PC Fans

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** or, fed 3.3V: 680Ω (4.9mA)

* 24V: 3KΩ (6V, 2mA)

Arduino 5V internal pullup resistors are several tens of thousands of ohms, so they can be used to simplify the circuit (on a 3.3V MCU, you'll need hook up the tachometers to at least a 5V V<sub>CC</sub>, so this won't help you there). You'll then want an RC circuit going to ground, ~~something like a 1nF ceramic capacitor and a 15kΩ resistor (this results in~~ a [https://en.wikipedia.org/wiki/Low-pass_filter#RC_filter low-pass filter] at 10.~~6KHz (f~~ = 1/2πRC)~~; you could go lower)~~. The tach will pull the signal low. It's best to detect this via hardware interrupts:

Arduino 5V internal pullup resistors are several tens of thousands of ohms, so they can be used to simplify the circuit (on a 3.3V MCU, you'll need hook up the tachometers to at least a 5V V<sub>CC</sub>, so this won't help you there). You'll then want an RC circuit going to ground, yielding a [https://en.wikipedia.org/wiki/Low-pass_filter#RC_filter low-pass filter]. Given a time constant RC, a low-pass filter blocks signals over 1/2πRC. Pick something a fair amount above your expected frequency. Let's say your fan maxes out at 3000 RPM. That's 3000 / 60 = 50 RPS. The Hall sensor triggers twice per revolution so you're expecting 100 transitions per second. If you're using a 680Ω resistor on input, and want to pass signals below 500 Hz, a 470 nF capacitor will do the trick: 1/2πRC == 1/2π(.0003196) == 497.98 Hz. The tach will pull the signal low. It's best to detect this via hardware interrupts:

static _Atomic(uint32_t) LowerFanPulses, UpperFanPulses;

PC Fans: Difference between revisions

Dank (talk | contribs)

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 ** or, fed 3.3V: 680Ω (4.9mA)
 * 24V: 3KΩ (6V, 2mA)
-Arduino 5V internal pullup resistors are several tens of thousands of ohms, so they can be used to simplify the circuit (on a 3.3V MCU, you'll need hook up the tachometers to at least a 5V V<sub>CC</sub>, so this won't help you there). You'll then want an RC circuit going to ground, something like a 1nF ceramic capacitor and a 15kΩ resistor (this results in a [https://en.wikipedia.org/wiki/Low-pass_filter#RC_filter low-pass filter] at 10.6KHz (f = 1/2πRC); you could go lower). The tach will pull the signal low. It's best to detect this via hardware interrupts:
+Arduino 5V internal pullup resistors are several tens of thousands of ohms, so they can be used to simplify the circuit (on a 3.3V MCU, you'll need hook up the tachometers to at least a 5V V<sub>CC</sub>, so this won't help you there). You'll then want an RC circuit going to ground, yielding a [https://en.wikipedia.org/wiki/Low-pass_filter#RC_filter low-pass filter]. Given a time constant RC, a low-pass filter blocks signals over 1/2πRC. Pick something a fair amount above your expected frequency. Let's say your fan maxes out at 3000 RPM. That's 3000 / 60 = 50 RPS. The Hall sensor triggers twice per revolution so you're expecting 100 transitions per second. If you're using a 680Ω resistor on input, and want to pass signals below 500 Hz, a 470 nF capacitor will do the trick: 1/2πRC == 1/2π(.0003196) == 497.98 Hz. The tach will pull the signal low. It's best to detect this via hardware interrupts:
 <syntaxhighlight lang="c">
 static _Atomic(uint32_t) LowerFanPulses, UpperFanPulses;