Specifications

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The LED (DSE part number Z3235) and Infrared Receiving Diode (DSE part number
Z1956) are mounted in an outer casing that mounts the motor to the gearbox. A 4mm
hole is drilled through the motor drive shaft which aligns the LED with the Receiving
Diode twice for every revolution of the motor. From this circuitry it is possible to obtain
a 5 volt drop for every time the sensor is aligned.
Figure 4.6 Single Hole odometer sensor
To obtain the 5 volt drop it was necessary to fiddle with resistor values on the 5 volt
input rail to the receiver. The resistor value was dependent on how much of the hole
aligned, the intensity of the infrared beam coming through the hole and also the
brightness of the outside light. (Incandescent light bulbs and sunlight contain masses of
Infrared.) To overcome the variance of the outside light, a film of white gap filler was
applied over the receiver to completely isolate all of the surfaces from any source of IR
other than the emitter. A guess and check of values found that when a 68 kilo ohms
resistor was placed in series in the power supply for the receiving diode, a five volt drop
(5v to 0v) was obtained when the sensor aligned. See circuit below for details.
This circuit can be used to generate an interrupt for use in the microcontroller. It is used
on the high to low edge however can be used in either edge sensitive case.
Figure 4.7: Odometer Sensor Circuitry