Fig 1’s circuit, which was developed to monitor the traffic of bumblebees into and out of the hive, differentiates “a-to-b” motion from “b-to-a” motion. When used with an optical decoder, the circuit distinguishes clockwise from counterclockwise rotation and provides a resolution of one output pulse per quadrature cycle. The circuit is simple and inexpensive.
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| Figure 1. | Each output pulse from this circuit represents the direction of an object’s movement past the two phototransistors, Q1 and Q2. |
Q1 and Q2 are mounted so that a moving object first blocks one phototransistor, then both, then the other. Depending on the direction in which the object is moving, either IC1B or IC1D emits a negative pulse when the moving object blocks the second sensor. An object can get as far as condition 3 (see Table 1) and retreat without producing an output pulse; that is, the circuit ignores any probing or jittery motion. (If an object gets as far as condition 4, however, a retreat will produce an opposite-direction pulse.) The time constants R3C1 and R4C2 set the output-pulse width. A 100-kΩ/100-pF combination, for example, produces 10-μsec pulses. You select a value for pullup resistors R1 and R2 from the, 10- to 100-kΩ range, according to the sensitivity your application requires.
| Table 1. | Logic states for a-to-b motion (interchange a and b for b-to-a motion) |
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