Cycling can be dangerous, particularly in American cities where most commuters drive cars everywhere they go. There are many reasons that riding a bike is dangerous, including that they offer no protection in the event of a collision. But in almost every accident involving a bicycle and a car, you will hear the car driver say something like "I just didn't see them." The obvious solution is to do everything in your power to make yourself as visible as possible while cycling. This LED-lit warning sign designed by Tegwyn☠Twmffat will help you increase your visibility.
This is a large sign that attaches to the back of a bicycle. It has 180 LEDs that form the classic caution sign and the words "KEEP CLEAR." Those LEDs are normally dim, but will illuminate when a car gets too close to the back of the bike. The sign uses an ultrasonic sensor to detect the presence of cars. A linear slider-style potentiometer lets the user adjust the brightness of the LEDs, so you can turn them up during the day and down at night. It might have been possible to use a light sensor for automatic adjustment, but the headlights of tailing cars could have affected the readings.
The entire sign is one huge PCB, which you can order from any modern PCB fabrication house. An Adafruit Feather M4 Express development board controls the LEDs through L293E drivers. The ultrasonic sensor is a high-end model from MaxBotix and it is the most expensive component in this build. Power comes from a standard 5V USB battery bank. If there is any oversight here, it is that this sign is not weatherproof. As shown, the unprotected PCB mounts straight onto the bike. But if you only ride in sunny weather or can find a way to protect the sign from the rain, this is a great project to make cycling a bit safer.