For any add-on boards such as VR conditioners, optos and OEM interface boards
By Falcao
#49068
Hi, I am working for a while now on a digital dashboard, I am currently designing the tachometer input. As I want my display to work with as many signal types as possible, I was wondering if the following schematic would work with either active hall effect sensor, open-drain hall effect sensor, pwm from ecu (5v) and direct signal from low side coil (from 5v to 50v let's say).

So let me know what you think of the first schematic, and if the seconde scheme is well designed to protect the level switching mosfet from too high voltage (5.1V zener diode).

Thank you very !
Attachments
schem.png
schem.png (6.16 KiB) Viewed 4438 times
#49092
Leave the 100K resistor away from its position in the second schematic and put a 20k in line with the rpm signal. That way the input of the FET will not see more than 5 volt.

Have a look how others do such an input. It is better to copy something that works well, than to develop something you have no clue about and burn it through at every second try.
#49124
Thanks for the heads up !

The thing is that designs I can find don't cover all my requirements, the level shifting using a mosfet is already inspired from someone's design, and I like to learn things :)

So apart from moving the resistor, do you think this will work for all my needs ?
#49159
Here is an older document from MS that will show you what you are dealing with. Even as Speeduino is newer, MS has had any problem just the same. So MS is, with knowledge of the differences, a very complete source for working solutions.
Try to understand them and you can see why your idea´s may not work.
There are solutions to any input signal you may find. Maybe you will come to the conclusion the universal input does not exist. With a software that analyses the signal, you could do it, but in pure simple hardware there are limitations.

http://www.msextra.com/doc/general/tachin-v22.html
#49165
SilentBob wrote:Could use an optoisolator as a level shifter.
Yes, I thought about that solution, in fact, I'm already using optoisolator for +12v digital inputs but with a fixed resistor protecting the led, it's hard to handle voltage range from 5v to 50v :(
Chris Wolfson wrote:Here is an older document from MS that will show you what you are dealing with. Even as Speeduino is newer, MS has had any problem just the same. So MS is, with knowledge of the differences, a very complete source for working solutions.
Try to understand them and you can see why your idea´s may not work.
There are solutions to any input signal you may find. Maybe you will come to the conclusion the universal input does not exist. With a software that analyses the signal, you could do it, but in pure simple hardware there are limitations.

http://www.msextra.com/doc/general/tachin-v22.html
Thank you for the link, a great source of informations there !
#49166
Falcao wrote: Sat Mar 20, 2021 10:27 am
SilentBob wrote:Could use an optoisolator as a level shifter.
Yes, I thought about that solution, in fact, I'm already using optoisolator for +12v digital inputs but with a fixed resistor protecting the led, it's hard to handle voltage range from 5v to 50v :(
If it's just to pull a mcu pin low something like a 2.4K resistor would cover from 5 to 50V. Absolutely no protection in that circuit i posted though
#49170
SilentBob wrote:If it's just to pull a mcu pin low something like a 2.4K resistor would cover from 5 to 50V. Absolutely no protection in that circuit i posted though
Yep, I'll make a testboard with both the mosfet and the optoisolator solutions to see which one works best for my needs

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