reading the frequency of a sine wave

Question

I'm trying to build a speedometer for my car, using the output from the cars speed sensor. The speed sensor outputs a sine wave. here it is on an oscilloscope:

as I understand it, in order for the arduino to be happy, this signal needs to be between 0 and 5v.

It seems that the peak to peak of this wave is about 8v-ish, which means if I somehow "chop" the negative voltage off of this signal, I should be left with something like 0-4v which is what I need.

How do I go about reading this signal by the arduino? I'd like to keep the circuitry simple due to the harsh(ish) environment that it'll be sitting in.

I've seen some people suggest simply a diode and a resistor, and others suggest op amps, schmitt triggers, and other concepts I'm not strong on.

Any help is very welcome!

Answer 1

There are several ways. Using an op-amp and a few discrete components, you can get a zero-crossing detector that provides a pulse no larger in amplitude than the positive supply rail to the op-amp, which can be detected with an external interrupt or pin-change interrupt on the Arduino.

Below is one example of a zero-crossing detector.

^{simulate this circuit – Schematic created using CircuitLab}

Answer 2

From what I see in your scope picture, your signal spans a range from roughly −4 V to about +2.5 V. The main issue with this signal is not its negative part, it's the fact that the positive voltage does not go high enough. Most Arduinos are powered at 5 V and have Schmitt triggers on their digital inputs. The voltage threshold for the pin switching to HIGH is typically about 2.6 V, but this is only a typical value. The only thing the datasheet guarantees is that the pin will read HIGH if its potential is at least 0.6 V_CC, i.e. 3 V on a 5 V Arduino.

One option would be to use an Arduino powered at 3.3 V, like the 3.3 V version of the Pro Mini. With this, the digital pins are guaranteed to read HIGH at 2 V. This is, however, borderline for your signal. Your picture shows that some of the oscillations barely reach 2 V at their maximum. Thus I would avoid this option.

The other option is to add a DC offset to the signal in order to have it oscillate around V_CC/2 = 2.5 V. This way you have a shifted sine wave that goes roughly from −0.75 to +5.75 V. Then you are guaranteed to hit both the threshold for reading LOW, which is no lower than 0.3 V_CC = 1.5 V, and the threshold for reading HIGH. I would use a circuit like this:

^{simulate this circuit – Schematic created using CircuitLab}

The voltage divider creates a DC offset of V_CC/2. The capacitor on the left allows the AC waveform in. Together, the capacitor and the voltage divider make a high-pass filter with a cutoff frequency of 1/(πRC). If you choose, say, R = 100 kΩ and C = 1 µF, then you have a cutoff at 3.18 Hz. Your signal is about 300 Hz, thus you should be able to measure a signal up to 100 times slower than the one pictured.

As for the negative part of the signal, the Arduino inputs have protection diodes that take care of it. You just have to make sure that the current through those diodes does not exceed 1 mA. This is why the above filter has a current limiting resistor after the voltage divider. A 10 kΩ resistor ensures you can safely overshoot the normal input voltage range by as much as 10 V, i.e. you are safe as long as the shifted sine wave stays between −10 V and +15 V.

Edit: As pointed out by Dmitry Grigoryev's comment, when the car speed is zero this circuit will let the noise through. Having some noise on a digital input is often not an issue, as it is cancelled by the input Schmitt triggers. Your noise amplitude, however seems quite large, and is probably larger that the ~ 0.5 V of hysteresis you have on the Arduino inputs. Then you may detect spurious transitions that will be seen as a finite speed.

If this happens, a simple fix is to change the DC offset in order to move it away from the transition thresholds. For example, a 47 kΩ pull-down combined with a 100 kΩ pull-up will set the DC bias to 1.6 V. You can use more asymmetric resistors if you need to make the DC bias still lower. Note that changing the resistors will also change the cutoff frequency. The 47 kΩ/100 kΩ resistors combined with a 1 µF capacitor will have a cutoff of about 5 Hz.

Answer 3

Assuming the speed is proportional to frequency and your plan is to measure it by counting the edges, something as simple as a common-emitter BJT should work fine:

^{simulate this circuit – Schematic created using CircuitLab}

Most BJTs can withstand a few volts of reverse voltage with no problem, and 2V of positive voltage are plenty to drive them in saturation. The fact that the signal will be inverted and the duty cycle won't be 50% shouldn't prevent you from counting the edges properly.

Answer 4

Once you get the schematic figured out from the excellent answers so far, set up a rising edge interrupt: https://www.arduino.cc/reference/en/language/functions/external-interrupts/attachinterrupt/

This will let you execute a bit of code each time the waveform goes from negative to positive. I actually wrote some code to do this a while back in order to measure some PWM stuff ( convenient 0-5v input, none of the filtering issues you are dealing with). I chose to keep a FIFO list of crossing time deltas, and then to get the freq, added them up and bit shifted to divide them and get the avg of those samples. The arduino has no hardware division so that dictated the number of samples in order to be able to do the bitshift division. Without averaging, I found the signal to be very jittery.

Answer 5

You could use a resistor, diode and zener diode like the circuit pictured here:

I used "mains" voltage with a step down transformer to test the output of the circuit which looks like this: