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The Pre-Triassic Period
At the dawn of the present era, transmissive sensors in automotive systems took the form of an emitter-detector pair in through-hole packaging that was wave-soldered to a printed circuit board. The emitter and detector were facing each other so that if anything came between them, the output current of the photodiode or phototransistor would change. This change would be relayed to a controller and something would happen: a motor would start or stop, an indicator light would turn on or off, or a bag of chips would fall to the bottom of a vending machine.
The Triassic Period
The position of the discrete components could be difficult to precisely control. One might be higher than the other or at a slight angle; the leads might be bent, or during handling they would become disoriented. This posed a problem for the system because the output current from the detector would vary from board to board. The controller was looking for a certain signal level and, without an exact orientation, it wouldn’t get it. An evolutionary leap was ushered in by Vishay, which molded the discrete components in a common plastic housing to ensure exact orientation.
This required several different package versions, each with a different gap between the emitter and detector, with a photodiode or phototransistor output, and with a different lead bend for horizontal or vertical gaps. These sensors were (and are) called transmissive sensors or slotted interrupters.
The Jurassic Period
With more and more board assemblies going with pure surface-mount components, the leads of the through-hole packages had to be bent so they could also be surface mounted. The plastic used in the housings had to change in order to withstand a +260°C reflow solder temperature. Horizontal slotted packaging did not evolve, and retained the form of a through-hole package. While many suppliers stopped evolving at this point, Vishay continued to push the envelope of device capabilities.
Tune in for Part – 2: The TCPT- and TCUT1300/50X01 Period