How Green Is My Green Electronics ? Now With Improved Threshold Detection !




Are electronic devices really in a very low power mode when on stand-by ?
Also, how much energy am I wasting for powering devices when not in use ?

I wanted to check it out and decided to build a device capable to detect stand-by mode of my electronic devices and start metering energy used. All this had to be done automatically without my intervention. Metering would start upon entering stand-by and stopped upon exiting.
To do so I needed an energy meter and a current gauge to monitor the current of mains supplied devices and start/stop metering.
For the meter I first looked for silicon, of course. Energy metering is a hot issue with a lot of chip makers and many of them supply cheap solutions.
The project was proceeding when recently I was lucky enough to find two electromechanical meters at a local special trash collecting center (electrical, white goods, furniture).
Actually there should be a plenty of these scrap meters available here as the local electric company is replacing these meters with remotely controlled electronic meters.
I took them along with two VCRs and a printer. The first meter I opened and dismantled to satisfy my inner primary need. The second meter I decided to use in place of the silicon-based one; also, the ready made electromechanical meter solved the calibration issue.
This PopSci contest made me hurry and change priorities in my to-do-list, so here is my design.
Schematic V0.2 is an improved version of the electronic control box over the previous one. I added a potentiometer to set the hysteresis level. This helps discriminate power on vs. stand-by for noisy power supplies like some switchers are. This also helps get firmer metering on/off states.
The schematic shows in red the differences with respect to previuos version.

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Step 1: Caution, Safety First !

Before starting, you must be absolutely aware that this thing is powered from the mains and
as such it could kill you, cause damage or injuries. If you are not really skilled at mains
powered electronics and related safety building practice and are not well aware of the risk related, you are suggested to have a friend help you with this project.
Also, as a general rule, when you are working on dangerous things always have someone next to you instructed on what to do if something goes unexpectedly.
Most parts of the circuit should not be considered safe to touch when the circuit is powered on.
Keep low voltage and high voltage wiring as separate as possible. The relay is the point where the two worlds are closer. Choose a good relay and have the wires soldered firmly. Tape well and possibly use heat-shrink tube.
These notes are not just to scare or bother anyone, but I absolutely want that fun does not turn
into grief.

Step 2: How It Works. Electromechanical Meter

Electromechanical meters are described very clearly here.
The key point: metering is based on the product of two electrical entities, current I and
voltage V; power is the product of these two entities, V and I. Energy is calculated integrating
over time (that is adding together time after time) the V*I products. It is energy what we are
paying for.
Within electromechanical meters V and I feed two coils: the voltage coil (which is in parallel to
the line) requiring low power and a current coil (in series with the line) made of thick copper wire.
Metering can be stopped or started interrupting the connection to the voltage coil.

Step 3: How It Works. the Current Monitor

The current required by the electronic devices under test is monitored through a current transformer.
I made it out of a regular wall adapter: I removed the low voltage secondary winding and replaced it with 4 turn of paired 1.5 square mm insulated wire. Current flowing through the new winding produces a voltage across the untouched original 110Vac side of the transformer. The higher the current, the higher the voltage.
The untouched 110Vac primary is then connected to a filter stage, a variable gain amplifier
a full-wave rectifier, a peak detector and a threshold detector feeding a transistor and a low
power relay. The contacts of the relay are in series with the voltage coil of the meter.
The gain of the amplifier stage is regulated through a potentiometer to set the threshold stand-by / operational.
You can find the detailed schematic down here.
All the parts are pretty common and most of them can be salvaged from other electronic devices.
Almost all the electronic components I took from a VCR and an automatic IR night light.

Step 4: Putting It All Together

I had a piece of wood cut to measure to make a stand the size of 25x25x8 cm.
Attached to it the meter, an avory-white plastic box containig the input and output cabling and the
current transformer. Finally attached a gray plastic box with all the electronics. The relay I put
close the wiring points of the meter. This also adds to safety.
The wiring runs at the back of the stand. I dremel-led the paths for the wires in the wood and kept the wires in place with duct tape. Next step will be to add a 3-4 mm thick piece of plywood to cover everything.
Now, check the wiring again, make sure you did not connect in parallel anything supposed to be in series, this applies expecially to the meter.
Rotate both gain and hysteresis potentiometers counterclockwise (minimum resistance, i.e. minimum gain and wider hysteresis window).
Connect the electronics you want to test. Plug the meter.

1.) Turn off your electronics. Rotate the hysteresis potentiometer clockwise until the relay energizes (and the LED turns on) and the meter wheel starts rotating. if nothing happens rotate the hysteresis potentiometer clockwise until the relay energizes.
2.) Turn your electronics on and the relay should de-energize. If nothing happens, turn the gain potentiometer a little further clockwise (higher gain).

Repeat the 2 steps above again to trim.

Should you find difficult or impossible to set the threshold, likely the stand-by mode current is close to the operating mode. This means your equipment consumes almost the same regardless powered on or in stand-by.
Once you know the energy (in Watts per hour) used by your equipment, you may also want to know the average power dissipated by your electronic device. This can be done if time is counted along with the energy. That is, if 10 Wh were metered in 100 hours, an average of 10/100 = 0.1W is the power dissipated by the equipment. For this purpouse a battery operated wall clock timepiece can be connected at the points +/-1.5V in the schematic.
The timepiece will be powered (and then advance) only when the relay is on.

Step 5: Final Notes

If you just want to have an idea of the stand-by current with relation to operational power, you may just want to build the current transformer and connect the filtering resistor and capacitor. A multimeter at the RC filter gives readings in the range of tens of mVolt to a few Volt. Values depend on the transformer used.

