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Here are some tips on the electronics to read the wheels.

If using parts from a ball mouse, the optical chips that read the encoders have fixed aperture size and spacing built into them. Measure the existing wheels used in the mouse to get the values needed:

  • How long are the slots? (sl)
  • How wide are they? (wsw)
  • How wide are the bars? (bsw)
  • How thick is the rim? (bm)

      With this information in hand, find the variables (in parns. above) in the top area of the postscript file and edit them. Also set the radius of the wheel you would like (wr).

      This should make a wheel compatible with the expectations of the optical readers. Notice also the orientation of the wheel slots to the chips, this must be keep the same.

      Note that the parts in the mouse will only allow for the A and the B channels, for the index channel an opto-interrupter must also be added.

      Now to the circuit, for this example it will be assumed that the power used is +5V DC, thus there are 5 connections, ground, +5v, A, B, and index.

      There will probably be two LEDs used, one for the channel A/B reader, and one for the index. As these LEDs have about a 1.1 volt drop, they can be put in series, along with a resistor of about 47 ohms, all across the 5 volt supply.

      Now for the readers, they seem to come in two flavors:

      • Simple dual NPN transistors with a common emitter connection and open collectors, no base leads. These will have 3 leads. These could also be FETs. These will require pull-up resistors on the collector leads, to +5 volts. Some 4.7k resistors should work fine, the exact value is not real important as long as they do not go below about 500 ohm. Take the output from the collector leads, this will give a TTL compatible signal.
      • More sophisticated devices with four leads, these do NOT require pull up resistors, as they are NOT open collector. These chips may do some processing to cleanup the signals. The pin outs of the ones I've seen are (as looking at the front) channel A, +5 volt, ground, channel B. The output from these chips is likely TTL compatible.

      For the index, use a stand alone interrupter module, which is likely open collector and will need a pull up resistor as above.

      These signals can now drive the input lines of a parallel port to get the signals onto a computer. EMC may help with this.