Low fuel level warning light

[ECR]

The alloy tonneau that I have makes reading the fuel gauge difficult. I don't really want to relocate the gauge, but if it proves to be the best/only/easiest option it will fit in front of the gear lever in an auxiliary pod. Before I do that, is there a way to piggy back a warning light that illuminates at say 20% full. I'd put a switch in the line and only check it in the paddock.

🤔

[Jonathan Kay]

Circuit should be quite simple. Do you want to make one or buy one?

 

You'll need an ignition switched feed, an earth and the sensor output that goes to the gauge.

 

Jonathan

[ECR]

buy one or make if it's simple (I'm no electrician) ...

if I just connect a light to the sender, it will be permanently fed with presumably varying current ?

[Jonathan Kay]

First thoughts:

 

Add a high-impedance circuit in parallel with the gauge. It needs to be high impedance to avoid interfering with the reading on the gauge. Then have an LED with some sort of variable resistance on its driver so that you can adjust the threshold at which it comes on. But you want it to come on at low signal from the sender so may need something like an op-amp to get it to work the way round you want.

 

The sender resistance curve is in the archives here.

 

Jonathan

 

Edited by - Jonathan Kay on 15 Apr 2014 18:04:36

[BigCol]

There was this thread in 2005...

 

I contacted Chris Wyles late last month asking for the circuit diagram and he responded:

I have no idea of your electronics knowledge or background, but there are two basic ways to achieve a low fuel warning light.

 

The first is simply to use an op-amp comparator. If this means nothing to you, I fear that sending you the circuit won’t help. If it does, then you shouldn’t need a drawing as all one needs to do is to input the fuel sender voltage to one side of the comparator and set the voltage on the comparator’s other input (with an amplified diode arrangement) to that voltage (which you can measure at the sender) when the tank is low enough to be at what you would call “low fuel”.

 

The only disadvantage of this circuit is that the low fuel light will flash around a bit (as the fuel sloshes around) when the tank is getting low. One could add a monostable to stop most of this and reduce the flashing on the boundary of the low fuel point.

 

The better way, IMHO, using far fewer components but requiring programming knowledge, is to program a microcontroller with your own software which can sample the sender voltage, wait for say 2 or 3 minutes, and then sample the sender voltage again. If both readings are low, the low fuel warning illuminates. If not, it was just sloshing around and getting low but not actually low enough yet.

 

I have appended a diagram below showing the basic circuit for the first method. I have no idea what the actual sender voltage will be at low fuel, however the sender voltage increases as the fuel decreases (at least it did on my Caterham)

 

I didn’t put them on the drawing, but add a 100nF and a 470uF (25v) capacitor in parallel at the battery input to the circuit and put a 100nF across the op amp’s supply terminals close to the op amp. That should kill any ignition interference.

 

As this is a fairly basic circuit, the battery voltage will affect it so there will be a different result for when the engine’s on or off, owing to the alternator’s having a higher voltage than the battery itself. However…… so long as you tweak the “low fuel” lamp level when the engine’s running, that will be accurate. When the engine’s OFF, but the ignition ON, the sender input will be lower (owing to lower battery voltage), which the circuit will interpret as a higher fuel level. The reference voltage will be unaffected by battery voltage.

 

It wouldn’t be difficult to make it all independent of battery voltage, but frankly, I’m not sure it’s worth the extra complexity on the Caterham.

 

circuit diagram

 

I've bought the bits (Maplin stock the necessary) but have yet to make up the circuit.

[elie boone]

here you go

[ECR]

Thanks to all .... but particularly to Elie 😬

 

I'll buy one and see.

[ECR]

Well, my low level fuel light unit has arrived. It's small enough to fit anywhere and comes with an LED that is designed to fit in the gauge. Problem is, the LED is very small (about 3mm) and I would like to fit a larger remote one where I can see it. The instructions specify that any replacement LED must not draw any more than 20mA otherwise damage to the ic will occur. Can anyone point me to a suitable lager LED ?

[Jonathan Kay]

Have you tried putting something like a lens or fruit gum on top of it to enlarge the area of brightness? If that works you can the find a similar permanent solution.

 

Can you find any more information, possibly from a diagram or serial number, to identify the current LED?

 

Jonathan

 

PS: I suspect you're going to regret that typo! :-)

 

 

 

Edited by - Jonathan Kay on 23 Apr 2014 16:28:21

[elie boone]

Something like this but you need to phone them to see what the current draw is

[ECR]

Jonathan

Enjoying the lager, and wondering how to get the fruit gum to stick to the dash ..

😬

 

Elie

I'll do as you suggest, thanks

 

Edited by - ECR on 23 Apr 2014 19:40:32

[Jonathan Kay]

:-)

 

Same adhesive as on crowns for the wise men?

 

Jonathan

[Nigel Riches]

Can you add some flying leads to the led, so that it could be mounted where you need it?

I don't know the construction of this item, but a couple of bits of wire could be you answer.

Nigel.

[ECR]

Yes, no problem with the flying leads, it's sourcing a reasonably sized diode with a current draw of less than 20 mA ...

[Grim Reaper]

here

1600mcd should be bright enough if it's in your eye line

 

[ECR]

Forgive my electrical ignorance but which part of the specs say that it will not draw more than 20mA

🤔

[Richard Price]

Roger,

The LED on its own would draw more than 20mA, but will require a resistor wired in series with it to limit the current.

The size of that resistor will be determined by the other components in the circuit that drives the existing LED.

 

Edited by - Richard Price on 24 Apr 2014 12:07:31

[oldbutnotslow]

Mmm I have to say that I am interested in having one of these.

Ive sent a question to the manufacturers asking for a bulk buy price for say 10 and whether they can supply a bigger led.

I will update in due course.

[revilla]

For £4 more the same people sell this. Slightly larger but easy enough room hide behind the dash. Will drive 500mA enough for any LED or a bulb up to 6W.

[oldbutnotslow]

They have come back with a 15% discount for an order of 10 (£23 as opposed to £27)

I will move this to Bulk Buys and see if there is sufficient interest.

 

See here

 

 

Edited by - oldbutnotslow on 24 Apr 2014 13:20:43

[Grim Reaper]

Better specs given here

Forward current If max=20mA

I note that Farnells say the brightness is 2200mcd rther than 1600 at Maplins.

If you'd prefer a yellow one, the maplins 5mm here whilst rated at 2100mcd there, is rated 3300mcd by Farnell, and is still 20mA If.

 

Edited by - Grim Reaper on 24 Apr 2014 21:58:57