low fuel light circuit

[tornado]

Dear All

My first post here, I hope you'll all be kind to me as I'm a real novice on all things electrical!

To try and keep it brief..

I've recently changed the dashboard on a motorcycle, the down side to this is I've lost the low fuel warning light, this obviously comes one when there's a few litres in the tank, my interpretation of the wiring diagram is the fuel pump assembly contains the level gauge, a resister that gives low ohms when the tank is full and high ohms when empty, this resistance is [was] monitored by the original dashboard and switched on the light when a certain ohms reading was reached.

Now I’ve lost this feature, is there a simple circuit I can create to replicate this with an independent light? As I don't know the original ohms reading I'd also like to be able to adjust the trigger point somehow

Many thanks for help with this

Paul

 

[CDRIVE]

Is your fuel gage wired like this?

 

[tornado]

fuel circuit

it's very similar - picture of the actual circuit attached

"12" is the fuel pump

the green/white wire goes to the original dashboard, now - to nothing
the red/blue wire goes to the ECU

Paul

 

[CDRIVE]

So, you originally didn't have a gage + fuel low light, just the light? Did you toss the original dashboard?

 

[tornado]

yes, the original dashboard had just a light [no gauge, like you'd have in a car]

I still have the original dashboard, it'll probably go back on the bike someday [or on ebay!]

 

[CDRIVE]

Take a look at the indicator on the original dashboard. My guess is that the indicator isn't just a simple lamp. Odds are it includes a Schmitt Trigger or Comparator circuit, but at this point I'm just guessing. Take a look at it and tell us how many connections it has.

By the way, I'm surprised that the red/green from the sending unit didn't go to the ECU. It would have made sense. I would think that the ECU could measure the sending unit and provide an output to the low fuel lamp when fuel was below a certain point. I believe ECUs are microcontrollers and uC's do have (ADC) analog to digital inputs.

 

[tornado]

I've took the back off the OE dashboard and I've attached a photo, basically a printed circuit board that's confusing to me!

the [back of] the fuel low light is connector "R165" [LHS, just above centre]

any the wiser? [I know I'm not!]

Paul

 

[CDRIVE]

I'd like to say it helps but I'm afraid not. I don't suppose there's a schematic for that board?

 

[tornado]

No - just a blank circle to signify the dashboard

if it was a Schmitt Trigger, how would that be wired up?

 

[CDRIVE]

There are quite a few variations of a Schmitt Trigger but they all serve the same purpose.

http://images.search.yahoo.com/search/images?_adv_prop=image&fr=yfp-t-701-s&va=schmitt+trigger

http://pcbheaven.com/wikipages/The_Schmitt_Trigger/

While a common electromechanical relay is not a Schmitt Trigger it possesses the virtue of one. This is because the 'Pull-In' current required to energize it is higher than the current required to hold it in that state. This is termed "hysteresis" which it the primary function of a Schmitt Trigger.

What's the min and maxi resistance of the sending unit?

 

[tornado]

I've done some research and found that sender will read 100 ohms when the fuel tank is empty, so a trigger reading of 80 ohms would be ideal

 

[tornado]

I've got exact figures now.....

full tank - 20 to 26 ohms
empty tank - 134 -140 ohms

 

[CDRIVE]

How are your soldering skills?... Welding experience doesn't count!

I'll sketch up some circuits.

Chris

 

[CDRIVE]

This should work but you're working with fuel. Proceed cautiously!

Edit: Schematic was removed, updated and reposted later in this thread.

 

[tornado]

Do you know me? lol - I actually work as a mechanical engineer and we do do a lot of welding!

anyway, many many thanks for the diagram

Paul

 

[CDRIVE]

You're welcome Paul. Keep us posted on your progress. BTW, U1 is a quad comparator (14 pin DIP) on a single chip. You'll be using only one of them. Perhaps one of our members know a single 8 pin DIP version? Also, U2 (7805) is a TO220 package. A 78L05 can replace it and is much smaller in a TO92 package.

Chris

 

[tornado]

you mentioned that increasing/decreasing the R2 value will alter the trigger point, what's the math?

by increasing the R2 by 10% does it increase OR decrease the 112 ohms by also 10%?

[I'm going to order some extra resistors for R2 just incase]

thanks again

 

[CDRIVE]

You may opt to use a 10K pot in place of R2. This way you can play with the value with ease. When you find the trip point you like best you can then measure the pot and replace it with a fixed resistor.

 

[CDRIVE]

Here's an updated schematic and some plots that should answer your questions. R2 was replaced with a trimpot and R6 was added for safety. See notes!

Chris