Basically the idea is that the photo interrupter shoots a beam of IR light from an emitter into a collector:
When the beam is intact the output current is based on VCC being supplied and the resistor value the photo interrupter has, when the beam is broken the output is pulled from ground so no current is applied.
Now this leads into an current to voltage amplifier configuration: http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/opampvar2.html#c2
Basically this converts the current back into a voltage, the voltage is fed into the panel meter. When the panel reader reads a higher voltage it starts to creep up, when it reads a lower voltage it starts to creep down. The idea is that based on the output current by the photo diode the beam can be controlled.
My question is this: In the video he describes that when the beam is broken the output [voltage] of the amplifier is increased. So when the weight is put on it, which brings the beam down, the beam is broken which ends up increasing voltage output from the amplifier which when fed into the beam pulls it up. How does this work, since the current to voltage amplifier is based on the current wouldn't breaking the beam, which makes the current output 0, mean that there would be no current to amplify and thus no voltage so the beam should fall?
Likewise when the beam is not being blocked at all I'd imagine that the amplifier would reduce the voltage and bring the beam back down, but I'm just confused why this seems reversed logically. When the unblocked beam should give a higher voltage and a blocked beam should give none.
Also anyone know the difference between a typical 0-5v panel meter and one that's in the mv range? Is it just a smaller resistance in the coil? Since I see one that measures from 0-5v or 0-30v costing $5 while one that measures millivolts costs $40+.