When one of my friends gave me a warning about trailers being invisible, I decided it would be nice to have flashing tail lights. But would LEDs be bright enough to see in daylight?

In the picture above, the LED on the left is a 5 mm diameter 8500 mcd LED with a 30° viewing angle. The one on the right is a 10 mm jumbo LED, that automatically flashes at a rate of a few times a second. Each was 99¢ at Halted, a local electronics surplus store. Unfortunately, they were sold out of the non-flashing jumbo LED, which would have been only 19¢ or so.

Even discounting the intermittent nature of the jumbo LED, the smaller one was much brighter, and clearly visible in daylight with a 20 mA forward current. I will drive at the recommended current of 30 mA in the actual trailer.

It would have been fun to control the tail lights with the same processor as the power transfer controller, opening up sophisticated possibilities such as turn signals, brake lights, and exotic flash patterns. However, as a safety feature, the tail lights must function at night, which means that I must power them from the batteries. The architecture of my design prohibits me from powering the processor from the batteries, because one of the design goals is to be able to run without a battery attached.  So the flasher circuit will be separate from the digital electronics.  I might even use discrete transistors, to avoid the need for a voltage regulator.