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Electronics question (transistors)

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Electronics question (transistors)

Old 08-17-05, 06:18 PM
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Electronics question (transistors)

I'm restoring an old Heathkit weather station. Along the way, I replaced the infrared LEDs and the matching phototransistors. In between each LED / phototransistor pair is a disk which has clear or shaded portions. The phototransistor is supposed to be activated only when there a clear space on the disk, allowing IR light to reach it.

These phototransistors only had two leads: one short and one long. The first thing I did was to clip the leads down so I could solder them to the board. The trick is to not get them reversed after they've been trimmed. I could have screwed that up.

Now, one of the phototransistors is returning +5V, logic high, whether it sees the LED or not.

If I mounted a phototransistor upside down, would it return a constant +5V? Or did I get a bad phototransistor?
Old 08-18-05, 11:48 AM
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Just to be sure, you are testing this thing in the dark, right? IR light from the environment will make it "turn on" as well.

You may have simply gotten the polarity reversed. I would expect it to always return zero if it was in backwards, but what do I know?

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