Current Switch

Today I will try to create a simple electronic device that will recognize if a machine is on or off by "reading" amperes on feeding wire.
The first thing we will need is a CT (current transformer). For my project I use this:

current transformer

By reading the datasheet of this CT, we can see that the load current it would be 0-30 Amperes and the output it would be 0-1 Volts.  So using an opamp I will amplify the input signal and then if the signal is bigger than zero volts I will enable an optocoupler.
First lets see the schematic and then I will try to explain it.

current switch schematic

And now step by step let me explain you the circuit. CT1 and CT2 is the AC voltage that the CT producing which is 0-1 Volts. Then I am adding a small capacitor which will remove any dc voltage from the signal. Next step is to amplify the signal, this can be done with an inverting amplifier (wikipedia: An inverting amplifier inverts and scales the input signal). The output of an inverting amplifier (see below) is:
Vout=-(Rf/Rin)*Vin, so the maximum output voltage of my circuit it would be Vout = -[(30+1)/1]*Vin. In other words I would amplify my signal 31 times.

Inverting Amplifier (Wikipedia)

After the amplification using a big capacitor as decoupling capacitor I will try to convert the AC signal to pure DC voltage that will feed the optocoupler and will give a pure DC (5V dc) output.
Thats all nothing special but useful device...


References:

1. http://en.wikipedia.org/wiki/Operational_amplifier
2. http://en.wikipedia.org/wiki/Operational_amplifier_applications
3. http://www.ghielectronics.com/downloads/Gadgeteer/Module/SCT013-030V%5B1%5D.pdf

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