# Relay to detect .75V and turn on a light

Discussion in 'The Projects Forum' started by ddaytona1, Nov 5, 2008.

1. ### ddaytona1 Thread Starter Member

Nov 3, 2008
13
0
Hi,
I have a fuel guage in a car that has a voltage span of 0V for empty to 6V for Full (that's my best assumption based on research, I will take actual measurements this weekend).

The goal is to have a 12V light to come on as a low fuel signal when the voltage at the fuel guage is .75V (roughly 1/8 tank). I will connect between gnd and the fuel sender wire to the fuel guage. The rough schematic of the fuel sender looks like it's just a variable resistor that changes based on the rise/fall of the float in the tank

When the voltage at the guage exceeds .75V (fuel tank is filled up), the light needs to turn off.

Are there Relays from Radio Shack that can be purchased to do this?

Otherwise I was considering looking up a circuit to sense the .75V then turn on a 555 timer that will turn on the light.

2. ### mik3 Senior Member

Feb 4, 2008
4,846
69
Use a comparator IC to compare your signal voltage coming from the fuel gauge with a fixed voltage set by two resistors. When the fuel will be less than 1/8 (as you said) the signal from the gauge will be less than 0.75 and the comparator output will go high and turn on the indicator. Otherwise the indicator will be off.

3. ### SgtWookie Expert

Jul 17, 2007
22,201
1,803
I suggest that you use an LED for a low fuel level indicator, as the current requirements are low, they have extremely long life, and practically no heat is generated.

[eta]

See the attached. To simplify the circuit, I used an LM324 opamp rather than a comparator, which is what this type of thing really calls for.

The pot allows adjustment of the threshold level; as it is right now, it trips at 0.5v. The sine wave represents the signal from your fuel level sender. The square wave represents the supply voltage to the LED.

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Last edited: Nov 5, 2008
4. ### ddaytona1 Thread Starter Member

Nov 3, 2008
13
0
Thanks for the replies - and the circuit recommendation. I've been searching the forum for applicable circuits.

For the opamp recommendation, will the light remain off when above .5V and remain on <= .5V?

Is R2 an adjustable pot set at 1K to provide a threshold of .5V or is R2 a 1K pot that needs to be set at some value to achieve the.5V threshold?

Additionally, there is a 12V low fuel lamp that needs to be driven by this circuit. I believe in place of the LED in the opamp suggestion can be replaced with a transisitor that will drive a small relay?

Last edited: Nov 6, 2008
5. ### mik3 Senior Member

Feb 4, 2008
4,846
69
If the op-amp is ideal and the reference voltage does not change, it will depend if you have the reference voltage on the inverting or the non-inverting input and if the led is connected between the +ve and the op-amp output or between the output and ground. Think of it and have in mind that when V1=V2, Vout=0 ideally. In practise, nothing is fixed and the led will turn on/off at a slightly different voltage than the reference voltage.

6. ### SgtWookie Expert

Jul 17, 2007
22,201
1,803
That's the idea of the circuit.

You can adjust the reference voltage by adjusting the variable resistor (potentiometer, R2).

Note that the other three opamp inputs shouldn't be left floating.
For each remaining opamp:
1) Connect the output to the inverting (-) input.
2) Connect the non-inverting (+) input to a voltage divider. It would be OK to use a single voltage divider for all three.

You could use two 100k resistors connected in series between +14v and ground. Where the two resistors connect you'll measure about 7v.

Last edited: Nov 6, 2008
7. ### Bernard Expert

Aug 7, 2008
5,020
567
If you wish to use existeng 12V bulb, try a transistor as an emitter follower, current rating to match bulb and high gain [darlington ?], assuming that one side of bulb is grounded already.

8. ### SgtWookie Expert

Jul 17, 2007
22,201
1,803
Bernard, I don't know what in the heck you're talking about.