Simple way to convert LCD segment to LED  - Page 1
Forum section: electronics / projects

author: 001
author_rank: Super Contributor
timestamp: November 17, 2017, 03:51:03 pm
content: I have one stuff the LCD display damaged and cant be fit new one ,because no more in the market.. And now i have and idea to use few 7 Segment LED modules and additional LEDsIs it ok to use simple discrete diodes and transistors to drive LEDs from former LCD segment pins? What about some modern way to do it?

author: MK14
author_rank: Super Contributor
timestamp: November 17, 2017, 05:32:42 pm
content: My understanding is that LCD segments are almost always driven with a lowish frequency, AC (square wave) signals, which continually swaps polarity, to minimise LCD damage by DC. DC eventually destroys LCD displays by electrolysis.That can make this harder, than it might at first appear.You can buy individual LCD screens (seven segment), from various sources.For LEDs, using an MCU, to read in the LCD status and then drive the LEDs. Might be the most practical solution.In general, it is probably NOT worth spending the time, to make such a fix. But if you are learning from the exercise or the item is very important, then go ahead.There are other possible technical challenges, further making the MCU a favourable solution. They may be multiplexed at a fairly low frequency, e.g. 5 Hz (LCDs of this type, are usually rather slow to change). Which would be too slow for LEDs, which would flash somewhat horribly. An MCU could fix that issue as well.What gadget is this for ?

author: Gyro
author_rank: Super Contributor
timestamp: November 17, 2017, 07:19:07 pm
content: The way to drive a segment on a simple LCD is to XOR the DC input signal with the backplane drive. The way to get a DC segment output from an LCD drive is the same way, XOR the LCD Segment signal with the LCD backplane signal. You're going to end up with a few XOR gates though.

author: 001
author_rank: Super Contributor
timestamp: November 17, 2017, 07:31:43 pm
content: Is it usable?

author: 001
author_rank: Super Contributor
timestamp: November 17, 2017, 07:33:30 pm
content: OH! Even old Flukes!https://lous.home.xs4all.nl/fluke/Fluke8050Asite.htmlAnd more epic project (not wanted but looks good):

author: Gyro
author_rank: Super Contributor
timestamp: November 17, 2017, 07:45:24 pm
content: Nice conversion - but will only work if the LCD is being driven by discrete logic.QuoteFor a normal LCD drive, the backplane and segments are driven in anti-phase with the backplane and the backplane is driven with a 50:50 square wave. Segment signal opposite polarity to backplane= segment on, Segment signal same as backplane=segment off.As a result, all signals are going up and down all the time - hence the need for XOR gates. That's unless it's descrete logic where you can 'stop' the backplane oscillator.Displays with lots of digits / segments use multiplexing where the signals go through several voltage steps (no chance with those!).Edit: On the up-side, you can use the XOR gates to directly drive reasonable efficient LEDs.

author: Gyro
author_rank: Super Contributor
timestamp: November 17, 2017, 08:29:55 pm
content: ... actually a pair of SO14 packages (74HC86) and 8 resistors, per digit, might not be impossible with a little microsurgery. It's smaller than your suggested circuit.

author: MK14
author_rank: Super Contributor
timestamp: November 17, 2017, 10:35:50 pm
content: Quote from: 001 on November 17, 2017, 07:33:30 pmThat is really neat, and probably not that hard to do. Once you have read the LCD status, into your MCU or Arduino etc. You can plug a multiplexed-LED display (usually cheap and fewer wires than single 7 segment displays) or even a backlit LCD 16x2 or bigger. Or even a graphics display (just like in your picture), which can plug straight into an Arduino, so all you have to do is scan the LCD lines and send that information to the screen using the Arduino libraries.Although what is shown in your picture, looks like a lot of effort has been made, to make the picture so detailed and nice.EDIT:Woa. Apparently, he seemed to put a version of the Forth, programming language into a STM32 (in Arduino like/compatible, mode). That is rather impressive. Maybe that is how he was able to relatively quickly sort this thing out. Although I would have thought C would be fast as well.https://madnessinthedarkness.transsys.com/projects:fluke8050a:start

author: 001
author_rank: Super Contributor
timestamp: November 18, 2017, 12:55:52 pm
content: So it is only occasianol google picture. It is overengineered IMHOAll what I need is to convert LCD segments to LED no more