The challenge here to to provide a way for non-technical people to replace a failing display with a identical looking current technology equivalent without needing a soldering gun or any need to dig into the electronics. Much of this was easy. Physical layout and logic connections are the same, so in theory, the display could be replaced with a screw driver and unplugging 2 connectors, so I decided to see if I could design a "plug and play replacement." The display is basically a standard 4002 (40x2) LCD display. The original 1985 product uses the same logic configuration and supply voltage as the current products. The original backlight was much different- driven by +12vdc to the Anode and -12VDC to the cathode (24vdc). Current displays looks for 5V difference between the A and the K. I resolved that with a simple potted regulator circuit so the user could still use his +/- 12vdc source connector, but deliver 5vc to the backlight. Now I have everything resolved but the contrast circuit which unfortunately, is important. The original display uses -12VDC (minus) source for contrast control to pin 3 (yes, the logic circuit is still 5 vdc). Current technology uses +5 at pin 3. See attached schematics. The original display is a Sharp LM402B02 which I cannot find ANY data on except one "simplified" one page data sheet which lists a "secondary support voltage" requirement of -12.
So.... I think my question is.....is there a simple circuit I could put in line to pin 3 that would convert the existing variable -12vdc signal to a variable +5vdc signal at the display? Any thought? Thanks.
So.... I think my question is.....is there a simple circuit I could put in line to pin 3 that would convert the existing variable -12vdc signal to a variable +5vdc signal at the display? Any thought? Thanks.
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