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Topic: Taking values from a pot. that is powered separately (Read 804 times) previous topic - next topic

CTE_T

Currently, there is already a circuit build with a pot. that will adjust the speed of an electric car I am working on. The car goes faster as the pot is turned until it is full. Can I take the middle (part?) of the pot (the connection that isn't positive or negative/ground) and put this straight into an analog input on the Arduino? The car circuit is powered with two 12v batteries and I am wondering if I need to hook a wire from the 5v pin on the Arduino and another ground pin if the pot already has power. I am also curious to see what the number I read will represent. It should be 0 when the pot is all-the-way off and 1023 when the pot is all-the-way on but will I be reading the voltage of the circuit or the current, ect.?

Any help is always appreciated.

DVDdoug

First, if you put more than 5V into the Arduino, you can damage it.    Do you have any idea what kind of voltage you'tre getting out of the pot.   If the "hot" terminal on the pot is powered to 24V, that's a problem.

You can use a voltage divider (2 resistors, something like a non-adjustable pot) to knock-down the voltage, but the total resistance needs to be about 10 times the resistance of the pot or the voltage divider will affect the voltage coming out of the pot.

OK...  You just need a common ground (connect the two grounds togeher) and you can read voltage from a separate source.

CTE_T

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First, if you put more than 5V into the Arduino, you can damage it.


Yup, figured someone would say this. I'm trying to work around damaging the Arduino.

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Do you have any idea what kind of voltage you'tre getting out of the pot.   If the "hot" terminal on the pot is powered to 24V, that's a problem.


An "idea" of what I'm getting out of the pot is 24v, or two 12v car batteries, which I need to somehow, using the pot., read off current/voltage/ect. using the arduino. I thought maybe there would be a way to read off only one 12v battery but that would be less ideal because I believe I would need another board to read the other battery.

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You can use a voltage divider (2 resistors, something like a non-adjustable pot) to knock-down the voltage, but the total resistance needs to be about 10 times the resistance of the pot or the voltage divider will affect the voltage coming out of the pot.


A voltage divider would work as long as it won't affect what the arduino is reading.

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OK...  You just need a common ground (connect the two grounds togeher) and you can read voltage from a separate source.


Read voltage from a separate source?

DVDdoug

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A voltage divider would work as long as it won't affect what the arduino is reading.
Of course, it's going to affect the reading of the Arduino...  That's the whole idea, otherwise you're going to read 1023 as soon as you hit 5V and possibly fry the the thing...   But, the reading will be proportional so it's no problem.

The thing is, you don't want to  affect the speed of the car too much...  It the pot is set to half-speed, you don't want the speed to change when you hook-up the Arduino.  What's the resistance value of the pot?   (If it's 100k or more, you are probably going to need some active circuitry...  A high-voltage op-amp or something, in order to get a high-impedance load on the pot.)


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Read voltage from a separate source?
Voltage coming from a separate power supply.


CTE_T

Would it easier to use somehow?: http://www.sparkfun.com/products/8882

Although I believe the pot still would need to be involved because it is what controls how much current is flowing :/

Also our pot says: Alpha B5k? so to answer your question, it has 5k ? resistance.

mweisz481

If you're charging the batteries with the arduino hooked up, keep in mind the charging voltage is higher than 12v.

CTE_T

No, the arduino won't be connected while anything is charging, only while the batteries are in use.

GoForSmoke

Is the power to run the car going through the pot or is the pot used to control a power transistor?

I find it harder to express logic in English than in Code.
Sometimes an example says more than many times as many words.

CTE_T

The pot is connected to something similar to a voltage regulator. It "converts" the 24v of power from the two 12v batteries down to a value that the pot regulates. It is called a DC car regulator or something similar.

GoForSmoke

What do you get when you put a volt meter across the points you want to read analog? Something to ground?

I find it harder to express logic in English than in Code.
Sometimes an example says more than many times as many words.

CTE_T


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