resistor would only develop ½ the power but might partially switch on the load at lower currents.
    You’d Have to measure the current taken by standby appliances to see what is needed. 25mA @ 240V = 6 W, so consumption would have to get up to 18-20 W to have any chance of switching on the trailing loads. That should be OK.

  6. Allan, I checked with Flavio with this. He writes:
    “I re-checked the pcb layout and the circuit diagram but I found no error. The correspondent seems not aware that the pcb layout shows the underside of the board, whereas the parts layout shows the opposite side. Therefore, looking at both drawings left and right sides are reversed.
    In any case, looking at the pcb layout, the six holes at the top, corresponding to the 6-way terminal are, starting from the right:
    Neutral 230V
    Live 230V
    Control Socket Neutral
    Control Socket Live
    Switched Socket Neutral
    Switched Socket Live
    The Switched Socket’s wires can be connected to one or more sockets wired in parallel.
    Looking at the Parts Layout drawing, the connections listed before must be intended as starting from the left.”
    Hope that clarifies matters.

  7. There would appear to be something seriously wrong with the PCB layout. In the layout suggested the chain of 6 back to back diodes connect from the Control socket, (and on to the gate of the triac via R1), to the Switched Sockets, instead of between the Control Socket and Live Input.

  8. Whilst this is a good idea to simplify controlling multiple devices, there are commercial devices out there already.
    One that I have used is http://www.oneclickpower.com, they are intelligent and learn the quiescent current of devices and so can be used with computers etc.

Leave a Reply

Your email address will not be published. Required fields are marked *