Tag Archives: LEDs

Paparazzi Flashes

For last Saturday’s Oscar-themed prom, two of my students wanted to make a cardboard wall of photographers. Essentially, a collection of randomly-firing bulb flashes placed on a piece of cardboard.

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After experimenting with the capacitor-driven flash units of couple disposable cameras and recognizing they would be too dangerous, we settled on the use of some high-intensity LEDs we took off a string of holiday lights. We decided to use Arduino as our microcontroller platform. To simplify construction, each LED was wired to one pin, with the grounds soldered together.

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In testing, these LEDs were receiving about 4.4 V (rms) from their main supply (220 V), so we figured they could handle the Arduino’s 5 V for a short duration without damage. Three of the LEDs together made a light source too bright to look at directly, and that passed for a flash in the interior setting of the prom. Thus, for 8 “cameras” we needed 24 LEDs, and thus two Arduinos.

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A former student, currently studying computer science, wrote up the straightforward Arduino script. We considered drawing a random number (0 to 3) to choose which set of 3 LEDs to light, and another random number (0 to 2000) to determine the delay between flashes, but a constant flash interval of 1 second looked better. Indeed, it looked quite good, and after 2 hours of stripping wire and soldering connections, the students were justifiably proud of their work.

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LEDs in a Jar

Our Design Technology teacher asked for some technical help with a project involving LEDs. We tracked down a shop that sells LED strips, and I picked up a meter of high-intensity bulbs. As a physicist, I was curious about the stopping voltage, currents limits, and heat dissipitation, but it turns out these strips are designed with three LEDs and accompanying resistors on each 5 cm length, so the strip is simply supplied with a 12 V supply. The wall wart supplies have power ratings, and power is used to calculate the appropriate supply. As 0.12 W per LED, with a total draw of 7.2 W for the strip of 60 LEDs, I was able to use a power supply rated to 8 W, or 0.67 A at 12 V to my mind.

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A smart persin suggested that LEDs in a bottle would look nice, so after soldering the power supply to the strip, I dumped it into a glass pitcher. A clear bottle would probably look better.

Up next: students investigating aspects of lighting. I cannot wait to see what they come up with!