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Topic: Led backlight brightness control - dazed and confused. (Read 661 times) previous topic - next topic

hadoukengr

I am trying to control the brightness of a LCD screen of a hand held device using a PWM signal from an arduino. Because of reasons i can only disconnect and access the cathode pin (-) of the LED backlight strip, the Anode (+) remains connected to a voltage source as it originally was in the device.

You can see my circuit in the attached image.

Typically from what i can find online there would also be a second resistor between the Anode and the voltage source which i can not connect since i have no access to the Anode.

My questions are :

1) Why is my circuit wrong and what can i do about it.

2) Do i really need the transistor? Normally i would try to supply correct voltage and current to the anode pin and in that case i would connect a transistor emitter to the anode to amplify the current but the way it is now (since i can't use the anode pin) i am connecting the collector to the cathode (because that is what i was told to do) and i am not really sure why or how it works. Would be really thankful if someone could explain it to me in layman's terms.

Grumpy_Mike

Yes you do need a transistor.

Your circuit shows no common ground between Arduino, transistor and your device.


PaulRB

Use your multimeter on 200mA range between the cathode and ground to find out how much current the backlight draws. If less than 25mA ~ 30mA, then the Arduino pin can sink that ok. But it will probably be more than that, in which case go with your proposed circuit.

hadoukengr


This is what i have so far. Will this work correctly? Also how will i calculate R1 and R2?

Grumpy_Mike

Quote
Will this work correctly?
Hard to say just yet. Their is no labeling on the two connectors to the Mega. Nor is their any indication what the "device" actually is.
Not sure how you get 17V from a 3V battery, is their an inverter?
R1 should be 1K and R2 depends on how much you need to limit the current. Too many unknowns at the moment.

hadoukengr

#6
Oct 30, 2017, 06:39 pm Last Edit: Oct 30, 2017, 08:42 pm by hadoukengr
Hard to say just yet. Their is no labeling on the two connectors to the Mega. Nor is their any indication what the "device" actually is.
Not sure how you get 17V from a 3V battery, is their an inverter?
R1 should be 1K and R2 depends on how much you need to limit the current. Too many unknowns at the moment.
Thank you for taking the time to reply.

The device is a game boy advance. I believe getting a higher voltage than the battery can supply is pretty trivial, although in this case i do not know exactly how it is done. Doesn't really matter at this stage and i might be remembering wrong, could be 12V instead of 17V.

I was not clear in what i am asking and i apologize.

What i want to know is, if the general idea of this circuit is correct. i.e. using a Transistor and 2 resistors as i have, one resistor used to bias the transistor and one to limit the current that i want to sink from the cathode.

Why should R1 be 1 K?

I designed this circuit from what i could gather online and by looking to how other people did it so i am still not quite sure why i need the R2.

From what i can understand the transistor acts as an amplifier and sinks current from collector to emitter which is a multiple of the current that is passing though the base (Ic = β* Ib) and so depending on the value of R1 could be extremely high and fry the LED backlight, so that is the main reason i need the R2 to limit the current. Is my understanding correct?


PaulRB

Yes, R2 limits the current flowing through the backlight. For most displays sold for use with Arduino, the display has a current limiting resistor built in. With a salvaged display, there may not be one. So it's safer to assume there isn't, and use one anyway, at least until you can establish the truth.

As for R1, there is a range of values that would be suitable. But it's important to "saturate" the transistor, so that it's voltage drop and heat dissipation is minimised. A 1K will almost certainly saturate it, but still limits the base current to a modest amount, around 5mA.

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