Archaeology expedition - or WTF is inside a fuel level sensor, anyway?

[gjscott]

How about maybe, when I replaced the LM393 I installed it backwards and somehow reversed the supply polarity. Check out the attached diagram with the LM393 and supply polarity reversed, does it make sense then ?

[Bill in OKC]

It looks better to me.  If you call the bottom horizontal line ground, cut the emitter loose from it and tie it to the side of the lamp with the ground sign you might have a working circuit.

[tuxicle]

Looks like the first comparator (output on pin 1) will go low when the fuel switch is engaged. R2/R8 form a reference point at 12V / 2 = 6V. Noting that the LM393 is an open-collector output, this means that when its output goes low, it forms a discharge path for C2 (through R4) at a time constant of 1/(2.2k * 100u) = 4.5 seconds. The charge path for C2 is through R5 (I assume that's 345k, which gives a time constant of 35 seconds. The capacitor output is monitored through R6 (100k) by the second comparator, against the same 6V reference from R2/R8. The idea behind R6 is so that the leakage (bias) current of the LM393 input does not discharge C2. This comparator drives the transistor, which lights up the fuel lamp. I suppose when this happens, the LM393 loses power briefly, but the capacitors help the circuit "retain state" when the transistor turns off and the power is restored.

All I can say is "crazy effing Italians!" Adding a separate input for +12V would have made it a lot more sane, at least to my eyes.

[gjscott]

cut the emitter loose from it and tie it to the side of the lamp with the ground sign

Would that not make a short circuit when the transistor switches ?

[gjscott]

Looks like the first comparator (output on pin 1) will go low when the fuel switch is engaged. R2/R8 form a reference point at 12V / 2 = 6V. Noting that the LM393 is an open-collector output, this means that when its output goes low, it forms a discharge path for C2 (through R4) at a time constant of 1/(2.2k * 100u) = 4.5 seconds. The charge path for C2 is through R5 (I assume that's 345k, which gives a time constant of 35 seconds. The capacitor output is monitored through R6 (100k) by the second comparator, against the same 6V reference from R2/R8. The idea behind R6 is so that the leakage (bias) current of the LM393 input does not discharge C2. This comparator drives the transistor, which lights up the fuel lamp. I suppose when this happens, the LM393 loses power briefly, but the capacitors help the circuit "retain state" when the transistor turns off and the power is restored.

All I can say is "crazy effing Italians!" Adding a separate input for +12V would have made it a lot more sane, at least to my eyes.

Can you please elaborate on the separate +12V ?

[gjscott]

How about maybe, when I replaced the LM393 I installed it backwards and somehow reversed the supply polarity. Check out the attached diagram with the LM393 and supply polarity reversed, does it make sense then ?

So I've reworked the circuit to look like this and the gas light is still permanently on.

There is 12V going in and 9.5V coming out to the lamp regardless of switch state. Any ideas or things for me to check ?

[Bill in OKC]

Would that not make a short circuit when the transistor switches ?

No, the light bulb will be the load.  The way it is now only the fuel level switch turns the light on  with R8 inline - the rest of the circuit does nothing as it is. That might be what you are seeing during testing.   You only have one ground point for the entire circuit - everything grounds through the lamp.  You will need the bottom horizontal line to be ground to set up your voltage dividers, ground the negative side of your capacitors and ground pin 4 of the IC - the comparators will not work without that.  Put the lamp between the emitter and ground.  

[tuxicle]

Can you please elaborate on the separate +12V ?

Well on further thought, I was wrong. I thought the circuit would oscillate (as soon as the transistor turns on, it shorts out the supply to the comparators). I forgot that the lamp has a finite resistance, so the circuit will still have power even when the transistor's on.

[scaramanga]

The circuit supply common doesn't make sense to me. I know us italians want to be different but this borders on stupid design for such a simple control.
Please check the supply's gnd or negative and make sure that diagram really is that way.

[gjscott]

The circuit supply common doesn't make sense to me. I know us italians want to be different but this borders on stupid design for such a simple control.
Please check the supply's gnd or negative and make sure that diagram really is that way.

Which version doesn't make sense, the original or the reversed one?

Her are some pics of the board without some components so so can see the layout:

   



                     

[scaramanga]

reversed, sorry.
you see those 3 holes to the right of the black wire(second photo), they communicate to the other side and connect to the trace underneath. things are getting clearer...now get back to work 

[scaramanga]

and whats worse Ducati want ridiculous money for them ,, in UK£96   ( $150),,, shameful profiteering  !!!!!!!!! for something that costs $10 to make

small price to pay for  a safely engineered electric device that sits "IN" a fuel cell just below your face. 
 

[Dry Martini]

small price to pay for  a safely engineered electric device that sits "IN" a fuel cell just below your face.  
 

No it is too much to pay for a dumba$$ engineered device, when a mechanical float switch would perform the same function. This another example of a solution to a problem that did not exist.

[Howie]

No it is too much to pay for a dumba$$ engineered device, when a mechanical float switch would perform the same function. This another example of a solution to a problem that did not exist.

I'm guessing you don't remember the days of non electronic floats that would change greatly with approach and decent on hills and low fuel lights that would be constantly blinking once you used some fuel.

[Dry Martini]

I'm guessing you don't remember the days of non electronic floats that would change greatly with approach and decent on hills and low fuel lights that would be constantly blinking once you used some fuel.

Yes I do, however, those drawbacks can be dealt with.