LCD, possessed by the devil?

F

fred_b

Member
I have this LCD : http://www.411techsystems.com/Ebay/Specs/SSC2F08.pdf

Everything is connected and works fine. I have a 1000 ohm resistor between the LCD backlight power source (a picaxe output pin) and the LCD anode pin (pin 15).

The backlight does come on.

Trouble is, I disconnected the backlight power so that I could use that picaxe pin for another purpose and the backlight still comes on even with LCD pin 15 unconnected.


Anyone see how this could be?
 
Y

ylp88

Senior Member
Check to see if the backlight power suppply pins (both the anode and cathode, pins 15 and 16) are hard-wired to the LCD power supply pins (pins 1 and 2) using your multimeter and it's continuity or resistance functions. I've come across some LCDs where it is hard-wired.

Typically, you'll find that using a 1000 Ohm resistor in series with the backlight anode pin will not provide enough current to illuminate the screen. But of course this doen't really matter if the backlight sems to come on by itself anyway! :p

ylp88
 
W

westaust55

Moderator
LCD Backlight control

Many LCD modules have several sets of pads on the printed circuit board that give options for backlight power as follows:

1. onboard current limiting resistor

2. connection to the main power supply pins 1 and 2 as mentioned by Ylp88

3. taking the backlight supply from pins 15 and 16


Even for a given make and model, there can be several options available for backlight power. It is a case of what the retailer has ordered.

Nothing is shown to this effect on the datasheet you gave a link to but so checking with you multi-meter might enable you to determine what the circuit is.

If you look at the datasheet for the Ampire controller based LCD modules I have purchased
http://www.picaxeforum.co.uk/showthread.php?t=13661
You will see the jumpers/pad I mention on page 9 = section 7.
 
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