speed signal bounce

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mx5 na 1989 1.6

mx5 has mechanical drive speedo.

on the back of the later NA model MX5 speedo is a reed switch that gives an output 4x per rev. all the hardware is there on my 89, just no reed switch.

so I got one from jaycar. soldered it in and connected it to DI7. verified it is working. pullup resistor on, active edge rising, calibration 125.

however, there is a lot of variation in DI7 output at road speeds above 20 km/h

is there anyway to smooth it in real time ?

or any ideas on how to eliminate the variation

Thanks Marc

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There is no option to filter a DI speed input directly at present (only wheel slip can be filtered).  It would be nice to see an oscilloscope trace of the signal to see why it is not happy but my suspicion is a typical consumer level reed switch wouldnt be fast enough for a signal like this.

Have you got a part number or link to the sensor you are using?

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I am led to believe it gives 4000 pulses per mile, with equates to 160.9 pulses per 100m

if 100 km/h this is 5.8 pulses per sec

I think you might be right regarding the ability to handle the signal speed

what I can do, tomorrow, is to log DI7 and then produce a time plot... would that help?

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59 minutes ago, MarcD said:

I am led to believe it gives 4000 pulses per mile, with equates to 160.9 pulses per 100m

if 100 km/h this is 5.8 pulses per sec

4000P/Mile is common standard speedo signal, but I think your math might be wrong? (could be me, its late here).

4000P/M * 1.61 = 6400 pulses per kilometer.

assuming 100km/h speed, that would be 6400 x 100 = 640000 pulses per hour.

640000/60 = 10667 pulses per minute.

10667/60 = 178 pulses per second.  = a bit different than your 5.8!

Here's the data sheet below, I cant read whatever language it is but the only speed related spec is 4.8KHz?  To me that seems a bit optimistic for a mechanical switch that has moving components inside?

What does the mating "wheel" have to trigger the reed? Has it got some magnets embedded in aluminium or plastic etc?

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9 minutes ago, Adamw said:

4000P/Mile is common standard speedo signal, but I think your math might be wrong? (could be me, its late here).

4000P/M * 1.61 = 6400 pulses per kilometer.

assuming 100km/h speed, that would be 6400 x 100 = 640000 pulses per hour.

640000/60 = 10667 pulses per minute.

10667/60 = 178 pulses per second.  = a bit different than your 5.8!

Here's the data sheet below, I cant read whatever language it is but the only speed related spec is 4.8KHz?  To me that seems a bit optimistic for a mechanical switch that has moving components inside?

What does the mating "wheel" have to trigger the reed? Has it got some magnets embedded in aluminium or plastic etc?

its late here in Hamilton too!

1.61 km / mile so you need to multiply 4000 by 0.621

= 2484 pulses/km

thats 248400 pulses @ 100 km/hr

4140 pulses/minute

69 pulses /sec which is a bit different to both my 5.8 and your 178 !!!

aarrgh!!

its an aluminium disc with 4 magnets imbedded in it.

have found some pictures on the web - it might be that I have pushed it jsut a little too close to the spinning disc. I'll check again tomorrow.

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