While digging through my stores of old and obsolete parts I came across this 3.5 X 2.5 X 1.25 cm black box with 4 leads and no markings.
I can remember it being in my stock of parts back in the late 1960s and early 1970s when I was repairing equipment for the Radio Shack but I can not place the piece of equipment that it was used in. I also was not sure exactly what it was. With only 4 leads the puzzle seemed solvable.
An Ohmmeter check of the leads showed that there is a diode effect between the two leads on the left and no connection to the two leads on the right. The two leads on the right had a non-polarized resistance of 300K. This looked suspiciously like the readsing one might expect from some sort of giant optocoupler. I had to check the Wikipedia listing on LEDs to see if they had been invented by the late 1960 era and was pleased to see that practical devices existed as early as 1962. To test my theory I connected a 20 mA forward biased current to the left leads and once again checked the resistance of the right two leads. The resistance had dropped from the previous 300K down to about 50K. The non polarized nature and relatively high resistance of the right two leads seems to indicate a photo resistor output. The size of this device compared to optocouplers currently in use is a good testament to how far and fast the technology has evolved.
I am not sure if I will ever find a use for it but this is true of perhaps 95 percent of the components stocked on the shelves and in the nooks and crannies of my lab.
John
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