N3ZI Digital Dial
Instructions for installing Contrast Control

In general you can use a 10K resistor for R3 and the LCD display will look fine when using a 5v supply (or and unregulated power source with the installed 78L05 5 v regulator.  If you are using a lower Vcc such as 3.7 volts from a battery, generally you can remove R3 and replace it with a short to ground.

But there are some cases which you might want to variable contrast control.  Typically with portable radios where you might be looking at the display from different angles, and adjustable contrast control will allow you to optimized the readability at the angle you happen to be looking.  Here you would remove R3 and install a trimmer pot (10K) in the 3 holes for it.  This will give you the ability to change the contrast at different viewing angles. 

However, a slider pot gives you much nicer control.  A slider pot can be mounted straddling the top edge of the circuit board.  You would have to remove R3, fixed or trimpot.  There is a rectangular pad near the upper left cornet of the PCB.  The is a corresponding pad on the back side.  The one on the front is connected to Vcc, the one on the back is connected to the LCD contrast control.  The contrast control needs to be wired electrically between power and ground, with the wiper connected to the LCD contrast control pin.

You will need to scrape off the solder mask covering these two pads, and also at a point around the XTAL or C2 on the ground plane depending on how long your slider pot is.  (I recommend a 10K slider pot, I know 100K does NOT work, I have not tried 1K, but it should work too)  I got the one shown in the photos from electronics goldmine, but it was a surplus item, and I don't know how many they have in stock.  Mouser also sells one that works fine but is a little longer, in which case the ground connects a tad further down.  The solid plane on the front of the PCB is connected to ground so you can connect anywhere.

Slider Pot Front

Slider Pot Back
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