Testing the Difference Amplifier

The difference amplifier compares the signals from the two optodetectors, and amplifies the difference between them. How can you tell it's working? To read about the difference amplifier go here.

See if it's Working

There's not a large number of things to go wrong, but here are some things to check.

• We can't really get the third op amp to do much until the first two are detecting light changes successfully. So you've already checked out both opto-detectors and they're working, right?
• Measure the unblocked signal at pin 6 of this op-amp.
  This means you put both opto-detectors have been checked out and working. Turn on the power. Don't put any object in the light path, we're measuring the unblocked signal. Pin 6 should be 12 to 15 volts above ground.
• Measure the blocked signal at pin 6 of this op-amp.
  This means you carefully place something like your fingertip in the light beam, exactly where the suspended object should be. Be careful that you don't block the light path to the reference detector. Pin 6 should be between -5 and -15 volts.

With the above behavior, you have a sensitive comparator that detects whether the light beam is blocked or not.

Now the hardest parts are done! All that remains is a final stage of amplification, and the power control transistor.