Wanted, An inexpensive current sensor with logic level output.

A Quest for KISS

Current Sensor on my GE Water Heater
Current Sensor on my GE Water Heater















I will never forget the lesson Don Colvin, a Designer and Manufacturer of Electronic Devices  taught me. It was his collection of products designed to restrict unauthorized toll calls, which was a note worthy problem of the day.

Some of the devices Don had purchased for study looked like full blown micro computers of the day with specialized tone decoder boards hanging off the bus using phase lock loops ICs and other neat and expensive stuff. A power supply and steady state fan, blinking lights, and a terminal and keyboard to program it..

At the end of looking at more than a half dozen solutions, Don produced his own product, it was a circuit board just a little bigger than a 20 pin PIC processor with a few external components, and it was telephone line powered!  Furthermore, you could program it right from the phone, and hide it neatly inside a phone jack, or near anywhere you wanted to put it.

How could I ever forget!  It was a lesson as memorable as getting into the high voltage section of an old TV set! Don had spent his time researching how to build a KISS product. He expanded the market to folks who wanted to protect a single phone in their apartment!

Certainly you understand if you can make a product with the same functionality for one dollar that your competitor makes for $100, your competitor will soon realize he has a large marketing  problem.

So today, I’m spending time researching a way to detect whether I have about 4-20 amps of current, (more or less is good). 120-240 VAC 50-60hz  flowing on typical insulated 10-12 gauge wire with a motor or pure resistance load at the far end.

All I need is a yes or no.. and I don’t even need a quick yes or no. certainly it would be nice if we could attach our inexpensive current sensor without lifting a wire from a terminal block, but we can’t have everything.. or can we?

So of course, I Google (logic level current sensor) and similar, I finally arrive at this page:


I look at the unit price even at 1000 units, and I ask… are you kidding?

And here’s that neat little led on my water heater, one for the top element, one for the bottom element and powered directly from that mini transformer!





Yes, that’s an LED, and if we can light this led directly from that little coil, there’s little doubt we can use an opto isolator and give ourselves a clean logic level change into the input pin of a PIC, or maybe an 8 pin Atmel?

You might ask why we bother?  My answer is we are unwittingly doing all we can to raise the price of energy, and we’ll need smarter products. WE need to know when they are mal functioning early as possible.

Consider this water heater, what if we took the design one step further and warned when one of the elements was burned out?

There are other things far more difficult to detect than a water heater we normally have good access to.., we could  use a tiny micro to monitor patterns of use and alarm when anomalies pattern.

If you have an idea or source, please let me know, no way GE paid more than a dollar or two for these, and it sur would be fun to find some at surplus prices..

Added Note: Thanks for reading, scroll down for comments, in wordpress, you may see a truncated version of this story, and you may need to select the story from the menu bar on the right to see the comments. attached to this article are some great contributions under comments:




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10 Responses to Wanted, An inexpensive current sensor with logic level output.

  1. Dave says:

    The answer is yes: that circuit is about as simple (KISS) as it gets: current transformer output placed across an LED (there may be other devices to limit the current through the LED and the voltage output of the current transformer). Sub the large red LED for an opto-isolator like this one and you’ll have yourself a 120 Hz pulse output that can drive a PIC. Place a potentiometer in parallel with the LED of the opto-isolator and you can roughly control the minimum current required to make it fire. To save on the current transformer, the most expensive item in the combo, make one yourself.

    • George B. says:

      no doubt the solution is simple, and that’s exactly what we want.. the link I supplied, (I hope it worked for you) takes you to digikey where thye want over $10 for the transformer and led! I’d need to add an opto isolater as you suggest.. and that would likely produce costs too high for the market I’m after. As for driving the PIC, there’s AC present, so I’ll likely use a wall wart. The current sensor need be simple and inexpensive.. just a logic 1 or zero to a pin.. I was hoping to hit a price of about $4 in quantities of 1000 parts.. like I say.. 120 volts at 4 or more amps, if I can get a reliable logic level change there, it’s all I need.. I’m blown away this cheap part is not on the shelf.. I’m sure GE didn’t pay much for their monitor led driven directly from that coil..