You will find that a number of electronic devices do not have a real stand-by mode. Rather they turn a red LED to green and respond to remote control commands, but they are fully powered on when on stand-by as well. This consumes a lot of energy and you can have the feel touching them: they are always warm (quite warm, sometimes !).

I have my home entertaining system connected to a switched outlet I switch on only when we are going to use it, otherwise my 5.1 system would be always hot.

I am not sure that this will cut bills and solve related problems, as shown in the drawing Michele made a few moments ago, but I am absolutely sure that resources we save today will be handy in the future to Michele and his brother.

Ciao a tutti



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    71 Discussions


    How much does it really matter anyway when our clothes dryers, toasters, cook-stoves, ovens, refrigerators, irons, blow dryers, and air conditioners all draw 1 KW or more (mostly more)?
    Good instructable on building an AC current meter, though.

    green is a red herring. there is nothing green about electronics. I propose a title correction! How energy efficient are my electronics projects?


    11 years ago on Introduction

    I understand, the device requires too much work compared to (possibly cheaper) digital devices on sale ! I would never make it either (again !).
    It was a sort of divertissement.
    The plugs here are very safe as it is practically impossible to come in contact with the live metallic part while plugging in. Also, the sockets in the wall require that all three contacts enter the socket at the same time thus preventing young experimenters from putting wrong things in the socket.
    The female (right) can accept 16A larger holes and 10A (smaller holes) plugs. Ground is center and there's no polarity (Line and Neutral can be exchanged). We run on 50Hz while US and some other countries run on 60Hz.
    As we run on 220Vac, for a given power requirement we use smaller cords (less Amps are required).
    Hola !

    13 replies

    Reply 11 years ago on Introduction

    That does seem a lot safer than our American outlets, Because like sometimes if you are plugging in an electronic device and your hand slips you will get a nasty shock.


    Reply 11 years ago on Introduction

    Davvik no offense, but you are seriously mistaken. It make look safer (and cooler) but really it is no different, it is almost impossible to shock yourself if your "hand slips". It is safe by design. I am a certified electrician and I have worked in electric work shops for 20 years and I have never, ever seen or heard of someone electrocuting themselves with improper use of an "American" style plug.

    Bad Maxxgormly

    Reply 9 years ago on Introduction

    WOW! I beg to differ, no offense intended but:

    "CPSC document number 524 Electrical Receptacle Outlets" "Electrical receptacle outlets in walls and floors may present shock and electrical fire hazards to consumers. The U.S. Consumer Product Safety Commission estimates that 3,900 injuries associated with electrical receptacle outlets are treated in hospital emergency rooms each year. Approximately a third of these injuries occur when young children insert metal objects, such as hair pins and keys, into the outlet, resulting in electric shock or burn injuries to the hand or finger. CPSC also estimates that electric receptacles are involved in 5,300 fires annually which claim 40 lives and injured 110 consumers........"

    So if only 1/3 of the 3900 annual injuries received from electrical receptacle outlets are children, it is fairly safe to assume the remaining 2,601 shock receivers would indeed be adults using an outlet and through some type of mishap were indeed shocked, if not worse. 

     I know in my lifetime I have received two electrical shocks from "slipping" down to the metal prongs while plugging into an electrical outlet, and I know of other people who also have done so, I only mention this as none of us ever reported these incidents to anyone! This despite the first incident knocking me unconscious while plugging in a lamp while reaching behind the couch to do so.


    Reply 11 years ago on Introduction

    Arr, when I plug in our CRT TV into a multi board sometimes I notice a bright white/blue flash under the plug. I have also noticed this while plugging in a light to a completely different multi board in a different room.


    Reply 9 years ago on Introduction

    An arc is most common when unplugging a device that's turn on, but not unheard of when plugging the same in.


    Reply 11 years ago on Introduction

    Oops ! I notice your comment quite late and thanks to Derinsleep reply...
    Sorry for overlooking.
    Anyways, the blue flash you noticed is pretty common and is due to a very interesting (geek-ly speaking) physical law, the lenz' law where when an inductive circuit like those with motors or large transformers (like the ones in TVs) or some large light bulbs is connected or disconnected from power a large voltage is generated with the white/blue spike you saw.
    Physically speaking, the magnetic field generated by the inductive circuit (the motor, the bulb, the transformer) opposes to changes when the circuit is opened or closed generating a reverse voltage such that the current associated to the voltage and the circuit will be such that the flux it creates opposes the change in flux that created it ( quoting wikipedia in part)
    Hope this helped


    Reply 11 years ago on Introduction

    arcing,not that important,I just kick the power bar and my TV works again!


    Reply 11 years ago on Introduction

    I say American but I meant North American, I'm from Canada, and I've never really electrocuted myself, I've just shocked myself because I wasn't paying attention when plugging something in. and a lot of my outlets are broken and very unsafe, so it might just be because my house is old.


    Reply 11 years ago on Introduction

    Hmmm,they don't need the center pin since I can plug my laptop that has the other euro plug into the italy outlet. Italy is awesome,by the way.


    Reply 11 years ago on Introduction

    Ciao D., sorry for not replying earlier but I was on a trekking vacation and couldn't do any Internet stuff. Yes, live and neutral on Schuco plugs are the ones protruding out of the plug handle but plugging it into a regular Italian plug would leave the safety earth unconnected. Also, the diameter of Schuco pins is larger than the italian ones. Oh yes, Italy is awesome as many other countries around the world. Thank you ! BTW, I've just noticed that 'Italy' is not in the Instructbles spelling checker ! :-)


    Reply 11 years ago on Introduction

    That is true for sure,but I saw outlets that can accept both plugs with the earth always connected.


    10 years ago on Introduction

    all of this for a power meter? a hacked GFCI (ground fault circuit interrupter) outlet would give you better results. GFCI outlets work by measuring how much energy goes in and how much comes out, when there not the same it trips.