# Simple Single LED 'Low Fuel Light'

#### nizzkid

Joined Sep 7, 2009
3
Hi, I've been looking for weeks and its doing my head in!
All I'm after is a VERY simple circuit, just one led that lights up when the fuel gets low on my bike. (making a custom dash using the "Veypor")

I'm usually pretty good with electronics ect, but i need some help with this one!
I've been searching for ages and only come across crazy complex (for a noob) circuits with time delays and 100,000 led light bars!

Is this anywhere near a working cercuit?!

What i've "tried" to do is have the led switch on when the fuel drops below 90Ω this should be @ 6.2v that the zener is at. So when the fuel drops the zener has more voltage and switches the led on.

Dont realy need any sort of 'time delay' to combat fuel sloshing around and continualy ficking the light on and off. This is because as it stand the fuel guage sits at 'full' 90% of the time anyway, its not till the last 30km or so that needle moves down. (and when it does its a rapid decent!)

Also any feedback on the resistor ratings would be very apriciated.

Cheers Steve.

Click on pic below

#### JoeJester

Joined Apr 26, 2005
4,390
Your looking for a comparator circuit if all you want is a low fuel indication. The problem is your wasting your time if the sensor is defective in your vehicle.

#### Gustav180

Joined Aug 25, 2009
17
Hello

Your problem is not the circuit, it´s OK. It´s the sensor. I have not test this, but I should test with a NTC.

If you use a NTC with low value, i.e. 100 ohm, it will bee wam of the current from the 12 V supply. When it is in the fuel, it will bee cool of the fuel. When it comes over the fuel, it will bee warmer and the output of the OP will switch.

If you want the LED to light up when fuel is low, you have either to change + and - on the OP-amp or put the LED from output to GND by 560 ohm.

Best regards
Gustav

#### nizzkid

Joined Sep 7, 2009
3
Hello

Your problem is not the circuit, it´s OK. It´s the sensor. I have not test this, but I should test with a NTC.

If you use a NTC with low value, i.e. 100 ohm, it will bee wam of the current from the 12 V supply. When it is in the fuel, it will bee cool of the fuel. When it comes over the fuel, it will bee warmer and the output of the OP will switch.

If you want the LED to light up when fuel is low, you have either to change + and - on the OP-amp or put the LED from output to GND by 560 ohm.

Best regards
Gustav
Hi, thanks for the help I haven't built the circuit as yet I just wanted a little opinion/help to see if it 'will' work before I waist any time on it!

The level indicator is a float style not a thermistor.
Which explains below.

Your looking for a comparator circuit if all you want is a low fuel indication. The problem is your wasting your time if the sensor is defective in your vehicle.
The sensor works fine, the fuel tank design/shape is the culprit.
Its a 5 liter tank mounted under the bike with @ 1+ of those liters being in the fuel filler pipe (above the sensor). And @ a 1 liter remaining on empty.
So it's only got 3 liters to play with in a long thin tank.

#### JoeJester

Joined Apr 26, 2005
4,390
That circuit will energize the LED when the sensor is about 98 ohms ... which is within the specification for empty.

The best thing you can do is fill the tank with the amount of gas you want for the low level alarm, then measure the sensor. Then design the circuit to illuminate the LED at that resistance. At least you won't be pushing the bike to the next fueling point.

#### nizzkid

Joined Sep 7, 2009
3
That circuit will energize the LED when the sensor is about 98 ohms ... which is within the specification for empty.

The best thing you can do is fill the tank with the amount of gas you want for the low level alarm, then measure the sensor. Then design the circuit to illuminate the LED at that resistance. At least you won't be pushing the bike to the next fueling point.
Good point! lol
My final design has two LED's (so much for the simple one LED effort!)
First one being solid Red coming on at 67 ohms.
Second being flashing Red coming on at 93 ohms.

#### JoeJester

Joined Apr 26, 2005
4,390
My final design has two LED's (so much for the simple one LED effort!)
Why not a solid yellow .... then a solid RED?

They make bi-color LEDs and you can get dual comparators on one chip.

#### andrejarret

Joined May 29, 2021
3
Thank you for the reply. Yes, I had white LEDs that were Very bright. I switched to dual red LEDs to see the effect. They work fine also.
However, it sort of looks like a spruce holiday tree light string flashing on my dash panel. White or factory green probably are best.

