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Preventing a voltage source from stealing from AA batteries

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PowerJunkie

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How do I prevent a voltage source from stealing from AA batteries?

Hello everyone,

I'm making an "idiot light" for a couple of engine sensors in my engine compartment. The "idiot light" will be on the Mass Air Flow and Throttle Body sensors. I'll be switching out to dual intakes (which will be including larger Mass Air Flow sensors and Throttle Body senors on each intake) and I'm making this so that I know if the secondary intake is active. The sensor voltage is a constant 2V DC.

What I made was two simple AA batteries in series, with the negative side of the AA battery going to the MAF signal wire (which outputs 2V DC). The positive side of the battery goes to the positive side of a 2.1V 20mA Green LED, and on that same lead the negative side of the LED is going to the MAF ground wire.

If I touch the two wires on the battery, it's super bright. Measuring the DC voltage between the two LED pins, I'm getting 2.8v to the LED. Here's where my problem comes in: When I connect the "idiot light" to the MAF sensor, the LED lights up EXTREMELY dim, so dim that you can't really see it at all during the day time. I checked the voltage at the LED pins and it's getting 1.9v to the LED. What I'm thinking is that the MAF sensor is stealing my AA battery voltage.

When I ground the positive side of the AA batteries (which is now the negative side of the LED pin since the LED is on this lead) to the MAF ground and then ground the negative side of the AA batteries to the vehicle (instead of the MAF signal wire), it goes extremely bright due to a complete circuit. But, as soon as I connect the negative side of the AA battery to the MAF signal wire (keep in mind that the MAF signal wire is hot with 2V DC), the LED goes extremely dim and measures 1.9v between the LED pins.

So, what's a simple way to prevent the AA batteries voltage from flowing into the sensor? I've tried to play with some 1N4001 diodes but can't seem to get anything to work!

Again, I'm making this so that I know if the secondary intake is active so I'd really like to get this working! I'm a complete idiot at electronics and circuity in general, so laymen terms or pictures would help a ton!

Thank you for ANY help!
 
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audioguru

Well-Known Member
Most Helpful Member
The MAF sensor output is a voltage that does not have enough current to brightly light an LED.
You need to use a low voltage opamp to amplify the signal and drive the LED.
 

PowerJunkie

New Member
The MAF sensor output is a voltage that does not have enough current to brightly light an LED.
You need to use a low voltage opamp to amplify the signal and drive the LED.
Thanks for the reply! I figured that there is no current in the MAF voltage, so that's why I'm running two AA batteries to supply the voltage and current to drive the LED's. The only thing I'm using the MAF sensor for is to complete the circuit (i.e. continuity) so that it lights when it's fully connected and operating. Except the problem I'm having is that it's just not ONLY making continuity - along with continuity, the MAF sensor is taking around 1.1 volt from my AA batteries, so I need to stop this so that ALL of the AA batteries voltage goes straight into the LED, and not into the MAF sensor.

Thanks again!
 
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audioguru

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Most Helpful Member
The sensor is not reducing the battery voltage. It is limiting the battery current to the LED because the sensor cannot pass enough current.

If the sensor could pass enough current for the LED then the LED would burn out with 2V from the sensor plus 3V from the battery= 5V.
 

tcmtech

Banned
Most Helpful Member
What type of vehical is it and what engine?
I am not sure what you are tying to do with the idiot lights.
SOme more in fo is needed. Almost all of the MAF sensors I have ever worked with were 5 volt source systems.
 

PowerJunkie

New Member
The sensor is not reducing the battery voltage. It is limiting the battery current to the LED because the sensor cannot pass enough current.

If the sensor could pass enough current for the LED then the LED would burn out with 2V from the sensor plus 3V from the battery= 5V.
Ohhh.. that makes sense now. I appreciate the laymen terms! What would be an easy way to get enough current to the LED when using the sensor for continuity?

Thanks so much for your help!
 

audioguru

Well-Known Member
Most Helpful Member
I told you before.
You need a low voltage opamp (half of an LM358) to amplify the signal from the sensor and drive the LED. An opamp has an extremely low input current.
 

PowerJunkie

New Member
I told you before.
You need a low voltage opamp (half of an LM358) to amplify the signal from the sensor and drive the LED. An opamp has an extremely low input current.
I see what you mean now! So you use this OpAmp to amplify the continuity that's going through the sensor, correct?

I'm extremely new to this so I'm still learning on how all of this basic stuff works.. ;) Could you possibly tell me which Mouser part number for a LM358 that you would recommend (since there's so many different versions of the LM358), as well as briefly how to hook it up in my application?

Again, thanks so much for helping me.. I'd still be beating out my brains here without you guys! :p
 
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MikeMl

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Does the Map sensor have three pins?
5V input
Gnd
Out?

If you connect just power and ground, what is the voltage (measured with respect to Gnd) on the Out pin with the engine stopped (map=atmospheric pressure)?

What is the Output voltage with the nominal map (engine running so as to produce min/max map)?

If the Map sensor does indeed run on 5V, wouldn't you prefer to power it from the car's electrical system? If so, why not use an LM7805 voltage regulator to convert the car's 14V to 5V, and use it to power the LM358 and the LED as well?

On the LM358, get the cheapest one that is in a DIP8 package.

From your posting, it seems that you are forgetting to put a current limiting resistor in series with the LED?
 
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tcmtech

Banned
Most Helpful Member
I am still not sure what you are trying to do with the LED.
There are so many ways a MAP sensor can work.
Most are 5 volt source power but the outputs can be variable voltage, variable frequency, or PWM.
Without knowing what purpose you are intending to monitor it for or what type of vehical your working with we can only guess.
Being it is a variable output the LED is not going to tell you anything. It is not an on/off signal that sensors put out.

You said you are changing out your intake system to a dual intake, Correct?
If it was a single intake having two intakes now is going to mess up your computer and make the car undriveable and likely wreck the engine.

Or do you already have a computer that is compatible with a dual intake sensing?
 
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