    • George B. says:

      Great find, this is less money than the digikey part and we don’t need to lift a connection to add the current sensor. This is overkill compared to what we need, and a testament that we should be able to buy the more simple part for less.
      I ordered a few of these to experiment with,, and perhaps I’ll get started building a few appliance/equipment monitors. There is one type of failure I’ve been studying for years, and it’s one that cascades like dominions. Some problems caught early can be corrected with a simple adjustment, but typically they go unnoticed and create repair bills in the thousands of dollars.
      We see more application of monitors when death is highly likely with a failure.. an example might be a metal chip detector in the gear box of a helicopter. In other gear boxes, the first alarm might be smoke, fire, or the fact that the input shaft and the output shaft no longer have a close relationship. I’ve seen this with differentials that leaked lubricants from a seal, it took a while for the failure to happen, but when it did, the temps were way high enough to cook dinner on top of the differential. Now days, many trucks have monitors on their gear boxes and differentials..
      Of course the application of monitors will have a lot to do with cost, and perhaps your past experience. If you just replaced equipment with a $3000 repair job, you might spend a few more dollars for a monitor compared to the guy that hasn’t seen the failure yet.
      Where does a guy draw the line? The answer has a lot to do with cost and convenience of adding more protection.

  2. bob g says:


    back when i was working with the bs2 stamp processor i too was looking of a current sensor that could provide a logic yes or no output.

    my solution was to rob a little current transformer from a circuit board from a deceased apc ups system. it had two such current transformers, little buggers you could get a 10 gauge wire through with some effort.

    i connected it to drive a cheap little npn transistor which switched the logic for the pin of the bs2. by changing a resistor value i could fairly accurately adjust the logic “yes or no” (whats that? 1’s and 0’s?) anyway you get the idea.

    somewhere in my collection of crap from the move i have a few of these little jewels, if you need one i will see what i can find.

    being able to monitor a line for current, such as a motor running or a pump is something very useful when it comes to automation.

    about as kiss as i know of, and the price is right too!

    bob g

  3. George B. says:

    The key is finding the lowest cost part.. the whole project needs to cost about $10 to make it worth marketing.. I figure the sensor with an opto isolator might be half the cost.. best include the opto for safety..

  4. Erik Homme says:

    Something like this should work as a cheap current transformer. I don’t know if this one has the right number of turns for what you’re doing; if it has too many turns, you could unwrap some of them.
    By the way, you could also power your circuit from such a device and skip the wall wart. With a big enough capacitor, a PIC could run for hours with no AC power.

    • George B. says:

      Thanks Erik!
      I ordered some of these to test, it’s my intent to update this page with the results, this is certainly the right price! Amazing what better values you two found over my searches.

      Just to shed additional light on Erik’s comment, a PIC can run at 20mhz clock speed with the power it takes to keep an LED lighted.

  5. Gene W. says:

    George B.,

    Yes, that little LED element current sensor is an admirable KISS solution, but GE doesn’t think it pays its way. My SE50M12AA01 water heater had one on each element, and I valued them. It not only shows that the element is powered up, but that it is actually drawing power (i.e., is not burned out). The new (Dec 2011) GE SE50M12AAH does NOT have them, so I just switched them over.

    These parts are not trackable through GE or parts distributors, so a source site is handy.

    • George B. says:

      Gene, I value your post! It’s a great point you make, if that LED is not glowing AND the thermal switch is closed to provide power to the element, it’s likely a burned out Elemnet. The service life of the LED and transformer will likely be many times longer than the electrical heating element for sure.

      Another add, this little transformer is specifically designed to drive this LED at the current the element draws, it’s a custom part, and we’re not likely to find a part at radio shack as a substitute. I’d love to find one, as we know we could turn on an opto transistor with same. No part mentioned yet will drive an LED directly like this part does 🙂 Nice feature GE engineers, at least a hand full of us appreciate what you did!

      No doubt.. we can both see the argument between the engineers, marketers, and bean counters.

      The bean counters likely pointed out the reality of the situation.. “MPAI, they’ll have no clue we offered a handy and valuable trouble shooting tool that could save them a lot of time and money in a repair of the water heater, they’ll buy from the competition at $2 less”. “Most People are Idiots”, we tolerate our Social Security Payments going into the general fund where it is robbed at will.. isn’t that proof enough?

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