Speedometer Setup
Following a desire to monitor certain vehicle telemetry, it was determined that a speedometer was amongst the instrumentation that the team would like to have incorporated into the new car. Instead of finding a complete system for monitoring the vehicle speed, it was instead decided that a random collection of mismatched parts would be a more suitable substitution. After all, anything too easy is rarely worth doing.
A Hall effect sensor was selected as the triggering device. It works by sending a high output signal whenever a ferrous (i.e. made of iron/steel) object passes within close proximity. For this application it is being used as a gear tooth sensor, sending a high signal whenever one of the teeth of a sprocket passes through its field. The sprocket can be mounted on part of the drivetrain system of the vehicle, thereby allowing this sensor to monitor the speed of that shaft, from which the vehicle speed can be extrapolated.
The speedometer that was selected is a digital unit with an analog display. It has a backlight for all of the nighttime driving that the race cars see. It can be calibrated to accept different ratios of input signal to corresponding speed in MPH, though the documentation for doing that is notably missing in action.
The speedometer is calibrated to the system using a series of 10 DIP switches. It is believed that they can be interpreted as bits in a binary sequence specifying some scale factor.
Tools.
Batteries, the Hall effect sensor and speedometer, and a digital multimeter.
Various and sundry electrical supplies.
In order to not explode, the Hall effect sensor requires a 1.2kOhm pull up resistor between the input voltage and the output side of the sensor. This is achieved by connecting a 1kOhm resistor and two 100Ohm resistors in series.
Ever wonder how to identify a resistor? Those colored lines mean something, but what? Don't worry; Radio Shack has your back.
The resistor setup was prototyped on a breadboard, and the effective resistance was verified with the digital multimeter so as to avoid any smoke or small explosions.
The wiring was put together with crimp connectors and a barrier strip. The speedometer was fitted with a switch to power the backlight, and another to (supposedly) reset the device, though the functionality of the latter is looked upon with some skepticism based on the fact that it doesn't seem to do anything.
A sprocket was mounted on a shaft, which was in turn mounted in a lathe to provide a constant rotational speed. The Hall effect sensor was fitted in a tool holder, and the lathe carriage was used to locate the sensor in proper proximity to the sprocket, as required.
The spacing between the sensor and the sprocket is important to insure proper signal strength, while also mitigating possible (and catastrophic) collisions.
The entire speedometer system was setup on the lathe for testing and calibration. Calibration was carried out empirically, as proper documentation for the digital speedometer could not be obtained.
A video of the test setup in operation is available: Speedometer Test Video. The system is shown running at two different speeds, as controlled though the lathe gearbox, with the speedometer reporting an equivalent MPH for each. Subsequent calibration brought these values into agreement with calculated drivetrain speeds provided by the powertrain group. Additional calibration may be required depending on how the system is integrated into the greater powertrain design.
I welcome any questions, comments, or concerns.