Introducing the “Dimmer Not Dumber”
A novel approach to fading line voltage lights!
A client wanted me to control the brightness of a set of residential 110v pot lights. The only experience I have in controlling line-powered equipment is through using relays and opto-isolated solid state switches. However, these were just to provide simple ON/OFF control; and dimming lights is a whole different ball of wax! Plus, I did NOT want to mess with mains voltages!
There are devices on the market to do this but they are a) expensive, b) require electronics connected to the 110v load side and c) require real-time software to control (see discussion below). I wanted a simpler solution that used off-the-shelf residential electrical components, simple electronics with complete electrical isolation.
So, I came up with the “Dimmer Not Dumber” light fader. In this design, a servo motor is connected to the shaft of an unmodified residential light dimmer (Levitron, I believe) via a simple gear pair. This dimmer has a control shaft that rotates about 320 degrees to fade between full OFF to full ON. In addition, pushing the switch turns the whole unit ON and OFF. As the servo articulation is typically a maximum of 180 degrees, a 2:1 gear pair means that the full range of the servo translates into a complete rotation of the dimmer shaft.
Control of the servo position allows full control of the intensity of the light, so now all I have to do is connect the servo to a micro-controller and voila! Full control of the light intensity and complete electrical isolation!
The 8-tooth smaller gear attaches to the fader’s control shaft with a small set screw, while the 16 tooth servo gear attaches using a plastic servo horn. The body of the unit is made in two parts: a 12mm HDPE base and a 6mm HDPE servo mount. The whole unit bolts to the dimmer using the normal screw holes used for fascia plates.
“Dimmer Not Dumber” works like a charm, both in the light and the dark!
Further notes about dimming lights:
Dimming lights under the control of a micro-controller without flickering is not as easy as it sounds. One popular way to do this is called “leading edge cutting”. First you need a zero-crossing detector to provide a reference pulse when the AC line voltage changes phase (i.e. when it passes through 0v). In addition, you need an electronic switch that can turn on the load, typically using a triac. Both the detector and switch control need to be electrically isolated from the micro-controller. This is typically done using an opto-isolator which consists of an LED and photo-transistor pair that are optically connected but mechanically and electrically separated.
Controlling the delay between the zero-crossing pulse and the time when the load is switched on controls the “average” voltage applied to the load; which controls the power delivered to a light bulb, for instance. Switching on the load at or close to the zero crossing provides full line voltage to the load (full brightness), while delaying the time before switching on the load lowers the average voltage to the load, dimming the bulb. Delaying the time to switch on by a full 1/2 cycle means that no power is delivered to the load; the bulb is extinguished.
So, the micro-controller software is responsible for this control. It resets a timer upon receipt of the zero-crossing pulse, and triggers the switch on pulse some time later based on the level of dimming required. Less time is brighter, more time is dimmer. SImple.