IR sensor of some kind? How does it work?

I got these from a computer mouse, used for scrollingwheel movement detection, different components were facing aech other on opposite sides of a pierced wheel. (The mouse had two scrollingwheels - so there are two pairs on the picture. Manufacturer A4tech.)The ones in bottom right corner are probably IR LEDs -  no visible light when current flows through.
But the black things are a puzzle - light sensitive transistor is supposed to have two legs not three. Two transistors or diodes in one package? And I get no reading with multimeter - no resistance nor diode polarity between any combination of the legs. Tested both of them. (They have to be really heat sensitive for both to be broken after my desoldering.) And there is no component number on any of them.

Does anyone know, what exactly are the three leged components?

And can they be still in working order despite the no reading on multimeter?

Comments

[steveastrouk]

Photo transistor CAN have three legs like a conventional transistor, it depends precisely on what you are using them for.

I expect these to be DUAL photo transistors though - that's how the mouse detects which way its moving.

The Clear packaged IR LEDS still need current limiting resistors, if you didn't use them, you probably ruined the devices.

YOu can see IR on many mobile phone cameras.

Steve

[Jack A Lopez]

+1.  The clear 2-legged ones are IR LEDs, and the dark 3-legged ones are dual phototransistors. 

The rest of the clues, as to how these should be connected, and what size resistors to use, can be found by examining the circuit board from which they were removed.  

[Libahunt]

Thanks for answering.

I did use a current limiting resistor, so everything is in working order. Although the circuit on the original mouse PCB was confusing - too many points connected to the main chip, I got it all wrong at the first time. But when I cleared my head and set up a voltage divider with the transistor (one in the package) and a resistor then I got almost expected result. The LED seems to be very weak or me too cautios choosing current limiting resistor... But it will be usable for a line follower robot sensor.

[steveastrouk]

The IR led has quite a high forward voltage drop, compared to a visible LED.

Steve