1602A LCD contrast issue

I tried search, but it appears to not be working at all today. I have a white on blue 2x16 LCD with the text "1602A" on the back. I have this successfully working with my arduino UNO and a gps receiver to display coordinates and MPH. I have the contrast AND the backlight on seperate pots so I can adjust both. The problem I am having is the contrast (pin 3 on the LCD) doesn't seem to have enough adjustment, almost like I need to go into negative voltage, to get the letters bright enough. In a brightly lit environment, you can't see it, and the backlight is definitely on. The contrast is on a 10k ohm pot, but even with pin 3 going straight to ground, the letters are not bright enough. I did a google search and saw some mentions of certain LCDs requiring negative voltage on contrast. Perhaps this is the case, but I don't know how you would go about doing that. Any help is appreciated.

I did a google search and saw some mentions of certain LCDs requiring negative voltage on contrast. Perhaps this is the case, but I don't know how you would go about doing that.

  • You need a second power supply or battery.
  • Connect the positive terminal of this supply to GND.
  • Disconnect the end of the potentiometer that is currently going to GND and connect it to the negative terminal of the new supply.

Don

floresta:

I did a google search and saw some mentions of certain LCDs requiring negative voltage on contrast. Perhaps this is the case, but I don't know how you would go about doing that.

  • You need a second power supply or battery.
  • Connect the positive terminal of this supply to GND.
  • Disconnect the end of the potentiometer that is currently going to GND and connect it to the negative terminal of the new supply.

Don

Can the same power supply be used to feed the main circuit and the LCD contrast? Currently I'm using this device in a vehicle, so feeding the 12Vdc through a 12v-5v voltage regulator to supply the 5V to the circuit.

lewisdw:
Can the same power supply be used to feed the main circuit and the LCD contrast? Currently I'm using this device in a vehicle, so feeding the 12Vdc through a 12v-5v voltage regulator to supply the 5V to the circuit.

Well, yes and no, sort of!

The logic ground must go to the common ground with everything else, it is too difficult otherwise. You need to try a contrast voltage negative to that ground.

Try it first using a 6 or 9V battery to generate the negative voltage, connected as Floresta advises. If the negative voltage is useful, you need an ICL7660 (dirt cheap on eBay) or similar to generate your negative voltage from the positive supply.

Can the same power supply be used to feed the main circuit and the LCD contrast?

Yes. After you add the extra battery (or power supply) you are only changing one wire. The wire that used to go from one end of the potentiometer to GND now goes from the end of the potentiometer to the (-) of the extra battery (or power supply).

Don

Paul__B:

lewisdw:
Can the same power supply be used to feed the main circuit and the LCD contrast? Currently I’m using this device in a vehicle, so feeding the 12Vdc through a 12v-5v voltage regulator to supply the 5V to the circuit.

Well, yes and no, sort of!

The logic ground must go to the common ground with everything else, it is too difficult otherwise. You need to try a contrast voltage negative to that ground.

Try it first using a 6 or 9V battery to generate the negative voltage, connected as Floresta advises. If the negative voltage is useful, you need an ICL7660 (dirt cheap on eBay) or similar to generate your negative voltage from the positive supply.

Assuming a negative voltage from a battery works, how do I determine the value of the capacitors to use with the ICL7660?

I tried it with two 1.5V batteries in series, supply -3V for the contrast and it worked. So I got a ICL7660 from a local place, and wired it using 10mF caps as shown here (http://electronics-diy.com/electronic_schematic.php?id=603). When I powered everything on, I didn't get any characters on the display, but did smoke the ICL7660 (lukily, I have two). So, I verified the circuit is correct and made sure the arduino circuit and LCD still works (they do) using the 1.5V batteries to provide -3V for the contrast again, everything is still good. Could the LCD contrast be drawing too much current for the ICL7660? Caps the wrong values?

So I got a ICL7660 from a local place, and wired it using 10mF caps as shown here (http://electronics-diy.com/electronic_schematic.php?id=603).

You have to tell us how you interfaced the circuit in the link to your Arduino/LCD circuit.

Don

floresta:

So I got a ICL7660 from a local place, and wired it using 10mF caps as shown here (ICL7660 DC to DC Converter from 5V to +/- 5V).

You have to tell us how you interfaced the circuit in the link to your Arduino/LCD circuit.

Don

I got it working! I used this circuit as my example (http://www.sprut.de/electronic/lcd/kontras2.gif). The letters actually appear white and I have plenty of contrast adjustment now. Thank you all for your assistance.

lewisdw:
So I got a ICL7660 from a local place, and wired it using 10mF caps as shown here. When I powered everything on, I didn't get any characters on the display, but did smoke the ICL7660 (luckily, I have two).

And now you fully know the risks of the Internet.

lewisdw:
So, I verified the circuit is correct

Clearly it wasn't. An article by some rank idiot - well, some well-meaning guy - who had never bothered to study the ICL7660 datasheet which clearly states

Do’s And Don’ts

  1. Do not exceed maximum supply voltages.
  2. Do not connect LV terminal to GROUND for supply voltages greater than 3.5V.

... before implying in that article that it was somehow actually equivalent to the LTC1046 which does in fact require that connection for supply voltages less than 6V. :o

I am guessing you are not the only one to have your fingers - or chips - burnt.