Introduction: Master-Slave Current Sensor

Picture of Master-Slave Current Sensor

Here is a circuit I designed to be part of any number of projects. It is used to detect when an AC current is flowing and send a 5 VDC signal to another piece of equipment. It is a simple circuit that can be built with as little as five components.

There are many uses for the sensor. When used with a comparator, Schmitt trigger or optocoupler and a separate power supply, it can drive higher amp dc loads. It can also signal microcontrollers such as an Arduino, giving you even more possibilities.

For example, I am working on a project where I would like a shop vac to turn on as soon as i press the trigger on my mitre saw. when the trigger is released, I would like it to run for 5-10 seconds longer to clear all the dust. Another possible use would be to turn on a bathroom fan when the light is turned on, and off after a preset amount of time.

I can think of a few ideas, I'll leave a page near the end just for that purpose.

Step 1: Parts and Assembly.

Picture of Parts and Assembly.

Parts needed:

1 Current transformer.

1 Full wave Bridge Rectifier or 4 diodes ( I used 4, 1N4007s)

1 Electrolytic Capacitor, 10uF

1 Zener diode, 5.1V (use any voltage desired, this determines the HIGH signal voltage)

1 1/4W resistor, the value should be high. Although it is a balance between switch time and sensitivity. Too high and the circuit may not turn off very quickly. Too low and the circuit may not sense a current at all. (I used a 10k, but on some low loads, the current was not high enough to trigger. maybe a 20k would be better)

Assembly is pretty straight forward. Just be sure to keep the correct polarity on the capacitor and diodes.

Look at the photos for a second schematic. It describes the use of a trim pot instead of a pull-down resistor. I haven't tested that circuit but if my theory is correct, you should be able to adjust the sensitivity with the trim pot.

To use the circuit, just feed the black (hot) wire on the AC circuit through the CT and voila, current sensing.

Step 2: Alternative Ideas for Project

In the industry as well as some projects you may want to use a timer of some sort. In many cases, you have to buy one specific to the need. for instance, when dealing with banks of motors, you do not want all of them to start at the same time. if that were to happen, it could overload the wire and cause damage. Using this with a microcontroller you could delay the start of some motors to ease up on the whole system.

In another case, you may not want two things running at the same time. maybe you have two water pumps, a main one and a standby. While the main one is on, the standby is off, and vice versa.

Have a piece of equipment you want to monitor the runtime of. you could have it so that it will automatically shut down after a pre-determined amount of time, sound an alarm, trigger a separate system or any combination thereof.

This circuit coupled with an Arduino, and a few modules can be used for a limitless amount of projects.


Nobilis (author)2017-05-18

Nice, simple design. Some ideas:

- From a dead CFL lamp one can recycle the small transformer. It is for free, small and it's capacity is more than enough to serve in a sensor cirtuit like this.

- I would use this circuit in my home alarm system as a mains sensor. Alarm systems depend on mains power, their battery typically can last for just a few hours, and a mains outage may predicts and intrusion.

- This design also could be useful in DIY emergency exit lights which are automatically lights on in a power outage.

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