Author Topic: Question about LCDs  (Read 821 times)

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Offline MitiTopic starter

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Question about LCDs
« on: December 02, 2019, 01:30:18 am »
Hi,

I have a question about 7 segment LCDs. I'm trying to improve a weighing scale and reuse the load cell, LCD and case. The initial device was supplied by two AAA batteries, I have a boost converter on my board and I boost the 3V to 5V. I'm using a PIC16F917 and a HX711 load cell ADC.
The question is, are the LCDs built for a specific voltage? At this moment my code can display something on the LCD but depending on the supply voltage, I get lots of ghosting or not. AT 5V, the intended supply voltage, everything is turned on regardless of the LCD registers. At 3V, it works well. The ghosting starts after 3.1 - 3.2V.
The LMUX is 1/4, the bias is 1/3.
I wonder if a resistor between VDD and LCD Bias 3 to reduce the Bias 3 voltage to about 3V would help.

Thanks,
Miti
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Online coppice

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Re: Question about LCDs
« Reply #1 on: December 02, 2019, 01:37:17 am »
Most segmented LCDs only give optimum contrast over a narrow voltage, and the contrast degrades radidly as you move away from that. Narrow temperature range 4 mux panels are usually specified for somewhere in the 3V to 5V range. Wide temperature range 4 mux panels are usually in the 3.5V to 5V range. The bias resistor lets you tweak things for optimum contrast, but it can't usually overcome a big voltage error.

 

Offline MitiTopic starter

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Re: Question about LCDs
« Reply #2 on: December 02, 2019, 03:02:17 am »
Most segmented LCDs only give optimum contrast over a narrow voltage, and the contrast degrades radidly as you move away from that. Narrow temperature range 4 mux panels are usually specified for somewhere in the 3V to 5V range. Wide temperature range 4 mux panels are usually in the 3.5V to 5V range. The bias resistor lets you tweak things for optimum contrast, but it can't usually overcome a big voltage error.

Thanks, good info!
Aren’t those 3 bias resistors supposed to set exactly that, the voltages the mux swings between?
Just look in the LCD section of PIC16F917/946. That should make it independent of the supply voltage.
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