Ever wonder how to calculate revolutions per minute using a microcontroller? This project shows you how by purposing an IR emitter and detector and a computer fan. As the fan blades spin they disrupt the beam of infrared light between the emitter and the receiver. This results in a waveform on the receiver’s circuit which can be easily used to trigger interrupts in any microcontroller. In this case a PIC 18F452 monitors the detector’s signals for a rising edge. By measuring time data between interrupts the period can be established and RPM calculated. You can see a video of the test rig after the break.

So what can you use this for? It’s the method that most spinning POV displays use to stabilize the display. You won’t be limited to an IR sensor, but can use a hall effect sensor in the same basic fashion.

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We’ve had a few folks send us info about their vehicle display hacks after seeing [Will O'Brien's] motorcycle computer a few days ago.

On the left we have a display for an electric vehicle. [S1axter] is using a 4.3″ TFT screen to display charge information for each battery cell in the car. An ATmega88 collects the data and sends it to a breakout board with an LCD controller on it.

To the right is a display from a Formula Student project. a Matrix Orbital GLK19264-7T-1U LCD display provides a lot of real estate for displaying data. Right now [Alan] is still in the early prototyping stages, but the video after the break demonstrates the RPM readout using a function generator. It’s not shown in the video, but he tells us that he’s since tried it out with the engine and has a PIC 16f877 reading temperate data from the electronic control transmission sensors in addition to the RPM data.

Correction: Thanks to [j] for correcting our mistake. This is a Formula Student car.

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[fibra] has been slowly building a custom controller for his motorcycle. It’s an automated chain oiling system that varies application based on RPM. The LCD can show wheel RPM, voltage, time, date, air, and engine temperature. A separate driver board has a MOSFET for controlling the oiling valve. The real gold here is the attention to detail. He built a one off circuit board. The case is laser cut acrylic that he then shaped. The box is molded smoothly into the original instrument cluster using epoxy. It’s excellent work that could be mistaken for a commercial product.

Flow sensors are useful tools for collecting data on the rate of liquid usage, but they need a device to display the data they collect. This three digit frequency meter was designed by [Turbokeu] to do just that, converting a Swissflow SF800 flow sensor’s square wave signal (similar to fan RPM signals) into an numerical expression of liters per minute on a 3 digit LCD. Fan RPM is

[Turbokeu] provides detailed schematics of different configurations for the frequency meter as well as schematics of the layouts of the two PCBs that are used. Even if you don’t have an immediate use for a frequency meter, his clean and readable schematics are worth a look in their own right. The display is installed on front of a tower case along side a CPU speed display.

[via YourITronics]