FAQ
TL;DR: Use the tach pulse to count RPM, then drive LED brightness with PWM. A 4‑cyl coil gives 4 pulses per revolution; "it sends a square wave signal." [Elektroda, stanton Henkle, post #21663407]
Why it matters:** It’s a simple, low-cost way to make RPM‑reactive accent lighting without tapping the head unit or OBD-II.
For: DIY auto‑electronics tinkerers who want LED brightness to follow engine RPM using an ECU/coil tach signal.
Quick Facts
- Tach pulse type: square-wave from coil/tach line; frequency rises with RPM. [Elektroda, Calinoaia Valentin, post #21663406]
- Typical 4‑cyl coil: 4 pulses per crank revolution. [Elektroda, stanton Henkle, post #21663407]
- 2012 Subaru: dashboard data communicated over CAN; cluster receives encoded RPM. [Elektroda, Calinoaia Valentin, post #21663404]
- Control method: count pulses, convert to PWM, modulate LED brightness. [Elektroda, Adam Taylor, post #21663401]
- Beginner-friendly platform: Arduino boards with abundant examples and libraries. [Elektroda, Calinoaia Valentin, post #21663410]
How do I make LED brightness track engine RPM from the tach signal?
Count tach pulses to compute RPM, then map RPM to a PWM duty cycle that drives the LEDs. As Adam Taylor puts it, "count the pulses and hence determine the RPM," then output PWM for brightness control. [Elektroda, Adam Taylor, post #21663401]
Does a 2012 Subaru WRX use CAN for RPM, or a simple wire I can tap?
A 2012 Subaru routes dashboard information over CAN, so the cluster receives encoded RPM and other data. If you tap a cluster wire, confirm it’s a usable pulse and not just an internal control line. Otherwise, you’ll need CAN decoding to extract RPM. [Elektroda, Calinoaia Valentin, post #21663404]
Where can I find a tach signal to tap on a WRX?
Users report a single blue tach wire heading to the tachometer. The same line often feeds aftermarket tachs. As an alternative source, some tap the ignition coil negative terminal for a usable square wave. Verify with a scope before connecting electronics. [Elektroda, stanton Henkle, post #21663405]
How many pulses per revolution should I expect from a 4‑cylinder coil signal?
A typical 4‑cylinder coil tach feed produces 4 pulses per crankshaft revolution. Use this pulses‑per‑rev value in your RPM calculation and PWM mapping. This statistic helps size your measurement window and avoid flicker at idle. [Elektroda, stanton Henkle, post #21663407]
Which microcontroller is easiest for this—Arduino, PIC, or FPGA?
Choose Arduino for the fastest start. It has ready examples and libraries that simplify pulse counting and PWM output, ideal for beginners and quick prototypes. Any common Arduino board will work for counting and PWM generation in this use case. [Elektroda, Calinoaia Valentin, post #21663410]
I’ve coded in Java/Basic—can I handle this project?
Yes. With prior coding experience, you can follow Arduino examples to read frequency and output PWM. The ecosystem’s sample code and libraries reduce complexity, so you can focus on mapping RPM to LED brightness. [Elektroda, Calinoaia Valentin, post #21663410]
Do I need to decode CAN to get RPM on newer cars?
If your car sends RPM over CAN, you must decode that bus first to obtain engine speed. Without decoding, a random cluster wire may not expose a clean RPM pulse. Confirm your vehicle’s signaling path before designing the LED driver. [Elektroda, Calinoaia Valentin, post #21663402]
What is CAN bus in this context?
Here, CAN bus is the vehicle network carrying RPM, speed, temperature, fuel, and indicator data to the dashboard as encoded messages. The cluster interprets these signals rather than receiving raw analog RPM. [Elektroda, Calinoaia Valentin, post #21663404]
How do I convert RPM to LED brightness (simple How‑To)?
- Count tach pulses over a fixed window to calculate RPM.
- Map RPM to a duty cycle curve that fits your brightness goals.
- Output PWM at a stable frequency to drive the LEDs accordingly.
“Count the pulses and hence determine the RPM,” then generate PWM. [Elektroda, Adam Taylor, post #21663401]
Can I read from the coil’s negative terminal instead of the ECU?
Yes. The coil negative provides a square wave correlated to engine speed, and for a 4‑cyl it’s 4 pulses per revolution. Condition the signal before feeding a microcontroller input. This is a common alternative when the cluster line is inaccessible. [Elektroda, stanton Henkle, post #21663407]
What output signal controls LED brightness?
Use a microcontroller PWM output to command LED brightness. After converting pulses to RPM, drive a PWM signal that modulates the LED intensity in proportion to engine speed. Keep PWM frequency high enough to avoid visible flicker. [Elektroda, Calinoaia Valentin, post #21663406]
Why might tapping the blue tach wire not work?
If the dashboard relies on CAN, the blue wire may not carry a raw RPM pulse. In that edge case, the signal could be internal to a stepper driver or logic circuit, so you must decode CAN for RPM instead of using the wire directly. [Elektroda, Calinoaia Valentin, post #21663404]
