Compensating for variations and degradation in optoelectronic sensors

Optoelectronic switches consist of an infrared LED transmitting light across an air gap slot to a detector (phototransistor). But an LED’s light emission output degrades over time as the device is used and powered. As the LED emissions decrease with the same forward current drive, the output signal degrades.

OPTEK offers a pre-configured version of an auto-calibration circuit in the form of a design kit (above), OCB-100-Kit, and is available with several different types of sensors that can be used in setting up the circuit for a particular application.

As an LED’s temperature increases, it becomes less efficient and its output is reduced. However, on the other side of the device, the photo detectors sensitivity may increase with elevated temperature. Thus, while there is a partial offsetting effect with increased temperature, the actual variations can cause the output current to increase or decrease.

Variations in a system’s power supply can also create variances in the performance for an optoelectronic sensor. As the power supply output varies due to temperature, load and aging factors, the drive current for the LED may change as well as the output current of the phototransistor.

So what can be done to mitigate all these influences and have an operational sensor that performs exceptionally well?

An auto-calibration circuit can be added to enable system calibration at any point, without the need for external computer simulations or complex calculations. This could be done during any part of the manufacturing process or the system’s life-cycle, and would extend the product’s lifespan and increase its reliability. The circuit should also be designed to be able to store the calibration settings and initial set-up information.

The auto-calibration circuit can automatically change the LED forward current, which in turn will cause the phototransistor to reach its pre-set output level. This information can be stored by the system for later use, even if power is lost. This circuit can be used to calibrate either reflective or slotted sensors, provide a consistent output and eliminate the need to confirm the LED’s drive resistance or the phototransistor’s load resistor in order to maintain a consistent output steady state condition. Stored in a PIC microcontroller, the auto-calibration program routine could be initiated by momentarily grounding the "calibrate" pin.

Each time the system is calibrated, design engineers can narrow the expected startup output state of the device and ensure the device operates using that startup state over its lifetime, thus enhancing the reliability and consistency of the system.

The auto-calibration design circuit contains a series of shorting pins that enable users to change the phototransistor’s load resistor value from approximately 2.5 to 27 kohms by arranging the shorting bar to the appropriate location to increase the sensitivity of the device.

The amount of light radiated is adjusted by the forward current of the LED using a limiting resistor control (right). This current typically ranges from 5 to 20 mA.

An "analog out" pin allows for any reference point to be set by the design engineer to recognize an event change in the sensor. The logic output pins will change state once the preset LED light condition is reached, as set by the values of on board resistors. When the optical signal increases above two-thirds of VCC, the "logic out A" pin switches state; and when the optical signal decreases below one-third of VCC, the "logic out B" switches state.
Mark Miller is Product Engineering Manager for TT electronics OPTEK Technology.
www.optekinc.com