I was really never that happy about the blue light in the bathroom. It worked well enough, but the blue lights were a bit too bright at night. So this year, I decided to add a dimmer. I had originally wanted to modify a new fixture, but I could not find the fixture I used two years ago... it seems they are no longer available.

I then realized that there are actually two lights in the bathroom. The puck light that I modified, and a Facon SD1510-PIR motion detection light. Since those are readily available, why not modify that fixture instead? So I did just that; re-installed the original fixture and modified the motion detection fixture.

The way these light fixtures work is in the I position, it turns the light on. Then, flipping the switch to the II position, the light is turned on via the motion detector.

OK, that will work. Depending on how the motion detector is wired, I can use one side of the switch to turn on the white light, then the other side for the blue night-light. And there is enough room in the base of the fixture to install a PWM dimmer.

I suppose then the first order of things is to dissasemble the light fixture and see how it is wired. The results are shown below.

So the first surprising thing; the motion detector is a Low Side switch, meaning the LEDs must be attached on the ground side. Ok, no big deal (but it will cause the switch to work a bit differently). I mean, electrons flow from Negative to Positive anyway, but by convention, we tend to think of electricity flowing from Positive to Negative (but it is technically incorrect - even though New Age scientists tend to adhere to the "false positive" concept).

But... woah! What? The motion detector is wired directly to the 12V supply, bypassing the switch. Why? Can you say "residual load"? OK, my ammeter says the load is around 17uA, which is not much, but residual loads tend to add up... and why not just put the dang motion detector behind the switch?

Anywhoo... I did come up with a suitable switch configuration as shown above. When switched to the blue lights, they remain on all the time, controlled by the PWM dimmer. When the switch is in the opposite direction, the white lights are on, controlled by the motion detector.

This works fine. You really don't want the Blue Lights to be on the motion detector because you want to have them as a night light, which means they should always be on. But if you prefer, you can always turn the white lights on with the motion detector.

I added a RED 12V LED "Pilot Light" (with current limiting resistor built-in) to the fixture that is on whenever the blue light is on. The reason for this is to ensure I turn the light OFF when not needed. In the daytime, especially if the blue light is fully dimmed, it is hard to tell if the blue light is on or off. Having a RED LED at full brightness (i.e. not on the dimmer) provides daytime indication the light is on.

Unlike the 17uA load of the motion detector, the blue lights, even when fully dimmed, consumes around 20mA. So the RV should not be put into storage with the light switch in the blue light position.

For convenience, I used a 12V Panel Light style LED since the current limiting resistor is built-in. This prevents having a wiring mess (at least more of a mess) by having to add a resistor to a un-biased LED.

They are not necessarily the best light for night lights as they can act as a stimulant - keeping you awake. There have been studies done by the FAA to determine the best light for cockpit use, and Red/Orange have been determined to preserve night vision the best. But since you only have to reach the bathroom, night vision should not be an issue.

However, that is not the only consideration. Your eyes can see blue light better at night - so a lower intensity blue light works as well as a higher intensity red light. Also, for me, blue light tends to be softer, and red a bit harsher. But in the end, it is your choice, is't it. As I tend to sleep with my eyes closed anyway, I find that the blue light is not a problem - well, other than if it is too bright.

The good news is you can choose the color you want. The source I picked for the blue LEDs in the Bill-of-Materials also offers RED, GREEN, and YELLOW LEDs, all of which are interchangeable - so pick the color you like.

  The dimmer is simply a classic design NE555 based PWM (Pulse Width Modulated) type dimmer, driving a NPN transistor. This is a simple and reliable circuit, with the only downside being the duty cycle is around 5%~95%, which means you can never completely dim the LEDs into the OFF state (but why would you want to).

The available area to install the board means I had to use surface-mount technology (SMD) for the parts components - as well as leaving a relatively flat area on the board bottom to attach it to the light fixture. Either VHB tape can be used, or the board also has two holes for mounting screws.

An advantage to using a NE555 is it can be powered directly by 12V so no additional components are needed for a power supply section; again, resulting in a smaller board that is more fit-friendly into the fixture.

Notes:

1. Schottky diodes are preferred as they increase the range of the PWM duty cycle. A standard diode can have a 10-90% duty cycle, whereas a Schottky diode can have a 98~2% duty cycle

   If these parts are substituted due to lack of supply source, look for diodes with the lowest Vf (Forward Voltage) you can find, preferably 300mV or less.

2. Misc additional materials required include M2 sheet metal screws, VHB tape, Heat Sink compound, and hook up wire.

3. Optional Jumper (use wire if not using this item).

4. While I used 4 boards, you can use 3 boards if you want to. This cuts down on expense of ordering boards.

Install the components on the dimmer board as shown in the next two panels. Pay attention to the orientation of the NE555 and diodes. The transistor - while polarity sensitive can only be installed one way. The remainder of the components are not polarity sensitive.

Begin the construction of the circuit board as shown above. It is recommended to install the parts in the order shown so that larger parts don't obstruct the installation of smaller parts. Pay attention to the note on the NE555 dimple, or lack thereof.

After the installation of the above parts, the dimmer board is complete. You may test the board by connecting 12V and GND to the terminals marked as such, and a LED (strip or puck light) to LED+ and LED- terminals. This is optional.

Hint: if you connect 6" long wires to the board for testing, you can re-use them for the installation (i.e. not have to remove from the board). Test the board by applying 12V and turning the trimmer to ensure the dimmer works.

Construct 4 of the LED boards. One of the boards will have a Driver Diode on it marked in Yellow, and the other three will need a jumper in the same location. Optionally you may wish to use a Zero Ohm resistor for the jumper.

Install the LED boards on the light fixture as shown, using M2 sheet metal screws. Note. Orient the boards so the LEDs are 90Deg from each other. Since the LEDs are not centered on the board, the boards themselves will be offset slightly. If you orient the board centerline to 45Deg, the LEDs will appear slightly off-center. This is purely asthetics though.

Note that if you order 3 boards, simply orient them at 120Deg from each other. You will end up with one board having the Driver, and 2 boards having a jumper.

The panel light LED is optional, but it provides daytime indication that the light fixture is switched to blue lighting, which can sometimes be hard to see in the daytime if fully dimmed. Unlike the blue LEDs, this LED does not dim, so it (hopefully) prevents leaving the blue lights turned on in storage.

Final Note. It should be realized that especially with Chinese products, there can be some variation from mfg-to-mfg. For example the motion sensor might be different from one brand to the next. For this reason, double-check all of your work to make sure you are wiring the circuit correctly.