Crash-Alert Helmet: Signal Strength or Pulse Codes?

Building a Helmet That Calls for Help

1The bike-safety club at Harmon Middle School is designing a helmet that calls for help. If a rider falls hard, a sensor in the helmet should send a signal to the rider's phone, and the phone texts a family member the rider's location. Club members Yaretzi Calloway and Oskar Nwosu know that two kinds of mistakes matter. A false alert, a text when there was no crash, scares a family for nothing. A missed crash, with no text at all, could leave an injured rider without help.

2The sensor rates every bump from 0 (no bump) to 15 (a very hard hit). A score of 10 or more counts as a crash. A small radio in the helmet sends information to the phone, but on a city street the radio waves also pick up noise, small random changes caused by electric motors, power lines and other radios.

3The club built two versions. Version A sends an analog signal: the strength of the radio signal rises and falls smoothly with the score, so a stronger signal means a harder hit. The phone measures the strength and turns it back into a score. Version B sends a digital signal. A chip in the helmet picks one of four messages and sends it as a pattern of four short pulses, each either on or off. The phone only has to decide whether each pulse is on or off. If a pattern arrives that is not in the code table, the phone asks the helmet to send the message again.

4To test the two versions safely, the club strapped each helmet onto a foam test head and dropped it from a height that always gives a score of 7, a hard bump but not a crash. They ran an electric bike motor closer and closer to the phone to make five levels of noise, from 0 (none) to 4 (strongest). At each noise level they dropped each helmet 20 times and recorded what the phone showed. The drop height, the helmets, the phone and its distance from the helmet stayed the same in every trial.