Most of us have been on the road and stuck behind someone who is just driving too slow. We’ve also had someone tailgating us when we’re clearly going the speed limit (yeah… exactly the speed limit). It’s possible that these irritating situations are caused by miscommunication. Speedometers aren’t always accurate, and maybe your speedometer is showing a different reading than the other car’s speedometer, even though both cars are traveling at the same speed. That’s why Pierre Muth created a speedometer for his car’s rear window so that other drivers know exactly what speed his car’s speedometer is showing.
The speed is shown on a pair of gigantic seven-segment displays, each of which measures a whopping 7 x 10 cm (about 2.75 x 4 inches). Those are the LCDS101D40TR model made by Lumex, and they are controlled by an ESP32 board through a Microchip PIC16F19156 microcontroller. The PIC16F19156 has enough pins to drive the 14 total segments of the displays without requiring multiplexing. Speed is shown in kilometers per hour, meaning the maximum it can display is 99Km/h. But anything higher than that and any safe driver should be too far behind Muth to read the display anyway.
The ESP32 development board gathers the car’s current speed data through a cheap ELM327-based OBDII scanner. The car in question is a newer Volkswagen Beetle, so a great deal of the data from the car’s computer is available via the CAN (Common-Area Network) bus, which all newer cars have some variant of. The boards are housed within a simple 3D-printed enclosure that fits into the car’s rear cup holder, where there is a USB port for power. The seven-segment displays fit into a 3D-printed frame, which can be mounted by the rear windscreen. We’d be a little bit nervous about giving the fuzz such an easy way to determine our speed, but this is an interesting solution that could potentially deter tailgaters.