Hello colleague, here I send you a simple optocoupler tester. You can use a socket to place them. It can be one of eight pins and you use those that apply. You can adjust the resistance values according to the sensitivity of the Vometer. For the 3 volts you can use two batteries of any size as the consumption is minimal. You can also add a resistor between pin 1 of the PC817 and the potentiometer cursor, so that the 3 direct volts of the battery are never applied to you. (In my case I had to put one of 1,500 ohms.) The PC817 is working fine, if when you move the potentiometer the needle of the level meter moves. I hope you find it useful. It gave me a very good result. regards (The Vumetro is a sound intensity meter that usually comes in audio amplifiers, in this case it is analog, or needle, and it is very cheap. You can also use an analog tester at the lowest scale of Miliamperes.)