Speeduino Tachometer (rev counter) Connection

This video investigates the rpm connection to the Speeduino 0.3.x engine management system.
Below is all the text used in the video which can be used for translation, if needed. :-)
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• Speeduino Tachometer (...
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[@_vlnt_7800]

Nice job! You just got a new sub

[@lyndonevans1251]

Introduction
2/15
I wanted to know what the signal output from the Speeduino (0.3.x or 0.4.x) rpm pin looks like on a 4 pot 4 stroke engine
I assumed it would be a 5volt square wave at a frequency of rpm*2, which it is.
I decided to get the oscilloscope out and look at the signal and this is what I found
Speeduino/ Arduino Output Pin
3/15
The rpm out signal comes from the Arduino pin 49.
There is no specific RPM connection on the 0.3.x Speeduino board
Speeduino/ Arduino Output Pin Connection
4/15
Speeduino pin 49 is conveniently placed next to the proto area, so a simple bridge connection can be made between the pin base and an unused section of the proto area, thus
Connecting The Output
5/15
The RPM out is a direct connection to the Arduino so it can only sink a very small current and it is a 5 volt signal. (Tachometers need a 12 volt signal)
The following diagram shows the output connected through an Opt-isolator, this protects the Arduino and allows us to convert the signal to a more robust 12 volt version of the output.
This opt-isolator converts the signal from 5 volts to 12 volts
This is the circuit:
6/15
A87 4 Channel Opto-coupler Isolation Module High And Low Level Expansion Board - Banggood et al
Calculating RPM From Frequency First
8/15
From this you can see the wavelength time is about 25ms (5 squares at 5ms per square)
Frequency is 1000/25 = 40Hz (One second/ wavelength)
RPM = frequency * 60/2 = 1200 (convert from sec. To min.)
All good so far
The second picture shows the signal at just over 6500rpm
9/15
Calculating RPM From Frequency Second
10/15
From this you can see the wavelength time is about 4.8ms (just under 1 square at 5ms per square)
So frequency is 1000/4.8 = 208Hz
RPM = frequency * 60/2 = 6250
Still good.
Things to note
11/15
The OFF time for the wave is constant at 3ms (configurable in TS), the ON time decreases as the rpm/frequency increases.
TunerStudio config window>
The 3ms rpm pulse width is OK to about 9000rpm but you will need to decrease it if your engine revs above.
12/15
The next picture/video shows what happens to the signal as the rpm increases, if you notice a dropout at the top of the rev range this is my rev limiter kicking in
Connections 1
14/15
There are no components mounted on the proto area just two wires connected to the Speeduino board.
It uses the A87 four channel opt-ocoupler module available from eBay or Banggood. It costs about $2.
The two connections from the Speeduino are:
1. From ground on the Speeduino to IN Gnd on the A87.
2. And from Arduino pin 49 to IN4 on the A87 this is a 0volt or 5volt digital pulse, the input put are opto-isolated.
Connections 2
15/15
From the output side of the A87 we are are using only one of the four channels (three are still available)
The connections used on the output side of the A87 are:
1. Gnd is connected to the vehicle Ground.
2. Hvcc is connected to the vehicle +12 volts.
3. OUT4 this is the Tach signal output and is a 0volt/Hvcc digital pulse so when Hvcc is +12 volts the signal is 0volt/+12volt.
Hvcc can be in the Range 5 to 24 volts. So the device can also act as a protocol converter.
The 0/5 volt pulse from pin 49 is converted to a 0/12 volt pulse to the tach.
Test setup
16/15
My test rig is a Speduino 0.3.7 connected to a SpeedySim and a stock after-market tach with three wire connection: Gnd, +12volts and tach signal.
Here is a short video clip of the setup in action.
Thanks for watching
18/15
All the text in this video is listed below so you can translate it if you need to.