Introduction: USB Floodlight

This started out as a practice in SMD (surface mount device) soldering on standard prototype boards, and resulted in a very bright compact USB-powered flood light, great for camping or emergency lighting.

Most modern LED light bulbs contain in the inside SMD LED chips. These chips are mass-produced, very cheap and available to the hobbyist at very low prices. I bought 200 of the 5730 type for 1 EURO. The 4-digit number indicates their size: 5.7x3.0mm. They are rated for 0.5W (~140mA at 3.5V) each, although they will require a heatsink to run continuously at that power. Without a heatsink, they should either be run at a much lower current, or can be run in pulsed mode at full current, for example in multiplexed or stroboscopic mode.

This instruction details how to make a USB-powered flood light, but low price and small size mean that they can be used for many other applications, such as DIY 7-segment displays, mood lights, grow lights, projectors, drawing tables or any custom lighting solutions.

Standard USB power banks deliver 5V 1A, and the bigger ones can deliver 2A. The design presented here is for 1A, so it will work in any power bank, but by doubling the number of LEDs you can make one for 2A.

Step 1: Theory

Contrary to the old-fashioned incandescent light, the voltage drop on an LED depends very little on the current. The voltage drop for high-current white LEDs go from ~3.0V at currents ~10mA to ~3.5V at 100mA. So they cannot be connected directly to the 5V delivered by a USB power bank. The easiest solution is to connect each LED in series with a resistor. The value of this resistor determines the current through the LED, and thus the brightness. The exact current of a LED with resistor is difficult to calculate, but easy to estimate, and straightforward to measure.

For example, a 1 kOhm resistor in series with a white LED will mean that the current is very low, so the voltage drop over the LED is ~2.9V, leaving 2.1V over the resistor, and thus a current of 2.1mA through the resistor, and the same 2.1mA through the LED. A 100 Ohm resistor would result in 21 mA if the voltage drop of the LED would stay 2.9V, but it is likely to increase to 3.0V, leaving ‘only’ 2.0V over the resistor and thus 20mA through the LED. With a 10 Ohm resistor , the current would be 200mA if the LED voltage drop were 3.0V, but it’s likely to increase to 3.4V, and the remaining 1.6V drop on the resistor gives a current of 160 mA, which is slightly above the nominal current.

So you might think that to make a strong lamp from a 5V 1A supply, it would be sufficient to put in parallel 6 or 7 0.5W LEDs, each with a 10 Ohm series resistor. Each LED would consume 160mA*3.4V=0.54W and each resistor 160mA*1.5V=0.24W. That is close to spec for the LED and within spec for a 1/4W resistor. But if you try this out you will see that both the LED and the resistor get extremely hot (~100C). Even more so if you place all these components close to each other. Unless a heatsink and a fan are used, they are likely to die, and produce lots of toxic smoke in the process.

So I have tried the following setups:

10 LEDs with 22 Ohm series resistors. I measure 1.4V drop over the resistors, so the current is 64mA per LED, 0.64A total. With the LEDs and the resistors mounted close by it gets so hot that it hurts at touch, but it doesn’t melt or burn and it’s a nice compact light for occasional use.

24 LEDs with 47 Ohm series resistors. I measure 1.7V drop over the resistors, so the current is 36mA per LED, 0.86A total. Things do heat up after some time. Interestingly, the resistors feel hotter than the LEDs, despite consuming more energy and being smaller. Maybe be the LEDs manage to radiate away a large fraction of their energy as light? I wouldn’t use it in a tent since the temperatures reached can be painful and might raise to dangerous level if accidentally covered.

40 LEDs with 100 Ohm series resistors. I measure 1.9V drop over the resistors, so the current is 19mA per LED, 0.76A total. It gets noticeably warm, but definitely not hot. This makes a great lamp, similar to a 3W LED bulb (or 30W incandescent bulb). Very useful for photography of small objects, soldering or repair jobs, but also lighting up the BBQ or as an emergency light at home, on the road or on the camping.

Step 2: Required Components

The instructions are for the 40 LED panel with 100 Ohm series resistors, which I think is the brightest and the safest. The full thing took me about an hour to solder, but admittedly that was after I had gotten some experience and some confidence with two other versions of the board.

Required components (Total cost: less than 1 euro if bought in semi-bulk)

  • 40 white SMD ‘5730’ LEDs
  • 40 100 Ohm resistors, 1/4W
  • 1 5x7cm prototype board. Single-sided, 18x24 holes.
  • 1 male USB connector.

Tools: a soldering iron, solder, tweezers.

The LEDs have a polarity. From a distance their appearance may seem symmetric, but on close inspection you’ll see several differences. The most useful is on the yellow front-side: there is the oval part that actually lights up, but one side contains in addition a line. That’s the negative side, just as for diodes, electrolytic capacitors etc.

Step 3: Building Instructions

Start 40 putting blobs of solder at the place where the LEDs connect to ground. Next, solder the LEDs with their minus side on the solder blob: hold the LED with the tweezers, melt the solder blob and shift the LED into the liquid blob. Make sure that hole on the plus-side of the LED has some space left to put the resistor lead through.

One by one, mount the resistors on the back side of the board, following the regular pattern shown in the picture. Solder one side to the plus of the LED, and the other side to the center of the board. Cut off the excess leads on the ground side, but leave them on the plus side.

At the end, connect together also all the plus-side leads. Now is a good time to test if all LEDs work. I found that with the multimeter set in the 200 Ohm setting, the LEDs light up slightly, but distinctly enough to see if one is not connected well. Use some of the excess leads to connect all the points of both minus rails together.

Now attach the USB connector. I put four blobs of solder and soldered all four pins to the board, so that the connector is well secured to the board. Seen from above, the left pin is plus and the right pin is minus, and should be connected to the respective rails. The two central pins are for data and are thus unused. The connection to the left ground rail should go from the back side to allow it to cross the plus rail in the center. You can now test it on a power bank and if all lights up well you are done!

Step 4: Performance

It is notoriously difficult to show how strong a light is: autoexposure of a photo camera means the stronger the light is, the less will be the exposure. Pictures taken of the performance of 'insanely bright torch' are rather underwhelming. Nevertheless, I think the above picture gives an honest idea: nearby it's very bright, but it also illuminates well a couple of meters away. Notice also that the illuminations is very homogeneous, since these SMD LEDs, contrary to acrylic LEDs, have no focussing lens.

Last but not least, if you like these instructions, please consider to vote for it on the 'Make it Glow' contest!

Make it Glow Contest 2018

Participated in the
Make it Glow Contest 2018