Renewable Energy Innovation

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A couple of years ago I had accidentally purchased a batch of 'low temperature' 16 x 2 LCD displays. These do not work with the standard 5V supply as the contrast pin requires a negative voltage for the display to appear (around minus 1.6V seems to work well).

This is a post on getting them working with the Serial LCD display kit.

These displays are manufactured by Powertip and have the code PC1602-H. A data sheet is available here. The part number is very similar to their 'standard' range (only the -H part changes). It was also not very obvious from the suppliers website that these were the low temperature range version.

As you might have discovered when using these displays outside, liquid crystal displays (LCDs) suffer from slow response in cold weather. This is due to the physical properties of the liquid crystal. To combat this, some manufacturers produce displays specifically designed for use in lower temperatures.

In the case of the Powertip PC1602-H, they require a negative voltage applied to the contrast pin on the display. If you do not apply this voltage then the display appears to not show anything, although the text is there, it does not show (or is very feint). If you apply a voltage of around -1.3 to -2V then the display will suddenly appear.

This took me a quite a while of head scratching to figure out. I really wanted to use these in prototypes, but needed to have a standard way of getting them to work (I have a bag of around twenty or so).

When searching for an answer about these LCD displays, other people had got them working, but no-one seemed to fully explain their circuit or how they did it. Some had mentioned diodes to give a small voltage drop. I never got that method to work for me, but This email address is being protected from spambots. You need JavaScript enabled to view it. a circuit diagram if you have got a more simple solution.

I wanted to still power the circuit with a single 5V supply, so needed to create a negative voltage from this.

There are fancy ICs which will perform this function, but I wanted to do it without buying any more parts.

I knew of a standard 555 timer circuit which powered a charge pump to give a negative voltage. I found the circuit diagram here. I have embedded the circuit here as well.

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There are many versions of this circuit with slightly different component values. I built the circuit above on a bit of strip board and it worked well, giving me around -3V from a 5V supply. The circuit uses the 555 timer as a constant output frequency (of around 600Hz in this case). This 'pumps' current into the output capacitor, with the diodes controlling the flow of current to ensure the output voltage is negative.

I added a preset variable resistor and replaced the variable resistor (VR1 - used for the contrast) on the Serial LCD display PCB with the three connections to the negative voltage circuit.

I sent some data to the display and then twiddled the variable resistor and the display appeared! Great! Its a bit of a bodge, but now I can use up a bag of displays without buying any new components.

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This is the LCD display working, with the negative voltage add-on circuit.

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The circuit was built on a bit of scrap strip-board, with components from my junk box.

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The circuit was attached to the space for the contrast variable resistor. (The other two metal wires are from previous attempts to get a negative voltage using diodes (which I had not luck with), so please ignore them.)