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:

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  • 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!


1 Answer 1


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.

  • \$\begingroup\$ 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? \$\endgroup\$
    – Blender
    Mar 18, 2012 at 4:29
  • \$\begingroup\$ @Blender - the two connectors on the top (A & B) are the switch. \$\endgroup\$
    – JonnyBoats
    Mar 18, 2012 at 4:31
  • \$\begingroup\$ 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? \$\endgroup\$
    – Blender
    Mar 18, 2012 at 4:36

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