Alan Erzinger
EDN
Most object-sensing systemshave problems detecting the presence of an object. The system in Figure 1 uses an oscillator to ease detection problems and allow sorting. The oscillator reduces power dissipation in the photodiode by operating the diode at 50% duty cycle. The oscillator also enables a 50% increase in the noise-filter time constant, and it functions as a timebase to allow object sorting. The oscillator chops the photodiode's bias. The signal the photodetector receives is a square wave; thus, a filter can remove optical noise. You normally need filtering when light shines through fans onto the optical detector. When required, you can place a bandpass filter in series with Q3's output.
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| Figure 1. | An oscillator circuit allows a photodetector to both count and sort objects according to size. | |
The oscillator frequency has two limits: the response time of the phototransistor and the accuracy of the object-sensing system. Q4 and Q3 are connected in a cascode configuration to minimize the Miller effect in Q4. This connection reduces the pair's optical transient response to nanoseconds, thus allowing oscillator periods of submicroseconds. Objects on a conveyor belt travel at relatively low speeds. You can calculate their expected time in front of the photodetector, t, from
where s is the conveyor speed in feet per second and d is the object width in feet.
An object 2 in. long with a belt speed of 3.5 ft/sec blocks the detector for 47.6 msec. If the oscillator period is 250 µsec, the object blocks the detector for approximately 200 periods, so each period equates to 0.5% length accuracy. The system senses two objects – one 2 in. long and one much longer – so 0.5% accuracy is more than adequate. When the detector is unblocked, the inverting input of IC1 is dominant, and it keeps the output of IC1 low. The low state prevents the oscillator's output from reaching the counter (IC3). Blocking the detector allows C3 to charge to 5 V through R6, and the noninverting input of IC1 becomes dominant, starting the count. When the detector is unblocked, the one-shot comprising R11, C5, and IC4A pulses IC2B with an end-of-object pulse. The one-shot's trailing edge triggers the counter-reset one-shot comprising R12, C6, and IC4C.
C4 and R6 form a nuisance filter that rejects short optical noise. The timing is such to enable a 2-in. object to clear the detector while both Q6 and Q7 are high. When the end-of-object pulse coincides with Q6 high, Q7 high, and Q8 low, the output of IC2B indicates a 2-in. object. This situation is the only time window that can indicate a 2-in. object. If the object is longer than 2 in., Q8 goes high, indicating a large object. When the object clears the detector, the reset one-shot resets the counter for another cycle, and Q3 quickly discharges C4 in preparation for another cycle. With the component values shown, the system can sense and discriminate between objects as short as 2 in., separated by 0.1 in.
