Figure 1 in A powerful new light source for ultraviolet detection of scorpions in the field

Figure 1: Circuit diagram of the prototype LED array. The array has a scalable architecture, here with N = 56 parallel branches. Each branch includes three LEDs in series with a single current-limiting resistor (Rs = 39 Ω). Typical forward voltage drop measured across one diode was 3.4 V, giving a total diode drop of 10.2 V in each branch. During operation, voltage across the 12 V battery terminals was Vb = 11.5 V, so the current in each branch was (11.5-10.2)/39 A ~ 33 mA. This kept the current near the maximum rating of 30 mA specified by the manufacturer for 100,000 h lifetime. Each resistor dissipates ~ 1.3 V × 33 mA = 43 mW; the 3 diodes in a branch dissipate 3 × 3.4 × 33 mA = 337 mW. By using 3 diodes to maximizing total diode voltage drop, resistor power loss is kept down to about 13% of the total power. We have implemented the simplest possible circuit to drive the LEDs, one that can be easily built by biologists who have not trained in electronic engineering. More sophisticated driver circuits might be devised to regulate or balance current flow to different branches of the array, but they probably offer little additional benefit for the purposes of scorpion detection.