Battery Operated Wardrobe LED Light With Recycled LED Strip

In my previous Instructable “HOW to Recycle the LED Strips of a Spiral LED Bulb” I described how to save the LED strips of a 230Vac-Spiral LED bulb and to power the half of one strip with a low voltage supply (27Vdc). In this Instructable I show how to build a battery operated “wardrobe internal lamp” using this LED strip.

Supplies

• Pushbutton, like this one
• Timer 555, I bought it here
• 9V PP3 Li-Ion Battery rechargeable, like this one
• Relay TX2-9V, from Panasonic
• 2 x Diode 1N4001 or similar
• N-MOSFET IRF510 or similar (see text)
• JST male connector 2mm 2 pin, like this one
• Cable with JST female connector 2mm 2 Pin, like this one
• Clip for 9V Battery, like this one
• XL6009 DC/DC step-up converter, I bought it here
• Electrolytic Capacitor 47uF / 25V
• 2 x Resistor 22K
• Resistor 1.5K
• Resistor 100K
• LED Strip 18 x 9V-LEDs recycled (see txt)
• A tiny prototyping board (22mm x 45mm)
• 3D-printed enclosure (.stl files attached)
• L PVC-profile 10mm x 25mm (length according wardrobe/wall unit width)

Step 1: Project Outline

As short recap, the used LED strip is made of a series of three groups of 9V-LEDs, and each group in turn is composed by 6 LEDs in parallel. With a supply voltage of 27V the current draw is 155 mA. To provide the 27V-power supply I decided to use a Li-Ion rechargeable 9V battery followed by a voltage tripler based on an XL6009 step-up adjustable DC/DC converter. Consequently, the expected current draw from the battery should be 155 x 3 = 465mA (converter efficiency not calculated). Taking into account that the real voltage of a full charged 9V Li-Ion battery is about 8.3V, I set the converter output voltage to only 25V, which is anyway enough for a good illumination. The video included at the end of my previous Instructable shows that the current draw of my test circuit in this condition is less than 0.5A. Considering that the nominal battery capacity is 800mAh (not fully exploitable), that gives us just over an hour’s autonomy. Not much indeed, on the other hand I need to light on the lamp just a couple of minutes a day, so I decided that I can get by with it, provided that the current draw when the lamp is off is zero.

Another constraint I decided to place in the project was to use a pushbutton instead of a switch to toggle ON/OFF the lamp, for two main reasons:

• A pushbutton is easier to operate than a switch, especially if your hands are full of clothes or something else.
• A pushbutton is compatible with a timer in case you want to limit the ON time, while a switch needs to be manually reset to the OFF position before turning ON the light again.

The challenge here is that usually a toggle function is realized with an electronic (flip-flop) circuit that is continuously powered, waiting for the button to be pushed, but considering the low duty cycle of the system that would be a wasteful solution.

The circuit I propose overcomes this constraint limiting the overall leakage current in OFF state to few 𝜇A.

Step 2: Circuit Description

Let’s start with a basic design with no timer function, that can be easily implemented with just 4 additional components (see below).

Initially the 555 is not powered, and once the charging of C1 through R1 is completed only a small leakage current (< 10 𝜇A) flows through the capacitor and the MOSFET Q1, and V1 = Vbat = 8.3V.

When the button SW1 is pressed, Q1 is switched on, and relay K1 is activated. The 555 is now powered through the K1 contact 4-5 and immediately triggered. The output Q goes HIGH and keeps Q1 in the ON state. Q1 ON also discharges C1 through R3, but now the system is latched. The second K1 contact switches Vbat to the XL6009 converter input, the LED strip is powered and lights up.

When SW1 is released V1 = 0,7V (D1 forward voltage drop), which is below the Q1 Gate threshold voltage, therefore when SW 1 is pressed again Q1 turns OFF, relay K1 is deactivated and the LED strip goes out. That would also start a new charging cycle of C1, but as long as the button is pressed V1 can’t raise above about 2.5V. The reason why that happens is an - at last to me - unknown and unconventional characteristic of the 555 output stage: when the IC is not powered, the output acts as a “virtual path” to GND, with a voltage drop of about 1V. The block diagram in the 555 datasheet is not detailed enough to understand this behavior, but the value suggests to be the sum of two junctions (maybe protection diodes?). Anyway, this effect is reproducible on different samples (even the CMOS low-power version 7555), and in our case it is important to prevent the circuit to oscillate, what would happen if C1 was able to recharge while the button remain pressed. In this situation, R1 and R2 act as a voltage divider so that V1= [Vbat/(R1+R2) x R2] + 1 = (8.3/122 x 2) +1 = 2.5V. As long as it is guaranteed that the N-MOSFET gate threshold voltage is higher than 2.5V, it’s not important whatever device you use, given the modest load. I used an IRF510 just because at handy.

Step 3: Adding the Timer Function

Adding just 4 component (R5 - R6 - C2 - C3) you can get an automatic shutdown after a certain period of ON-time. With the suggested values for R5/C2 the time limit is = 1.1 x 220K x 220uF = 53,2 seconds. R6 and C3 are necessary to trigger the timer but differently from the basic circuit the trigger input returns HIGH to allow the output signal to go LOW once the time has run out.

The pushbutton mode is not affected by the change and operates as described above.

Step 4: Assembly

I’ve assembled the electronic components on a small prototyping board the same size of the XL6009 module, and combined electrically and mechanically the two boards connecting the respective grounds through a short piece of copper wire, to form a single module of size 22mm x 90mm. Then I placed the module in the 3D-printed enclosure box I designed.

See attached below the .stl files.

The pushbutton is mounted on the enclosure and connected to the electronics with a JST plug, the wires to the battery and the LED strip are directly soldered.

For the battery I found a convenient holder here (thanks Charlie!). I placed the holder on the cupboard wall in a way that the battery can be easily removed for recharging.

Finally, I fixed the enclosure and the strip to the PVC profile with a double sided tape, and I mounted the profile on the lower edge of a shelf above the area to be illuminated.