# Resistance Switching Circuit/Relay

Discussion in 'The Projects Forum' started by Cerveja, Dec 2, 2013.

1. ### Cerveja Thread Starter New Member

Dec 2, 2013
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Okay, hopefully that title isn't completely misleading, but I think that is what I am looking for. First off, new user to the forum, so thanks to all those who can lend a hand. Here's my problem:

I cannabilized a Hood Latch mechanism from a donor vehicle that has a built in switch to indicate that the hood is either open or closed. In a perfect world, it would have been too easy that when the hood is open the switch is open and when the hood is closed the switch reads closed. On the contrary, when the hood is open the switch reads 500 ohms and when closed it reads 1,000 ohms. All I need is the switch to be off when the latch is closed and to turn on a 12volt DC warning lamp (LED or incandescant) when the latch is open. I am trying to keep the circuit as low tech as possible and would like to avoid a logic circuit or IC chips if I can.

Ideally I would like a simple relay that trips when the input voltage drops as a result of the resistance change from flipping states. I have been searching on and off for a while for a simple off the shelf unit, but not even sure that one exists or that I am even searching for the right thing in the first place.

Any help would be greatly appreciated - Thanks in advance.

2. ### #12 Expert

Nov 30, 2010
17,840
9,178
Relays have a wide range between "on" and "off". Something like 80% to latch on and 10% to 20% to fall open. You are not going to get a dependable circuit with a two to one resistance range, therefore, you will have to use some kind of amplification circuit. "Ideal" is not going to happen.

There is also the idea that the power voltage has to be stable or it figures into the uncertainty. If you're using a battery that can be anywhere from 12.4 volts to 14.4 volts, that becomes part of the math.

Now, pick a relay, tell us about where the 12 volts is coming from, and we'll see what we can do.

3. ### mcgyvr AAC Fanatic!

Oct 15, 2009
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just forget about the built in switch and use a simple microswitch.

4. ### wayneh Expert

Sep 9, 2010
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I agree, unless you get a different switch as mcgyver suggests. Using this sensor will require at least a transistor, plus a couple resistors and I think a zener diode could be arranged to differentiate the two voltages, switch open or closed. It's not complicated but a pain if all you want is a switch.

Jul 18, 2013
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You can even get a proximity switch, there are types that just sense ferrous metal, no magnet required.
Both 2 wire or 3 wire.
Max.

6. ### inwo Well-Known Member

Nov 7, 2013
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315
With those readings it's not a switch.
Find out what it is and it may be simple.
Hall effect, pot, series/parallel diode, or bad switch.

7. ### inwo Well-Known Member

Nov 7, 2013
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Try a 12v test light if you haven't yet.
(only if you can risk the unknown device)

Might be better test than ohm meter.

8. ### #12 Expert

Nov 30, 2010
17,840
9,178
Mike ML has been playing with TL431s lately and I can see a way to use that for a very simple circuit if the relay current is below 100 ma.

• ###### Switch 12V at 500 or 1K.png
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9. ### MikeML AAC Fanatic!

Oct 2, 2009
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Right on cue, here is a TL431 circuit that lights the LED when the resistance is less than a predetermined value. Unfortunately, with such a simple circuit, the trip point is effected by the battery voltage. Here, I am simulating what happens as the resistance of R2 is varied from 500 to 1000. Note that the resistance of R2 is a function of the voltage at "hood". LTSpice has the neat feature of allowing a resistor to be a function of some arbitrary voltage, in this case, an artificial voltage where R2 is 1 Ohm per Volt.

Since the battery voltage can vary from 11.5 to over 14.5V, I ran the simulation at 1V steps (11.5, 12.5, 13.5 and 14.5V) to see the effect on the resistance at which the LED turns on. Note that the trip resistance is 860 Ohms at 11,5V and 640 Ohms at 14.5V

One thing that the OP didn't specify is how much the circuit can draw in the LED off state. Unfortunately, this circuit draws up to about 17mA when idle, and the current decreases slightly when the LED turns on.

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10. ### Cerveja Thread Starter New Member

Dec 2, 2013
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Thanks for the info, sounds like the easy solution keeps getting tougher. So in the end it will be going in my jeep, but for a different application other than under the hood. So the source voltage will still vary probably in the ten to fourteen volt DC range. So I ran a quick test using a jump box, grounded the latch, ran 13 volt (+) to the switch, cycled the latch, got 13 volts out of the switch in either position using a multimeter. I also did a continuity check from the ground to either side of the switch in the open and closed position, open circuit either way.

From the outside, it looks like a simple micro switch, and I considered just replacing it with a known micro switch, but it's a pretty small switch and it's going to be tough to get another switch in the same allotted space. And I know this one will pass corrosion and enviromental test, which might not be the case once I grab one from Digi-Key or Mouser and "make it fit"

Jul 18, 2013
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Why not use a reed relay and a magnet, you can get the small encased versions similar to used in alarm system at windows and doors.
One such maker is Hamlin for the smallest package.
No extra electronics or relays.
Max.

12. ### inwo Well-Known Member

Nov 7, 2013
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Here's a minimum component led indicator.
1- 500 ohm
1- 5.1 zener
1- led

Should work from 10 to over 15 volts.

Someone with a simulator could fine tune it.

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Jul 18, 2013
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14. ### Cerveja Thread Starter New Member

Dec 2, 2013
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Okay, so I did a little research and found the internal circuit for the switch.

I also discovered that the resistors are embedded inside the switch, so simply opening the switch body and removing one or more resistors from the equation is not an option either. By the time I remove the switch, cut out the resistor, and reinstall the switch, both the switch and the hood latch would be destroyed. So I either need to work within the confines of the exisitng internal circuitry, or remove and destroy the switch and replace it with a true on/off resistor'less switch which would work with a much simpler circuit.

15. ### inwo Well-Known Member

Nov 7, 2013
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The circuit in post 12 will work with the switch that you have and uses an led indicator.

16. ### inwo Well-Known Member

Nov 7, 2013
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Here's another way to draw it.

Forgot to change the 1000 label to 500.
All should be 500 with the corrected switch diagram.

• ###### ohmswitch1.png
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17. ### MikeML AAC Fanatic!

Oct 2, 2009
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Inwo's circuit has a couple of problems. The hood switch model is flawed, and even if replaced with the corrected circuit (two 500Ω resistors inside the switch), it doesn't supply enough current to the LED if the supply voltage can vary from 11V to 15V...

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18. ### MikeML AAC Fanatic!

Oct 2, 2009
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However, borrowing Inwo's idea and modifying it yields this:

Depending on what LED you use, you may have to diddle R3 a bit...

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19. ### inwo Well-Known Member

Nov 7, 2013
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I'll test it again.

Drew it wrong but tested it with op's original specs of 500 or 1000 ohms.

It still works ok, but may need values adjusted for real world.

Actually in retesting I subbed 3 series leds and no zener, works too.

I've got to get that simulator!

Last edited: Dec 4, 2013