Filtering Tacho signal to 5v pulses

[takai]

Ok, just doing a bit of work with an Arduino for an exhaust controller and i need to filter the 12v pulse for the tacho down to 5v pulses for the arduino to count.

I was planning on doing this:

Tacho output ----20k----> Arduino
.....................|
.....................4.7v 1w zener
.....................|
.....................Gnd


I was going to use this Zener for the ground pull: http://www.jaycar.com.au/productView...r&form=KEYWORD because i have a bunch lying around.

Will it work? Thoughts?

[RA35GT]

Wouldn't a voltage divider with two resistors be easier?

[takai]

Easier, yes. But the Zener provides a bit better overvoltage protection.

[wilbo666]

Another option is to have the tacho signal drive an NPN and have a pull up to +5V on Atmel side (Or use the internal pull up...).



Cheers
Wilbo

[river]

Hi,

Just run it through an opto isolator. 12volt into the opto part and run your 5v off the transistor output. Depending on the opto device you use, you may need to run a shunt resistor to limit the current, as per any normal LED running on 12v.

30mA is a good value for current through the LED on the opto, so you'll need to run a 470ohm resistor from the output of the tacho into the opto.

Use something like the 4N27 device. You can find its data sheet on the web Here it is.....

http://www.datasheetcatalog.org/data...ild/4N27-M.pdf

If you're not sure how to connect/use the opto, here's a link witha few circuits to help you out...
http://www.discovercircuits.com/O/opto.htm

seeyuzz
river

[takai]

Another option is to have the tacho signal drive an NPN and have a pull up to +5V on Atmel side (Or use the internal pull up...).



Cheers
Wilbo

Yeah, but surely the voltage limiting is easier?

Hi,

Just run it through an opto isolator. 12volt into the opto part and run your 5v off the transistor output. Depending on the opto device you use, you may need to run a shunt resistor to limit the current, as per any normal LED running on 12v.

30mA is a good value for current through the LED on the opto, so you'll need to run a 470ohm resistor from the output of the tacho into the opto.

Use something like the 4N27 device. You can find its data sheet on the web Here it is.....

http://www.datasheetcatalog.org/data...ild/4N27-M.pdf

If you're not sure how to connect/use the opto, here's a link witha few circuits to help you out...
http://www.discovercircuits.com/O/opto.htm

seeyuzz
river

Seems quite overly complex when all i have to do is drop the voltage from 12v to 5v.

Tacho is currently spliced into the IGF pin on the ECU.

[river]

Hi,

Yeah, a single resistor and a 6pin chip can over complicate things.

seeyuzz
river

[takai]

A single resistor and a diode does nigh on exactly the same thing.

Also the 4N27 has a ramped output, rather than a single pulse. Depends on the edge detection of the Arduino, but it could get quite finnicky.

[oldcorollas]

the opto isolates it tho.. so rather than relying on the zener to pull voltage down and save your arduino from smoke, the opto/NPN removes the possibility?

how ramped is the tach pulse compared to the output of the other options?

[river]

Hi,

Yes, the extra advantage of the opto is it isolates the system from 12V. If the zener goes AWOL you'll get 12V into the Arduino.

seeyuzz
river

[takai]

And if the resistor goes i lunch the opto... not much difference. The Arduino has polyfuses incase of overvolting.

Also, getting the opto is a pain. Its either Farnell or RS components, and ill almost be paying more for postage than an entirely new Arduino.

NPN is readily available. Not sure of the ramping characteristics though.

[RA35GT]

I used IGT as the input to my tacho corrector. On the UZ it was already 0-5v.

[takai]

Interesting about IGT already being 5v, ill have to check what it is on the 2JZ ecu.

NPN looks easy enough to implement, especially that they have added code to allow for interrupts on falling.

Any suggestions for an NPN to handle a 5-12v input?

[river]

Hi,

Why not just use an open collector buffer? Run 12v into the device and tie the output to 5v with a resistor. That's simple and also keeps the unit isolated. You can use a TTl or CMOS open collector chip.

seeyuzz
river

[takai]

Because again, getting parts sucks. Farnell charge through the nose for their "service" and Jaycar stocks buggerall.