# Can anyone identify this component?

I'm trying to solder together a small LED array with a dimming knob out of a bunch of old parts I have laying around, but I can't seem to identify how to wire this component:

• The two connectors at the top are labeled A and B and it says ALPHA in the middle.

• The red plate is labeled 1 3.

Any help would be appreciated. Thanks!

This looks like a potentiometer with a built in on-off switch on the back.

When you either push on the knob or turn it to the end of it's range does the resistance between A and B change from infinite to 0 ohms? If so, you have confirmed that it is a switch.

The resistance between 1 and 3 should be fixed and between 1 & 2 (or between 2 & 3) vary as you turn the knob.

These are (were) commonly used as combined volume controls and on-off switches on radios before the digital age.

In response to the comment.

Since you measured an exponential change in resistance you have what is known as a logarithmic taper potentiometer. The other type is a linear taper. Logarithmic taper is used for volume controls since human hearing does not respond linearly but rather on a decibel scale.

If you measured across 1 & 2 instead of 2 & 3 you would have perhaps found the switch as "on" when the potentiometer is "on" as you put it. Looked at another way, the resistance across 1 & 2 will increases as the resistance across 2 & 3 decreases. In fact the resistance between 1 & 3 should remain constant as you turn the knob. Also for any place you put the knob, if you add up the resistance between 1 & 2 with the resistance between 2 & 3 that total should equal the resistance between 1 & 3.

• The thing on the back is just a metal plate that connects to the body of the entire component. It seems to be a potentiometer (thanks), but what are the two connectors on the top for? Mar 18, 2012 at 4:29
• @Blender - the two connectors on the top (A & B) are the switch. Mar 18, 2012 at 4:31
• Thank you! I just tested it and the resistance varies (exponentially?) across the second and third connectors. Is the switch "on" only when the potentiometer is in the "off" state? Mar 18, 2012 at 4:36