Hi,

My MSnSE and relay board are not driving my Smiths tacho. I vaguely remember reading about a little circuit that can be made to overcome this issue but can't find it. I am using wasted spark via 2 VB9.. coil drivers. Any idea what I need to do and what parts I need to buy?

Cheers
Steve

mhhh...

it will all depend a bit on what kind of signal the smitshs needs...

maybe you can use direct coil...

else you could use the signal of the VB driver and turn a 2nd transistor with it on/off and create the pulses that way.

maybe you just should test it.

you will need an BC547C
and 1x 15K resistor
and 1x 1M resistor

drop a wire from the base of your driver (where the signal comes from wich it fires)

to the 1M resistor hook up that resistor to the base of the BC547C.

Hook the 15K to 12volts the otherend of it to the input of the gauge...

then the Collector of the BC547 to the knot where the 15K comes together with the gauge...

end the emitter of the BC547C to ground.

what you build is a signal amplifier
wich generates a waveing signal proportional to the coildrivers fireing order....

the reason for the 1M is that you don't want to interfere with the signaling of your spark.

the reason for the 15K is that it isn't nececary to destroy to much energy.

its a pulldown principle....

Tks

You need to make sure that there is a connection from the board to the 36 pin socket, you may need to add a wire from a pad on the pcb if it has not been set up for a tacho.Then as TKS says you need a signal amp made from a transistor and couple of resistors, after which you will need to go into megatune and turn on the output in the tuning software and select the correct output terminal that you have used.
You will need to do a search in the megasquirt and spark web site for tacho settings where it is explained fully.
Paul.

OK, I don't recall adding a tacho output during the build of my MSnSE so I need to do that now but I have a question first.

in Megatune it says use either

X2(JS0)
X3(JS1)
X4(JS2)
X5(JS3)
Output3
Pin10

Well I need to connect to either pin 25, 27 or 31 on the DB37 to get the correct path to my relay board. So my question is, do I need to add a wire from either of these outputs to the relevant pin on the DB37 or is there already a connection in the PCB?

Cheers

You will need to connect internally from the pad on the pcb that you choose to match that set in megatune.you will just need to thin cable and a bit of carefull soldering, does the relay board have a driver transistor ? if not you will need to add one and a couple of resistors as already mentioned.
Paul.

I made a connection on the board to JS0 and it worked. I didn't need the resistor / diode etc which I presume is ok? Thanks for the advice though, it really helped.

Cheers