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Parallax Serial LCD Display, External Contrast Control — Parallax Forums

Parallax Serial LCD Display, External Contrast Control

tbarebowtbarebow Posts: 7
edited 2010-03-11 19:38 in Accessories
Has anyone determined the value for the contrast pot and which of the 16 pins it is connected to?· My project would be greatly enhanced if it had external contrast control.··I would prefer not to have to cut off the pot to expose the pins then experiment with pot values, that's really ugly.· Input greatly appreciated.

Comments

  • Mike GreenMike Green Posts: 23,101
    edited 2010-03-11 04:59
    Which display are you talking about? The 2 x 20 serial display doesn't have a contrast adjustment.
  • tbarebowtbarebow Posts: 7
    edited 2010-03-11 05:54
    Mike, I reread the spec sheet fo the 27976, 77 & 79. Page 5 of 11 specifically refers to 27977 & 79 as having contrast control. Is the Parallax documentation incorrect? In the Introduction it does refer to a 4 x 16 display which is not shown on the website or in the documentation.
  • Mike GreenMike Green Posts: 23,101
    edited 2010-03-11 06:18
    The documentation is correct, sorry. Parallax hasn't published the schematic of the serial LCD, so I have no idea of the value of the pot or where it might be connected except to use the 2 x 20 Parallel LCD as a model. That uses a 10K external pot for the contrast adjustment connected to pin 3 of the display. Typically the "different" pin is pin 1 which has a squared off PCB pad. Measure the voltage there with different settings of the contrast pot. You should see a voltage ranging from 0V to +5V on that pin. If that's what you get, then that's probably the contrast voltage input. You could set the pot to its midrange setting and override the voltage at that pin. Be careful to have the pot at the midrange setting or you could damage your voltage source (or the pot) since the ends of the pot are likely connected to 0V and +5V. Obviously, you're on your own if you try this. I'd have a close look at the PCB and try to use an Ohmmeter first before trying to remove the pot, check its value, and substitute an off-board pot or voltage source.
  • tbarebowtbarebow Posts: 7
    edited 2010-03-11 14:04
    Thanks for the reply. Parallax hasen't published a schematic because according to tech support they don't have one, nor do they have factual pot value info. Based upon the documentation the 2 x 16 and 2 x 20 contrast pots are board mounted. Removing one would require a LOT of unsoldering then resoldering or carefully sawing one apart to expose the pins. I did some investigating before posting and agree that the pot is connected from the +5V to pin 3 but its value appears to be closer to 3K, min R=1.6K max R=4.6K. Basically I'm on my own and clearly some emperical testing and experimentation is on the horizon.
  • MikerocontrollerMikerocontroller Posts: 310
    edited 2010-03-11 15:12
    · I looked at the pot on my Parallax ·2x16 serial LCD display and· "K 103"· is printed on the side.
  • tbarebowtbarebow Posts: 7
    edited 2010-03-11 15:49
    Have you checked terms 1 - 3 for resistance? When I checked I got the values stated earlier, min approx 1.6K max approx 4.6K
  • Roy ElthamRoy Eltham Posts: 2,998
    edited 2010-03-11 17:16
    Mikerocontroller, the 103 on there usually means "10 with 3 0's after" so 10k. It's similar to cap numbers.

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  • tbarebowtbarebow Posts: 7
    edited 2010-03-11 19:38
    Thanks for the invite. I'm not into the Propellers YET. Right now I'm having enough fun with the BSsx
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