App note: Photologic

Photologic components and applications from TT Electronics.

Historically, optoelectronic components such as phototransistors have been analog output devices. The application engineer had to design with an output current from the phototransistor generated by a given input bias circuit. The advent of the integrated circuits and microprocessors has required the electronics world to turn digital. The sophisticated electronics today communicate by logic levels of 1’s or 0’s. This means that the application engineer must now convert the analog light current of the phototransistor to a voltage level in order to communicate the sensing function to downstream processing electronics. This signal represents additional system cost in components and performance specification guard banding which can be reflected in the unit pricing of the optoelectronic component or assembly.

Photologic discrete components consist of an input photodiode, an amplifier (with gain), a high speed comparator (with or without hysteresis), and a power output device. The photologic sensor is used in many applications to convert input light to a logical high or low output. Its current applications include encoder, non-contact reflective object sensing, non-contact interruptive sensing, assembly line automation, machine safety, end of travel sensing, object sensing and paper sensing.