The Sigma Brakelight is a device that illuminates when the bike's brake calipers close together, alerting nearby motorists that your bike is about to brake.
The light is water-resistant and is attached using an Allen key to the rear brake cable between the two brake calipers. It is compatible with any cable-activated mechanical rear brake. When the calipers are squeezed together while braking, they provoke a mechanism to complete a circuit and illuminate a red LED light.
The Brakelight weighs only 7 grams and it is powered by a fully replaceable 1025 button cell battery that is good for as many as 100,000 braking instances. It is available in a range of five colors, and is priced at a very reasonable $9.99, making it an exceedingly good option for the cyclist on a budget.