Using TIMM pot with 2 pins

[anishkgt]

Thought this would be somehing beyond i can learn from. I did try reading several app notes and examples but they were just silly application that were not applicable in the real world. If it was something i am doing perhaps i could grasp it better. I was thinking of working at the AC side hence looking at peak detectors.

My project is based on ar arduino. The open voltage at the secondary is 8vpp if i remember it correctly ( at work now) and 700mV rms. How is the voltage at the said "2 stages measured" ?

[JS]

Just measure the voltage at the secondary when open, idle, when open and triggered and when idle and closed. This would determine the detection you are after and the peak handling it should have. You might not need amplification at all or even attenuation, before plugging the opamp you need to measure all of this.


JS

[ArthurDent]

I believe that the following modifications to your original circuit would work, although I haven't actually tried it. In your original circuit, as others have pointed out, the pot is just changing the current through the optocoupler but it will be either on or off and you will not be adjusting a voltage point where it turns on or off. Also there is no safe current limit to the optocoupler so it is very possible to burn out the LEDs in the optocoupler. Also note that if the clamping diodes are regular diodes they will probably clamp at 0.6 volts but the LEDs require maybe 2 volts to turn on. A couple of back to back zeners in series across the pot might work but these diodes probably aren't necessary if the current limiting resistor limits the current to a safe value.

If you connect the 3rd pin on the pot to neutral, the pot will now become a true voltage divider which is what I believe you want. Now to protect the optocoupler when the pot is set to max, choose a series resistor that will limit the max current through the LEDs to a safe value. As a guess I would think it would be 1500+ ohms but that depends on the LEDs, the value of the pot, and whether the input voltage is measured as RMS or peak.