For a while now I have wanted some type of current sensor for use with my Homevision home control system. Originally I wanted a device to indicate when a television was on since most remote controls just toggle the power on a TV and don't know if the TV is off or on. I tried several video sensors but I couldn't get any of them to operate reliably. I sort of lost track of the effort until recently when I need some way to track the hours on my air compressor to know when it was time for service and on my furnace to know when it was time to change filters.So I decided to design a current sensor which would be simple, reliable and easy to interface to my Homevision controller. Before I go any further I have to say that, although I have quite a bit of experience with industrial electrics, I am not an electrical engineer. All of my knowledge in this area is self taught. So I'm not guaranteeing anything and, if you decide to build any of these, you do so at your own risk. All I can say is that they work great for me.
The design uses a Coilcraft CS60-101 current trransformer which basically converts the current in a wire passing through the center of the coil into a voltage. Although this voltage is quite small, it can be amplified and then rectified to get to a useable voltage level - and that's just what I've done. Here's the circuit diagram:
The voltage created by the coil in the above drawing is amplified through the OPA340 op amp based on the ratio of R2/R1. In the drawing, R2 is the resistance between terminals 2 and 3 of the 200K pot and R1 is the resistance between terminals 1 and 2. By adjusting the pot the gain can be adjusted over a wide range. The output voltage from the amp is half wave rectified by diode D1 and smoothed out by the 100uf capacitor. Resistor R2 pulls the base of transistor Q1 to ground preventing it from conducting. When the voltage at the base of Q1 exceeds approximately 0.6 volts, the transistor will conduct and the signal from the controller, which is normally pulled up to 5 volts, will be pulled to ground and we get a change on the controller input signal which is connected to terminal 2 of TB1.