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Floating base?

Liamlambchop

Sep 19, 2011
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Hey guys I have the setup, as seen in the attached photo, and the circuit is not functioning as I'd like. The infrared emitting diode is not emitting as far as it should be, hence I'm concluding that it isn't getting enough current through it.

I am hoping to have about 150mA going through the infrared emitting diode, which is pulsing at 38kHz (on for 13us, off for 13us). The pulsing is controlled by an arduino microcontroller which is giving the 5V output on the base of the transistor.

I am concerned because the arduino (5V on the base) and my 12V power supply have different grounds. I think this may be causing my problem.

Can you guys help me with this please?

Thanks a lot!
 

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(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
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Jan 21, 2010
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If you can't arrange to have a common ground, you'll have to look at something like an optocoupler to connect the arduino output to the base of the transistor.
 

Liamlambchop

Sep 19, 2011
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Is it possible for me to arrange to have a common ground?
What sort of issues would I have if this was the case?
 

(*steve*)

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Connect the 2 grounds together. Then you'll have a common ground.

This may "just work" or you may get sparks, smoke, and flame (although that would be most unusual). However without more information as to what these power sources are, I can't really say anything more definite.
 

Liamlambchop

Sep 19, 2011
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Well. The 5V power source is from the pin output from an Arduino microcontroller. The 12V power source is a battery.

Won't joining them be a big problem? Where will all the current go, to the battery or the arduino?
 

(*steve*)

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Neither, if the only connection is the common ground
 

jackorocko

Apr 4, 2010
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How are you powering the arduino? You could use an external 5V regulator off the 12V battery and use that to power the arduino instead of it's usb port. Then you would have your common ground using one power source if you are that worried about it.
 

Liamlambchop

Sep 19, 2011
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The arduino is powered by the 12V power source itself. It has a Vin pin and a GND pin for that input. The arduino also has a GND (ground2) pin for the circuit it is controlling. So if I connect the emitter of my transistor to ground2, will that be ok?

Steve, I don't understand your point....
 

(*steve*)

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Steve, I don't understand your point....

If the power supplies are otherwise isolated from each other, connecting their grounds together will provide a common reference for voltages produced by either.

If they are not isolated, the grounds may be at different potentials and connecting them together could short something out.

An easy way to test is to connect them via a (say) 1K resistor and measure the voltage across the resistor. If this is zero then you're almost certainly safe. If not, you need to do some more investigation.
 
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