I am ordering Amber and Green super bright LEDs. My factory turn indicator dash lights are some sort of low level of green painted(?) lens covers. They are not very bright, IMO. You can get green C*** 53,650 millicandle (mcd) LEDs. I normally get 25K to 35 K mcd.

These small relay coils for turn signal clicking are hard for me to hear. So, I decided to investigate 6-9 solutions. This solution is the easiest and safest I can design based on 4 decades of using a low fuel real time flashing LED on the dash.

To make the circuit safe, the 51K resistors should be installed at the turn signal connector(s) so that any wire further away and grounding out to the frame, would have 51K ohms to limit fuses blowing or damaging the relay system, for example. The two input series resistors can be much higher but I used what I had that were larger diameter wire leads to back probe the connector pins without bending them or jamming the leads in.

You can mount all the parts in a gray electrical box available at any big box hardware store. $2 tops. I used molex 6 pin connector push mounts and carved out a solid gray cover plate for a plug for the six wires. You can use anything like a grommet, et al. Just a few details on the construction of my device. HTH, Andre #### andrejarret Joined May 29, 2021 3 Thank you for the reply. Yes, I had white LEDs that were Very bright. I switched to dual red LEDs to see the effect. They work fine also. However, it sort of looks like a spruce holiday tree light string flashing on my dash panel. White or factory green probably are best. I am ordering Amber and Green super bright LEDs. My factory turn indicator dash lights are some sort of low level of green painted(?) lens covers. They are not very bright, IMO. You can get green C*** 53,650 millicandle (mcd) LEDs. I normally get 25K to 35 K mcd. These small relay coils for turn signal clicking are hard for me to hear. So, I decided to investigate 6-9 solutions. This solution is the easiest and safest I can design based on 4 decades of using a low fuel real time flashing LED on the dash. To make the circuit safe, the 51K resistors should be installed at the turn signal connector(s) so that any wire further away and grounding out to the frame, would have 51K ohms to limit fuses blowing or damaging the relay system, for example. The two input series resistors can be much higher but I used what I had that were larger diameter wire leads to back probe the connector pins without bending them or jamming the leads in. You can mount all the parts in a gray electrical box available at any big box hardware store.$2 tops. I used molex 6 pin connector push mounts and carved out a solid gray cover plate for a plug for the six wires. You can use anything like a grommet, et al.

Just a few details on the construction of my device.

HTH,

Andre
Ops! I have many circuits on different postings and thought this was the turn signal posting. Sorry.

Joe,

You can use anything that you can create. Some LEDs are very bright. My original LED was 21 MCd, not about 30,000 mcd.

Why not a solid yellow .... then a solid RED?

They make bi-color LEDs and you can get dual comparators on one chip.

The selection of LEDs now, 4 decades later, is almost endless.

Yes you can use a dual 741 comparator.

I wanted to isolate the two circuits in case I had some wiring problem on the IC plug board or proto board I used. I didn't have a dual chip.

I find the bicolor suggestion is of interest. You could decode the gas tank voltage as nearing empty on yellow (1/4 of a tank) and then nearing fumes when on red.

You could also use a quad 339 comparator and have a "ladder" of different LEDs and comparators tied to a series resistor voltage divider string. At the top of the resistors would be an adjustable 100K pot that would be tied to the 14+ V line. Four 1K resistor would each be tapped and the voltage sent to one of the four + non-inverting op-amp pins. The - inverting input pins would all be connected together and be feed from the gas tank voltage line as their input voltage sensing line.

Some call this a Bargraph Voltage indicator circuit. The op-amp output signal would go to the cathode (banded end) of an LED and then through a 1K resistor. The four 1Ks would all be tied together and go to the 14+ voltage line (or the battery, if the engine is OFF).

You would have one LED on at first and then they would start lighting up more of them as you got to below the gauge E mark.

In the circuit I once built using a ten LED bar, there was that top resistor pot of 100K attached to the Alternator 14.+ volt line and adjustable.

My ten LED bar graph display was sort of annoying but nice to watch it like some audio amplifier bargraph. I would watch it bounce up and down to waves in the gas tank and not pay attention to the road. I switched to one wave induced flashing LED for safer operation.

I didn't see my schematic on this posting. I will attach it again.

HTH,

Andre

#### Audioguru again

Joined Oct 21, 2019
4,701
Beware. Some cheap bright LEDs cheat by putting a dim LED in a housing that focuses the beam into a very narrow angle. Pointing directly at your eyes it is bright but a little to one side it is barely seen.
Good LEDs are very bright over a wide angle.