Just one small thing. The backlight brightness is a little low. Can I turn it up somehow? My guess is that I need a new resistor, but bigger or smaller ?
So what was the solution to the backlight problem? (just so others with the same problem have the solution).
A smaller value resistor will make the backlight brighter, but you must not go below a certain resistance (which depends on LCD model) or you'll damage the LEDs. The max current is typically 200mA for a 4x20 display, but the datasheet for your LCD (from the ebay seller's page) says 80mA max at 5V. Do you have an external resistor connected in the backlight supply? If so, what value is it? (eg 100ohm pot). It may be OK to reduce or even remove this resistor.
There is often a series resistor mounted on the PCB to limit the backlight current. This shouldn't be changed, as it sets the maximum safe current with a 5V supply.
If you have a multimeter that can measure current (DC amps/milliamps), connect it in series with the backlight power and see what the current is. It looks like you can safely have any current up to a max of 80mA.
mattcro wrote:So what was the solution to the backlight problem? (just so others with the same problem have the solution).
Actually I did find the problem. I cut the wires and connected the circuit on a breadboard. I worked as it should. Then I soldered the hole thing back together on a piece of veroboard. Still worked perfectly. My guess is, that I had a small short somewhere on the old veroboard.
A smaller value resistor will make the backlight brighter, but you must not go below a certain resistance (which depends on LCD model) or you'll damage the LEDs. The max current is typically 200mA for a 4x20 display, but the datasheet for your LCD (from the ebay seller's page) says 80mA max at 5V. Do you have an external resistor connected in the backlight supply? If so, what value is it? (eg 100ohm pot). It may be OK to reduce or even remove this resistor.
I have the 100ohm resistor, as the schematic says. I have nothing else connected, only the things from the schematic. Is it the 100ohm resistor, that should be smaller?
I'm sorry, I don't know how to measure the resistance in the circuit. (I'm a newb in this sort of thing)
OK, I see what you mean now. You can indeed reduce the value of that 100 ohm resistor - just try resistors of decreasing value until you find one that gives a good brightness. Try something like 47 ohms (twice as bright) then 27 ohms and so on in smaller steps.
If you already have a bunch of 100 ohm resistors, try connecting one or more in parallel across the existing 100 ohm resistor to reduce the resistance gradually.
According to the data sheet for your LCD, you can have the backlight power 5V connected directly to the module (ie no external resistor), but I'd recommend using some external resistance, at least until you can make sure you don't exceed the max current. I run my 4x20 (green LED backlight) at about half of maximum brightness, except in bright daylight.
