Jiglu: BLOKART-NZ: Discussion: Re: Speedos and why they stuff up when immersed!

Originally sent by: Mike Lansdown

Hi Grant and Phil.
I should ahve explained...
You are quite right, the 0.3V is not high enough to inputy a "high" into the speedo computer, but in fact inputs a "low", and makes the speedo think the magnet is there all the time.
This is why when you first connect the sensor to the computer, after a dousing in salt water, the speedo "wakes up", but then never dispalys anything further, until the electrollosis has died away.

Phil, you are also right, the sensor is nothing more than a simple reed switch, but I am gussing that the leads from the glass encapsulated switch are either gold plated (very thinly) and the combination of the solder joints (containing the lead and tin solder) create the conditions for the salt water to wreak havoc.

Regards,
Mike

----- Original Message -----
From: "Grant Crawshay" <...>
To: <...>
Sent: Monday, December 20, 2004 9:59 PM
Subject: RE: [blokart] Speedos and why they stuff up when immersed!
Hi Mike
My speedo got intermittent before the "total immersion" on Sunday and
the immersion finished it off. I noticed the cable had gone "low res",
but I never thought to check for electrolysis. I had a connector on the
bottom of the kart that suffered from a few too many stream crossings –
so I now have one reasonably straight connector less run there, and it
is working fine at the moment.
I'll check for voltage the next time I have a problem, and try the
"bleed resistor". The input must be quite high impedance. 0.3 V would
be enough to hold the input high.
Sorry to hear about your camera though.
I'm glad I didn't have the GPS on – despite it being shower proof.
Regards
Grant C2
-----Original Message-----
From: ... [...]
Sent: Monday, 20 December 2004 7:31 p.m.
To: ...
Subject: [blokart] Speedos and why they stuff up when immersed!
Hi everyone,
I notice from a past comments, people who have had strange readings from
their bike speedos when bkartn on a beach.
I have used a Smart type modified bike speedo with good success for the
last two years, that is until Sunday...
I got caught in a feak wave at Karotahi beach, which immersed the sensor
(and my digital camera, which has now been retired).
The speedo stoped reading, and failed to start again until I had a
closer look at home that night.
Being an electronics tech by trade, I had the tools to investigate
further.
The speedo computer unit itself was fine, and sensor reed switch still
functioned as a switch no problems, but what I found when I measuered
across the sensor switch with a voltmeter was extrordinary.
I found there was 0.3Vdc across the switch, without it connected to the
speedo unit.
When it was connected to the speedo, it would read erratically.
I then waited overnight to allow the sensor to fully dry out and then
measured 0.03Vdc.
Now when it was connected to the speedo everything worked as normal.
My conclusion is that there is an electrollisis action with the
copper/solder (lead and tin) coated reed switch wires and the salt
water/moisture, similar to the "lemon battery" we used to make for
school science projects.
This self generated voltage is enough to upset the speedo input when it
is up around 0.3V or more.
The answer is to fit across the switch a resistor to swamp/absorb this
voltage, but it has to be high enough in value to not interfere with the
reed switch action.
Go too high in value and you won't be able to "discharge" the salt water
"battery".
I have now fitted a 470K resistor across the switch, inside my connector
half way up the cable lead.
Unfortuantely I haven't been able to prove its effectiveness, as I'm not
game yet to get swamped again in the near future.
Thought you might find this interesting.
Regards,
Mike
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