# A little assistance from a knowledgeable soul.

Discussion in 'Automotive Electronics' started by Hecmac, Jun 29, 2015.

1. ### Hecmac Thread Starter New Member

Jun 29, 2015
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0
Evening, A newbie to the world of electronics and I am shamelessly on the scrounge for some urgent assistance,

To cut a long story short I have a home build kitcar going for its IVA test in a week and I have discovered a flaw in braking system circuit. I am sure the fix is an easy one but with the test looming I am pushed for time. To cut to the chase the circuit it is as follows;

A brake reservoir with a float has two wires. One to the vehicle ground, the other to a dash unit. With the float up, or reservoir full, there is no continuity between the wires(tested with the continuity tester on my multi meter). With the float down, or reservoir empty, there is continuity. This in theory then grounds the connection at the dash unit, completing the circuit and illuminating a dash warning light.

My problem is this this, its not opearting like I expected it to, ie the switch in the reservoir does not appear to be simple on off switch, When the circuit is energised as described with the 12v from the car electrical system the dash warning LED remain illuminated at all times, regardless of where the float is, unless I physically remove the earth wire. I think the reason for this is because the switch is actually a variable resistor and not a switch, with the float up the resistance measures .505 K ohm. With the float down ( reservoir empty) the resistance drops to 0. The system draws 3.5ma when the LED is illuminated. So basically what i am looking for help with is a circuit that provides the ground connection, the continuity when he resistance is zero (the float is down) but when the resistance remains greater than that, ie at .5 kohm (the reservoir full) the connection with ground remains broken and the warning lamp off. Can anybody help me with this

In programing terms I would be needing Float resistance = X

if X > 0 ohm then Ground to dash is Off
Else
Ground to dash is on ( and connection to earth is made, lamp illuminates).

I hope this makes sense and thank you for taking the time to read this.

Kind regards

Hec

2. ### ian field AAC Fanatic!

Oct 27, 2012
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Not so long ago, automotive topics were forbidden on this forum - now there is an automotive section, which is where you should have posted.

3. ### #12 Expert

Nov 30, 2010
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Of course, you might replace the sensor with the right kind.
Does the resistance "glide" down to zero as the float lowers?
or does it switch suddenly to zero?
What color is the LED?
Is it labeled as a 12V LED or is it a raw LED that you had to add a resistor for?

I have cheated this kind of circuit before with a mere resistor to feed the sensor resistance AROUND the LED.
If the resistance of the sensor glides, you will get a slowly brightening LED as the fluid level slowly drops. WHERE it starts glowing depends on the size of the resistor.

The other way is a lot more complicated.

ps, I'll tell the moderators to move this to the right Forum.

4. ### Hecmac Thread Starter New Member

Jun 29, 2015
7
0
Thank you for the reply, I appreciate what your saying, the fact that this circuit application is on a vehicle makes it hard to argue with your point however this circuit could be applied to any system monitoring the position of a level sensor in a tank. My question is about the design of a circuit rather than an automotive query. In any case I will move it, Thank you.

Hec

5. ### #12 Expert

Nov 30, 2010
17,887
9,294
No, you won't move it. You don't have the authority. You will merely end up making a Duplicate (which is also not allowed).
Yes, it's automotive because auto systems have very queer electrical system responses that need to be considered.

6. ### Hecmac Thread Starter New Member

Jun 29, 2015
7
0

Thank you for taking the time to address my question, The switch does not appear to vary the resistance on a scale, just .5 open and 0 closed. The LED is red and part of an integrated circuit in the aftermarket dash unit. The manual states all lamps are illuminated by grounding so I can only assume its resistor is fitted on the circuit.

Thanks

Hec

7. ### Hecmac Thread Starter New Member

Jun 29, 2015
7
0
Ok, thanks again for your help

8. ### #12 Expert

Nov 30, 2010
17,887
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OK. First run on the math shows too much power would be used up cheating the circuit with a mere resistor. You're going to have to use the right sensor or add an automotive relay that will not trigger with 500 ohms in the way of its coil. Have that relay ground the LED circuit.

A quick Google tells me that any automotive relay will work in this position.

@Hecmac

Last edited: Jun 29, 2015
9. ### Hecmac Thread Starter New Member

Jun 29, 2015
7
0
I
I tried the relay theory with a bog standard relay without success, again it just energised the coil. Unless there is a specific higher ohm coil available. I was looking at an npn transistor circuit I think it's alone the right lines but I can't quite get my head around it. , hec

10. ### #12 Expert

Nov 30, 2010
17,887
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Why is this necessarily Automotive?
Because the charging voltage on a battery might be as high as 14.5 volts, not 12 volts. Tiny electronic things care about a 20% difference in voltage.
Because the sudden unloading of the generator by, for instance, turning off the headlights, causes a voltage spike.
Stuff like that.

When it gets down to a mere relay, it's not so important, these differences.

11. ### #12 Expert

Nov 30, 2010
17,887
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The first relay I found has an 80 ohm coil. With 500 ohms in series, it has only 1.9 volts on the coil, and 1.9 volts will not fire a 12 volt relay.
Did you measure badly? Did you try a miniature (high resistance) relay?
Until we resolve this, we are guessing. Go measure again.

Hecmac likes this.
12. ### WBahn Moderator

Mar 31, 2012
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MOD NOTE: Moved from Projects to Automotive (Automotive Projects belong in Automotive).

13. ### Lestraveled Well-Known Member

May 19, 2014
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#12
The TS, in post #6 said, "The switch does not appear to vary the resistance on a scale, just .5 open and 0 closed." Perhaps there is leakage around the switch or a hidden resistor in parallel with the switch. This can easily be dealt with.

Edit; never mind you are there already

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14. ### #12 Expert

Nov 30, 2010
17,887
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It's hard to get ahead of me when I'm having a good day.

Last edited: Jun 29, 2015
15. ### Hecmac Thread Starter New Member

Jun 29, 2015
7
0
I will check again in the morning. Thanks for assistance its much appreciated.

16. ### Hecmac Thread Starter New Member

Jun 29, 2015
7
0
Gentlemen, Thanks for taking an interest in my wee dilema. I have revisited the project armed with a relay and its working as it should,. It seems the relay in my test rig was a 5v relay and was being opened with 2v. Silly me!

Thanks again for your help and advice, I can head for the IVA without the worry of him checking the brake circuit.

Thanks

Hec

17. ### ian field AAC Fanatic!

Oct 27, 2012
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No idea how its done nowadays - but last time I looked, the tank sender was a rheostat with a float on the end of a lever operating the wiper.

Almost all the voltage sensing instruments were actuated by resistance wire wound round a bi-metal strip - having a large thermal inertia meant it didn't really matter that the instrument console voltage regulator was also resistance wire round a bi-metal strip. It was basically a miniature version of the simmer stat found on electric stoves.