Reflective object sensor works in bright areas

Vladimir Rentyuk

EDN

When using a reflective object sensor, counting and identifying objects is sometimes difficult in the presence of electrical noise or bright ambient light. The circuit in Figure 1 shows an inexpensive solution to this problem using three independent and simultaneously working reflective object sensors. The circuit is suitable for many types of objects, but it targets use with objects such as cards.

Figure 1.	Infrared sensors and logic circuits detect the presence of an object.

The circuit uses three OPB704 reflective optical sensors with Schmitt-trigger NAND comparators IC_1A, IC_1B, and IC_1C on each output. IC_1D functions as a clock generator, and counter IC_6B functions as a divide-by-eight counter that divides the clock frequency. That signal drives IC_4D, which acts as a buffer to drive transistor Q₁.

To understand how the circuit works, consider Sensor 2. IC_1B’s output will be low if the sensor’s phototransistor doesn’t detect IR rays reflected from an object. Both of IC_1B’s inputs are high; therefore, the D1 input of IC₂ is low. In any case, if the sensor’s phototransistor detects IR rays reflected from an object, the D1 input of IC₂ is high. The level corresponding to the current situation transfers through IC₂’s Q1 output (Pin 7) by a write signal on the C input (Pin 9). The write signal is a leading edge of pulses from the clock generator. The signal from divider IC_6B becomes the D3 input of IC₂. A level of the divided clock signal transfers to IC₂’s Q3 output (Pin 15) upon receiving a write signal from the C input (Pin 9).

The signals on the Q1 and Q3 outputs have equal duration except when the sensor’s phototransistor detects IR rays reflected from an object. Figure 2a shows the process of this normalization. Exclusive-OR gate IC_3B compares the Q1 and Q3 outputs from IC₂. If they have the same logic level and duration, then IC_3B’s Pin 6 is low, and IC_3B generates pulse signals. If signals from outputs Q1 and Q3 on IC₂ are unequal, you must reset counter IC_5B’s reset signal, and its output 2Q2 at OUT₂ is low. The Q2 outputs of counters IC_5A, IC_5B, and IC_6A are low whenever the input signals of comparator circuits IC_3A, IC_3B, and IC_3C are unequal. This situation occurs if Sensor 2 doesn’t detect an object or receive any external signals – for example, IR noise from fluorescent lamps or interfering ambient light, alternating light, or flashes.

Figure 2.	Traces show the process of normalization (a) and three cases of using the presented device (b, c, and d).

The outputs of IC_3A, IC_3B, and IC_3C are equal only when all phototransistors detect a signal from their respective IR emitting diodes – that is, when a card is presented in front of Sensor 2 (Figure 2b). You must choose a clock frequency with regard to a delay time of the system. A leading edge triggers IC₂, a 74HC175, and a falling edge triggers IC_6B, a 74HC393. Because of the counters, this system automatically adjusts itself after any changes of frequency in its clock generator.

Thus, if counter IC_5B does not have a reset signal during a period equal to four periods of a reference signal, its output (Pin 9) is high, and the counter latches through R₈. The logic-high level appears on OUT₂ until you remove the card. In this case, the detected inequality signal from the sensor with the reference signal and the counter, IC_5B, causes a reset signal. Figures 2b, 2c, and 2d show three cases of using the presented device.

Figure 2b shows a case of normal operation. You can see the results of comparing a reference signal (Trace C) and a signal of IC_1B’s output (Trace D). The signal of IC_1B’s output (Trace B) is low when no card appears. When the card enters the zone of vision of a sensor (Trace B), it is a sequence of normalized pulses. The output of the device at Pin 9 of IC_5B (Trace D) changes its level from low to high after four cycles of both signals, but it will immediately change to low if you remove the card.

Figure 2c shows operation of the device under strong IR noise. The signal of IC_1B’s output (Trace B) contains some high-frequency signals if a card isn’t present and is a sequence of normalized pulses when a card is present. The output of the device at Pin 9 of IC_5B (Trace D) indicates the presence of a card by changing its level from low to high after four cycles of these signals. It immediately changes to low if you remove the card from the zone.

Figure 2d shows operation of the device under ambient direct lighting. You can see the results of comparing signals. In this case, the signal at IC_1B’s output (Trace B) is constant high when a card isn’t present. When the card enters a sensor’s zone of vision (Trace B), the signal is a sequence of normalized pulses. The output of the device at Pin 9 of IC_5B (Trace D) indicates this condition by changing its level from low to high after four cycles of both signals. It immediately changes from high to low when you remove the card from the zone.

Capacitors C₁, C₂, and C₃ are optional. They protect input circuits from electromagnetic noise when, for example, long wires connect the sensors and the device. Capacitors C₉, C₁₀, and C₁₁ provide performance reliability by protecting the counters from short pulses.