Wrong. It would kill the backlight of the LCD. Read the specs on max ripple for the supply and you will see that they have a very low tolerance. I did this once, (by mistake? / ignorance) and the backlight gave up after a month! Kyrre ----- Original Message ----- From: "Chris Hunter" To: Sent: Saturday, November 23, 2002 11:23 AM Subject: Re: [EE]: Automatic LCD brightness control > You're right - most photocells are in the k ohm range, and are quite > non-linear. As I recall, the resistance vs illumination curve is roughly > square-law. You could use a small power-FET in series with your 5.1 ohm > resistor, with the gate controlled by the photocell. This will work, but > there are better ways of doing it.... You could use a variable mark / space > oscillator to drive your backlight, which would give you variable > brightness, with good efficiency. > > Chris > > ----- Original Message ----- > From: "PicDude" > To: > Sent: Saturday, November 23, 2002 6:17 AM > Subject: [EE]: Automatic LCD brightness control > > > > Need some assistance with using a photocell to control the > > brightness of an LCD backlight. > > > > I don't know the current drawn by the backlight, only that > > the datasheet says to use a 5.1-ohm resistor from 5V. I've > > determined that the proper brightness I need is 10 ohms at > > night, and 5 ohms during the daytime. The actual curve or > > transition from the high to low value is not critical, but > > I want to use a photocell (rather than a phototransistor, > > etc) so the transition is gradual as it gets darker. > > > > The resistance of Cadmium Sulfide photocells goes down with > > increasing light, which is the correct trend. However, the > > resistances I can find are all in the K-ohm range. So now > > I need to figure out how to (simply) translate this into the > > correct resistances. > > > > I can think of many ways to do this "digitally" using PICs, > > PWM, comparators or switching-transistors, etc, but can't > > seem to figure out what to do for a simple linear circuit. > > > > Any ideas? > > > > Thanks, > > -Neil. > > > > --- > > [This E-mail scanned for viruses] > > > > -- > > http://www.piclist.com hint: PICList Posts must start with ONE topic: > > [PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads > > > > > > -- > http://www.piclist.com hint: PICList Posts must start with ONE topic: > [PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads > > -- http://www.piclist.com hint: PICList Posts must start with ONE topic: [PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads