How can I amplify a drive signal for an LCD watch display  - Page 1
Forum section: electronics / projects

author: kraychek
author_rank: Newbie
timestamp: February 13, 2023, 02:45:05 pm
content: Hello,I am trying to take a voltage drive signal for a single segment of a small zebra-connected LCD panel (pcb side of course) (like the ones used in vintage lcd watches). With the aid of an oscilloscope I was able to locate the segments of interest, and with an lm358 op-amp configured like a schmitt trigger comparator, I was able to power an led when the LCD segment of interest got "lit". Yes, my only goal is to use a couple of LCD segments of interest to power a couple of different LED diodes, depending on which display segment is lit. Has anyone tried this kind of experiment? Maybe anyone has some good and proven solutions? Any help, schematics, or off-the-shelf component recommendations or ideas would be greatly appreciated.Kind Regards,Kraychek

author: Gyro
author_rank: Super Contributor
timestamp: February 13, 2023, 07:24:43 pm
content: Non multiplexed LCD displays still use a square wave AC drive (segment and backplane alternating low and high) to prevent any net DC offset, which would seriously limit their life.The traditional method for driving segments (for instance decimal points or annunciators, not controlled by the main driver IC) is to XOR the DC logic input signal with the backplane (bp) signal to produce a segment drive which either follows the bp (segment off) or alternates with it (segment on). This method would normally use a CD4070 quad XOR package. To recover a DC level from a segment signal, the same method can be used - XOR the segment signal with the bp signal. You should be able to use a 74HC4070, with supply voltage depending on LCD drive voltage.I would advise using a CMOS input device rather than a bipolar due to the risk of introducing DC offsets from their input bias currents and (long term) damaging the LCD.P.S. 74HC logic is more than capable of directly driving modern LEDs to decent brightness.

author: kraychek
author_rank: Newbie
timestamp: February 13, 2023, 08:18:24 pm
content: Thank you very much for your expertise, Chris.I have one more question if I may: Is the 74HC4070 ic not going to introduce too much load to the driver ic driving the LCD, causing the display to lose contrast?Kind regards,Kraychek

author: Gyro
author_rank: Super Contributor
timestamp: February 13, 2023, 08:26:57 pm
content: You're welcome. No, you should be fine. The CMOS input of the HC logic is very high impedance and low capacitance, and the LCD drive frequency is only a few 10s of Hz at most.You may need to drop the supply voltage to match the display driver and meet the correct input logic thresholds - 74HC logic will accept 2-6V supply (although you will have trouble driving anything other than a Red LED below around 3V).Edit: I forgot to say, you can also use the more common 74HC86 XOR gate. This is available in HC and HCT variations - HCT might be helpful if the display drive voltage is low and you want to maintain a 5V logic supply voltage.

author: kraychek
author_rank: Newbie
timestamp: February 13, 2023, 08:37:48 pm
content: I understand now.Once again, thank you Chris for your expert advice. It's most helpful.Best regards,kraychek

author: Gyro
author_rank: Super Contributor
timestamp: February 13, 2023, 08:41:05 pm
content: @kraychek, just in case you don't notice it, I just added an edit above. I should have thought to suggest the 74HC86 as it is far more common than the 74HC4070.

author: kraychek
author_rank: Newbie
timestamp: February 13, 2023, 08:49:03 pm
content: I would have probably noticed it eventually. Thank you very much for the edit and your effort in helping me.Kind regards,kraychek