Re: Shack clock contrast adjustment


KI7MWA
 

If I am reading the schematic correctly, the three options (no connection/jumper/resistor) produce three different results:  with no connection (meaning neither R4 nor a jumper is installed), the contrast control pot is connected to ground, therefore Vcon is connected to ground (through more or less resistance), and the pot does nothing at all; with the jumper, the contrast control pot provides a variable voltage to the Vcon pin between 0 and +5 volts; with a resistor installed, the contrast control pot is one element of a voltage divider, therefore providing Vcon a voltage of between 0 and a maximum determined by the ratio of the maximum resistance of the pot to the sum of that resistance plus the resistance of R4 - in the case of R4= 220k, the maximum would be 5V * (100/(100+220)) = 3.44 volts.

In my experience, with no connection, the LCD demonstrates two states: no contrast (with the contrast control pot fully clockwise I can see characters in the blocks of dots if I look at the LCD from the side, but not if I am looking straight on), and blank (with even a slight shift of the pot, neither characters nor blocks of dots are visible).  

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