Low fuel level sensor (reserve)

BiKenG
Posts: 53
Joined: Tue Feb 10, 2015 12:08 pm
Location: Surrey, UK

Low fuel level sensor (reserve)

Postby BiKenG » Sat Jun 13, 2015 7:59 am

Motorcyclists will be familiar with having a light come on when fuel level gets low. On the bike to which I am fitting the D2P there is a single line from the OEM dash to the sensor which also has a ground connection. The bike version of the D2P I think uses the fog light connection for this, but will it work directly with that original connection from the sensor?

I've not measured the sensor output, so not sure yet how it operates, but I suspect it will be basically the same as all other reserve/low fuel level sensors as used on bikes. Can anyone advise how to interface such a sensor to the Dash 2?
You can't have too many bikes

BiKenG
Posts: 53
Joined: Tue Feb 10, 2015 12:08 pm
Location: Surrey, UK

Postby BiKenG » Sun Aug 09, 2015 9:59 am

This is what I plan on doing.

Set up the sensor as normal for the D2P with suitable resistor etc. Then set an output to PWM mode for 0% when fuel is over the sensor and 100% when fuel is low. Then use a transistor to control the power to the Low Fuel light input (normally Fog) so it will be off normally and start to come on as the fuel gets down to the sensor level and fully on once level is below the sensor. This should work. But...

The sensor is a 1K thermistor that works by heating up when exposed - i.e. no longer covered and cooled by the fuel when the resistance drops to virtually 0. As far as I know, these are normally 'driven' from a 12V supply to a tell tale light which grounds through the thermistor. It occurs to me that if the voltage applied to the thermistor is too low, it won't heat up sufficiently when exposed to measurably change its resistance. Hence I thought I should use 12V rather than a 5V reference from the D2P. But what value for the series resistor?

I want it as low as possible to provide the maximum voltage to the thermistor. But I also want it as high as possible to keep the actual current draw as low as possible. I was thinking 120Ω would be OK. So even when fuel was low and thermistor at 0Ω, that would only draw 100mA which I think is acceptable.

Anyone see any problems with this? Have I got my figures wrong?
You can't have too many bikes

BiKenG
Posts: 53
Joined: Tue Feb 10, 2015 12:08 pm
Location: Surrey, UK

Re: Low fuel level sensor (reserve)

Postby BiKenG » Tue May 10, 2016 3:18 pm

Returning to this in the hope others may join in and comment.

The bike originally applies power (I assume 12v) to an LED tell-tale in the dash that is grounded (or not) through the sensor (thermistor in the tank) which when cold is about 1kOhms, but as the fuel level drops and it becomes exposed, heats up and resistance decreases to about 0 when fully exposed (i.e. fuel level now very low) and the tell-tale in the dash is now fully lit.

The problem is that the D2P expects power FROM the sensor and this is earthed through the tell-tale LED in the dash. IOW, it's the opposite way around.

My simple solution is to remove the gnd. wire from the sensor and replace it with a 12v supply. The other side, as before, connects to the tell-tale LED, now in the D2P which will therefore start to glow as the fuel level gets low, exactly as it should.

So by the simple expedient of swapping one wire at the sensor, it should work. Can anyone see any flaws in this? If the sensor when cold (i.e. full tank) measures 1k, will that be sufficient to keep the low fuel LED off? When the fuel is low and sensor is virtually 0 Ohms, the LED will be receiving 12v, but I can't think this is a problem as it is originally the fog warning light which expects to light up when it receives 12v due to the fog lights being switched on.

Am I missing anything here?
You can't have too many bikes


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