where to get a tacho signal with carby

[mikie_121]

Hi

I'm trying to make a little tacho using a Picaxe microchip and a simple lcd. I'm trying to work out where to get a tacho signal from.

I know that some tachos you buy have a lead that goes to the coil, but what type of signal would that give? Would it go low every time it sparked?
Or would it be better to use a hall effect sensor with a toroid around a spark plug lead?

If i get it made i'll post up plans and hopefully it'll be cheaper than buying a simple tacho.

Cheers

[Jorrs]

when i had a carby rolla, it was simply joined of the wire that runs from the coil . it was fine

[RA35GT]

The Coil- on the coil is periodically switched to ground, but when this is switched off there is a high voltage induced in it. (This is your only option if its carby and has points, if it has electronic ignition you may find the module has a dedicated tacho output that is more usable).

You would need to have some sort of signal conditioning circuitry to interface with the PICAXE. If you can find the circuit diagram for the Jaycar tacho you can copy their input conditioning.

Using a 4MHz clock, the resolution would be approx. 53rpm / (timing interval [10us @ 4MHz]) using the pulsin command.

You could use the count command also, but you will have to use a long count period to get reasonable resolution. Using a period of 0.25s, would result in a resolution of ~500rpm.

It may be easier to use a LM2917 and then connect the output to the ADC input of the PICAXE. (So for a 4cyl it would be 4V @ 8000rpm & approx. 39rpm/bit (8bitADC)). I'm not sure how well this works at low rpm (I never got around to building a prototype). Some factory tachos use this method so it should be ok...



What did you have in mind for the display ? I know you said LCD, but that seems overly complicated for a tacho? A number of seven segment displays would suffice? You could multiplex them into the picaxe to save pins. (You didn't mention which PICAXE your using so you may be limited in Inputs/Output pins).

Rudi

[mikie_121]

Thanks heaps for the info, Rudi.

I am going to use an lcd with a serial interface (got one lying around and it only needs one wire from the chip), so it's really easy to use. I'm going to try and use the 08M (which gives me 8 pins total, 4 in,4 out at most), the 08 doesn't do 'count'. I'm not sure the pulsein would work and a resolution of about 500rpm would be fine for my simple application.

I'll definitely look at Jaycar's kit, and i'll read up on the 2917.
Just a note: the 08M has a 10bit ADC (1024 steps) which increases accuracy a little more. and if i ever get this engine to 8000rpm i would be amazed if it didn't completely die. it's done 260k and it's only a 1300.

Thanks again, i'll do some more reading and see what i come up with.

Cheers
Mike

[RA35GT]

No worries Mike.

You can do count on the 08M. (According to the manual at least).


If your not already aware the PICAXE website is http://www.rev-ed.co.uk/picaxe/
Lots of info is available there.

Rudi

[mikie_121]

Yeah, I just built a robot that used ultrasonics and bump sensors to avoid objects for a uni course, so I'm familiar with the great little Picaxe chips.
I downloaded the detailed instructions for the Jaycar tacho kit, I just need time to read through them and build the thing. Damn exams...

Mikie