Help me to build a fuel level gauge for a motorcycle using a 10-segment LED bar graph

[Trevor]

Dear All

I want to build a fuel level gauge for a motorcycle using a 10-segment LED bar graph operating from a standard LM3914 driver.

The input signal to the driver IC is 0 - 5v so if I connect a bench power supply to it, and vary the voltage up and down from 0 to 5 volts, the bar graph lights and tracks the input signal voltage as it should. That's the simple bit.

The tank sender in almost all vehicles (I will have to buy or make a suitable one) is usually just a variable resistance which varies according to fuel level. It could also be made using a set of fixed resistors in 10 steps. One possibility is to have a set of reed switches operated by a float with a magnet that operates each switch in turn as the float goes up down. Each switch can be used to switch in a fixed resistor.

Now here's my question, can anyone give me a basic description of the electronics needed to interface a variable resistance (or set of fixed resistor steps) to the input of the LM3914? I tried just putting various resistors across the 5v signal but this didn't work. It obviously needs something more complicated.

I know the very easiest way is to simply have each reed switch in series with the DC supply to the LEDs but this means running 10 separate DC lines to the bar-graph and doesn't seem the proper way to do it.

Anyone out there an advise, I'd be very grateful.

Trevor

 

[techie]

lm3914 fuel gauge

Simple. Just make a voltage divider. A series resistor from the fuel gauge to the LM3914 input and a fixed resistor from the LM3914 to 0volt.

 

[Trevor]

lm3914 fuel

Simple. Just make a voltage divider. A series resistor from the fuel gauge to the LM3914 input and a fixed resistor from the LM3914 to 0volt.

Hi

Thanks for the reply. Would you be able to give me a bit more detail? I'm afraid it's not that simple for me as I'm sure it is for you. What is the "fuel gauge" you're referring to? There isn't a fuel gauge. Do you mean the sender? Sorry if I'm being stupid.

Trevor

 

[techie]

voltage divider as fuel level sensor

Sorry, I ameant the fuel sensor, not the gauge. The fuel sensor is a variable resistor. You connect one end of the variable resistor (fuel sensor) to the positive supply and the other end to the input of LM3914 and also to another fixed resistor. The other end of the fixed resistor is tied to the negative terminal of the power supply. That way you have a simple voltage divider that will provide voltage in some proportion to the fuel level.

 

[Trevor]

resistors for fuel gauge

Techie, many thanks for your help. One last question I promise!

I used a spare 500 ohm pot to simulate the sender. One end of the track goes to Vcc, the other end goes to GND and the wiper goes to the signal input of the LM3914.

This works fine but without the fixed resistor. If I read your last post correctly, the fixed resistor goes between GND and the input of the LM3914. Why do I need this and what does it do?

Regards and thanks

Trevor

 

[techie]

fuel gauge lm3914

Well if your fuel sensor has 3 wires, it is already in a voltage divider configuration just like the 500ohm pot that you are using. You therefore dont need a fixed resistor.

But if the Fuel sensor has only 2 wires, it is just a variable resistor and you need another resisor (fixed) to make it into a voltage divider.

 

[1_C_E_M_A_N]

fuel gauge how to simple

Just be careful on tweaking your gasoline tank or else you end to your nearest graveyard, again be careful it can be very dangerous