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is there any known type of device will do the following:

[I have read the bi-directional potentiomenters posting LINK but I just joined and can not reply to that posting.

I am looking for a rotation rheostat/potentiometer to control a 110v 25w lightbulb.

  • top-dead-center [slider at top-dead-center] = bulb at brightest
  • rotate-clockwise = bulb dims [no need for a full-off clockwise position]
  • rotate-counter clockwise = bulb dims [no need for a full-off CCW position]

My chiropractor son and I are trying to make this device YouTube LINK on our own

  • The left bulb is always on [simple SPST switch]
  • The right bulb dims if the plastic spine is rotated right, or left, from the dead-center [full bright] position

Thank you for your considerate assistance & recommendation.

Jim [Mech-Engr & tinkerer, with very little EE knowledge so please dumb-down your answer ]

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Dimming AC-powered bulbs isn't really the simplest task, especially when you want a custom brightness curve like that.

I would suggest you use bright white LEDs instead (and not the AC light bulb replacement types, but simple DC ones powered from about 3–5 volts), and control the dimming from an Arduino. The Arduino IDE already comes with two very relevant and quite simple examples: LED dimming, and reading the position of a potentiometer. All you would need to do on the software side is combine the examples so that the position is turned into a brightness value. On the hardware side, you would probably need to add a simple transistor ciruit (such as just a single logic-level FET and a current-limiting resistor) to drive the current to the LED, since bright ones probably need more than the Arduino can directly supply.

This would be a pretty cheap and simple solution, and there would be no chance of frying yourself, unlike when working with mains AC circuits.

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  • \$\begingroup\$ Did you look at the YouTube link to see how the item functions? You provided a fine answer but even though you said "all you would need to do is ..." I find those instructions a bit intimidating as I've never worked with an Arduino nor built a 'simple transistor circuit'. ... continued on next commment [i got too wordy] \$\endgroup\$ May 11 '15 at 17:27
  • \$\begingroup\$ ... continued QUESTION: how much for materials, shipping + labor if you build 3 for my son? As in the YouTube video, the LEFT LED would be always full bright, the RIGHT LED would be bi-directionally dimmable. If the price is reasonable, we'll discuss details. I'd make the 3 wooden boxes. We leave for Panama in 3 weeks (May 30th) so if you have time-available, please quote 3 delivered dual-bulb systems to the 63376 [St. Louis suburb] area. Thanks again, Jim \$\endgroup\$ May 11 '15 at 17:31
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This seems fairly simple but does require a special potentiometer. Alternatively, you can use a rotary switch if that gives you enough resolution.

The pot that you need is a linear pot with a fixed tap at 50%. These used to be readily available in decades gone past but I honestly don't know where to find one now.

You would start with a commercially-available light dimmer. Remove the existing pot from the dimmer and measure the maximum resistance. That would be the two end terminals if the pot has 3 terminals on it. Otherwise, connect the ohmmeter to the two terminals and rotate the pot until the resistance is maximum. Record that resistance reading.

What you want is a center-tapped linear pot with a total resistance DOUBLE the resistance that you measured above. Connect the the center-tap and the wiper of the pot to the dimmer circuit. The two end terminals remain not connected to anything.

In use, the resistance is minimum when the wiper is at the center-tap location. The resistance increases as the wiper rotates away from the center position and the bulb dims.

If you can't easily find an appropriate pot, find a single-pole rotary switch with as many positions as possible. The most common rotary switches have 12 positions but I have seen rotary switches with as many as 27 positions. You would then choose appropriate resistors to give you the desired brightness at each switch position.

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