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Topic: Delay LCD backlight turn off (Read 3 times) previous topic - next topic


I have a project where I use a LCD. To save power, its backlight is normally off, and I have a button that, when pressed, will turn it on for a few seconds to allow reading the display.

Now, this is done on the software side, and works great, but consumes two pins of the Arduino. Is there a *simple* way to achieve the same effect in electronics, I guess using a condenser?


Tom Carpenter

Aug 18, 2012, 02:39 pm Last Edit: Aug 18, 2012, 02:41 pm by Tom Carpenter Reason: 1
Google "Monostable Multivibrator". I believe that is what you are looking for.

If you use one of those to drive the gate of a MOSFET which is connected between the cathode of the LCD backlight and ground ("MOSFET Switch"), then you should get the effect you are looking for.


Thanks a lot, Tom, seems what I need: http://en.wikipedia.org/wiki/Multivibrator#Monostable_multivibrator_circuit.

Would you recommend some precise model of transistors I can use to implement the circuit? I've never used any, before.

Tom Carpenter

Aug 18, 2012, 06:12 pm Last Edit: Aug 18, 2012, 06:15 pm by Tom Carpenter Reason: 1
There isn't really a specific one, any "NPN Epitaxial Silicon" Transistor will do. The BC547 is a good choice - its sort of your bog standard NPN transistor.

An alternative would be to use an NE555 timer chip. These are 8 pin IC's which are dirt cheap and can be used to make a monostable circuit.

In this circuit a 555 is setup as a monostable circuit with an output pulse time determined by the equation:
Ton = 1.1 * R1 * C1 as shown in the image. Here is a graph to help select the resistor and capacitor:

R2 is just a pullup resistor for the push button in that diagram, so a resistor of anywhere between 10k and 100k should do nicely.

The 555 is capable of sourcing 200mA of current which should be more than enough for the backlight. This means you could just connect the anode of the backlight to pin 3 of the NE555 and the cathode via a suitably sized resistor to ground.


If you use a 555 its best to avoid the original 555's and choose a CMOS version instead, the original 555's need massive decoupling capacitors (100uF electrolytic + 10uF ceramic is about right - the chip glitches upto 0.4A !!).  ICM7555 can only provide 100mA only though but doesn't crowbar the supply voltage in this way.
[ I won't respond to messages, use the forum please ]

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