# 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.

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.

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.

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.

# 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.

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!