Tacho Circuit

[jkutia]

Hi guys, I have built a custom dash display/logger for my car. The tacho signal conditioning uses the Megasquirt 3 design here http://www.msextra.com/doc/general/ms3v3schems.html , using the negative side of the ignition coil as its source.

The issue I am having is heat in the resistor; I started with a 390 ohm 1/2 watt resistor as seen in the schematic, which got hot after running for a while. So I played around with the values, before settling at 3k. This too got hot so I replaced it with a 5W wire-wound resistor. After running the car for a while, the resistor only got a bit warm so I put it back together and left it at that.

All was working fine until after around 1hr of use (racing), the car ignition died momentarily, then came back again, along with a burning smell - I immediately knew it was from the tacho circuitry. I am yet to check each component but there has obviously been a failure on the "car side" of the optocoupler.

Has anyone else experienced this? If so, how did you get around it? I could probably tap into the crank sensor wiring, but I don't really want to load any ECU sensors.

Thanks

[jsmcortina]

Why not just connect to the tacho-out circuit from your Megasquirt?

James

[jkutia]

There is no Megasquirt, the dash display is a standalone unit I built (using an ATMega MCU).
I have used the Megasquirt tacho signal conditioning circuit, where the optocoupler output is fed into a timer capture pin.

Cheers

[jsmcortina]

This support forum is for Megasquirt customers only.

As per forum policy, your account will now be deactivated.

James