Fun with LEDs


I have recently stumbled upon some LED strip at my local electronics shop and decided to give them a try. I bought some which I used to replace the spot lights in the kitchen. It is cold white, which is surprisingly good, especially for night time illumination (think moonlight like hue). It works at 12V and consumes about 0.25A per meter.

After installing the strip, some automation proved to be necessary, and so the following circuits were built. The goal in mind was to keep things as simple as possible and use only parts I had at hand, which is why the solution might not be the best.

The hallway spotlights got new white LEDs as well and a light sensor. Tiny PIR sensors will turn on the lights in the kitchen and bathroom when someone comes in range. The sensors are rather popular modules using a BISS0001 IC; they provide a 3.3V level for an adjustable time when motion is detected.

Overall the results are great. The hallway is lit at night, the there is a small automatic light for the bathroom and the automatic kitchen light is bright enough even for day time illumination of the sink and counter. The slow turn off provides both a visually pleasing effect and a warning in case someone stood still long enough to make the light go off. I am still looking for a simple solution to produce the same effect on turn on, but without the delay.


Simple light sensor

The circuit is used to turn on the hallway lights. T2 will turn off when it is dark enough, allowing T1 to turn ON and light up the LEDs. The threshold is determined by the divider formed with a photo resistor and a potentiometer. I gave up using a constant current supply in the favor of a simple current limiting resistor because at around 100mA of current (set with R3) there is no danger of thermal runaway.

The circuit was built inside a small remote control case, here it is at first tests:


Simple PIR sensor

The PIR sensors themselves cannot turn on a strip of LEDs so, for the bathroom light I have used a simple transistor amplifier. The light is made up of just 20cm of light strip consuming about 50mA, but it produces enough light for a sleepy person to be able to use the bathroom without the need of the bright waking main light.

Driving the transistor through the diode insures and instant on, but once the PIR turns off, C1 will discharge through R2 slowly turning off the LEDs during a few seconds. My initial desire was to have the same effect at turn on, but the time required to charge the capacitor up to the threshold voltage will produce a few seconds of delay which in practice causes undesirable effect.

Note that I have symbolized the LED strip through its basic cell, 3 LEDs in series with a resistor.

And the assembled circuit on the cap of a wiring box:

PIR and light sensor

For the kitchen strip I wanted both of the functions above: a dim light when it gets dark and motion sensor turn on. The circuit is not exactly the combination of the two since I had to take into account different things.

First, the light sensor was simplified since I found out that 10mA (limited by R7) worth of current through the strip produce enough light for the night (yes, that little current makes the strip give enough light to see what it is in front of me). This means that a single transistor is enough.

The light sensor turn on circuit became a little more complicated, mostly due to the parts I had available. The transistor has to drive more current (close to 0.5A) and support the heating produced during the slow turn off when it will dissipate more power. The only suitable transistor I had available was an IRF9530. This required another transistor to drive it. Just like the previous circuit, turn on carried as fast as possible, but the light turns off in a few seconds. R6 was initially 150K, until I discovered that this produces a delay in turn on. It may be simply shorted.


I first fixed the PIR lens and then inserted the photoressitor inside it:

The finished module (some resistors were still changed  afterwards):

Installed in the kitchen (still needs some to get some wires arranged):

The mounting conditions forced me to use an aluminum profile for first fixing the strip onto it and to split the whole thing in two pieces. Unfortunately, Murphy caught in:

Other notes: The strips were sprayed with a few layers of acrylic to ensure good insulation, just for added safety, as the power supply is insulated from the mains.


The math on consumption

I did some math on the consumed energy. The power supply spec says that it should consume less than 0.3W unloaded. So, per month, that gives:

0.3 * 24 * 30 = 216Wh;

Assuming the night lights turn on for an average of 12 hours per day, this gives:

(0.1 + 0.01)  * 12 * 12 * 30 = 475Wh;

2 hours (exaggerated)  of daily kitchen use:

0.5 * 12 * 2 * 30 =  360Wh;

2 hours (exaggerated)  of daily bathroom use:

0.05 * 12 * 2 * 30 =  36Wh;

Monthly total:

216+475+360+36 = 1087 Wh, or approximately 1 KWh per month., which is very little.

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