Ordinary electromechanical switches are not the only choices for applications demanding long life. To get a long-lived switch it is possible to use a photointerrupter in lieu of mechanical fixed and movable contacts. Photo interrupters also avoid the ancillary limits of lubricants and internal I2R thermal constraints.
Switches based on photointerrupters have no contacts, no lubricant, and only one spring. The switching action takes place by depressing a button that interrupts the optical path between an infrared LED and a photodiode or phototransistor. This provides a momentary "low" or "high" output state change. Internal circuitry converts the resulting signal into logic levels compatible with TTL circuits. The TTL circuits, in turn, control loads to be switched such as solenoids, relays, and so forth.
Switches that use a photointerrupter generally have high reliability and ratings of at least 3 million operations. Though photointerruption is widely used as a location-sensing technology, its use in pushbutton switches is relatively new. Such devices first became available in Asia about five years ago. These switches feature rugged construction, often with positive overtravel control, i.e., a stop that prevents possible damage by overtravel of the actuator. This allows repeated and rapid actuations as often arise in emotionally charged video games or vending machines that malfunction. Though its actuating force is typically quite light (about 0.75 N or 0.169 lbf), the switch can withstand operating forces to the button that would otherwise damage a typical electromechanical switch.
The switch is illuminated with either a red, green, or amber LED. It can be made highly visible by using a transparent lens that matches the LED color. The illumination can be subdued, for low light environments, through use of a translucent white lens.
The switch is equipped with a six-pin connector that eliminates soldering and makes for a simple connection. The switch snaps into a keyed round panel cutout.
Illuminated photointerrupter switches tend to be more expensive than other kinds of lighted pushbuttons. But their long operational life and easy replacement (no soldering involved) may outweigh the additional cost. Though the switches' absence of mechanical components well suits potentially abusive uses, other promising application areas include machines exposed to corrosive gases as arise in chemical and steel manufacturing plants.
Information for this article was supplied by NKK Switches, www.nkkswitches.com.