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'[EE]: appliance current sensor'
2005\08\20@200656 by Marcel Birthelmer

Hello all;
random project time. This is the situation: of the three people I'm
sharing a house with, I'm the only one whose room is not adjoined to the
garage. This means that I can never hear when the washer or dryer are
done, and so my laundry scheduling is always suboptimal. Now of course I
could just set my watch or whatever, but where's the fun in that. One
additional criterium that will come in is that these are not my
appliances, so I can't open them. Now, I was thinking that it should be
possible to pick up the current draw of those appliances and then, via a
small embedded system or maybe an old computer, forward this information
to the local network or something. However, I'm not sure it's such a
good idea for me to be screwing with high-voltage high-current lines. Do
any of you have experience with a similar project?
After some initial investigation I'm guessing a hall sensor would be
best, but I'm not sure which of the wire strands it would wrap around
and so forth.
Any input would be appreciated, both about the actual current sensing as
well as the delivery mechanism from the garage (no cat5 drop; wireless?)
to my room (my computer or otherwise).
For extra credit, Rube Goldberg Machines are also welcome.
- Marcel

2005\08\20@211655 by Timothy J. Weber

Marcel Birthelmer wrote:
> This means that I can never hear when the washer or dryer are
> done, and so my laundry scheduling is always suboptimal.

Baby monitors are pretty cheap...  I use mine to monitor my washing from
other floors of the house all the time!

Or if you want something more fun, make a low-power RF transmitter bug
and tune into it with an FM radio.
Timothy J. Weber       

2005\08\20@214122 by Mike Harrison

On Sat, 20 Aug 2005 17:08:59 -0700, you wrote:

{Quote hidden}

A neat solution is to use a linear GMR sensor - NVE make some. These are more sensitive than hall
devices can easily sense the sort of current you are looking at, without needing to split the 3 core
mains cable - the field is sufficiently assymetrical to get a decent signal if the sensor is clipped
onto the outside of the cable  - you may need to slide it along the cable to get the best signal due
to the core twist. From memory a small amount of gain (10x) plus a data-slicer style comparator will
give you a nice 50/60hz signal when current is flowing and none when off.

2005\08\22@070246 by Howard Winter

picon face

I don't want to spoil the fun by suggesting an off-the-shelf solution, but over here you can get multi-way
socket strips where one (the master) is sensed for current flow, and the others are only switched on when the
master is in use.  They're intended for computer use, where the computer itself plugs into the master and the
monitor, printer, modem, speakers etc. are plugged into the others, so turning off the PC itself turns off the

So  you could just run a small table-lamp or similar from one of the slave sockets - it will be on while the
machine is running, off when it stops.


Howard Winter
St.Albans, England

2005\08\22@153557 by Peter

picon face

My turn to chip in: I once made an operation sensor for a high power
heater (~15kW). I was not allowed to connect to the circuit in any way.
Including not open boxes to get at the cables etc. So I used a
differential thermistor sensor with an opamp. One thermistor was on the
board and the other was pvc-taped to the high gauge conductor that fed
the heater control unit (3 phase). The heater cable would run hot.
Voila. Inertia was half a minute give or take a little, the cable was
2/3 inches external diameter PVC 5 wires (3ph + gnd). It would heat up
2-3 degrees above ambient with the heater on. That gives more than 2% R
change with any thermistor and is easy to detect.

Another thing I tried was to use a cassette tape playback head and
preamplifier. The head would be brought in contact with the cable. The
output ac would be huge when the load was on. The tape head picks up the
stray field near the cable. The direction and orientation of the head
were important but not that important. The output ratio with/without
load was 1:100 or larger.


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