Sorry for my English, this is a automatically translated message.
Thank you so much.
Original message in:
(Spanish
)
Gp32Spain
Warning!
I have made this mod with my old knowledge of electronics and what I have been learning on the internet as I go.
There may be unforeseen side effects, since I have applied the trial and error method.
I must also warn that the plastics that make up the Psion have become very brittle and break very easily over the years.
The Psion for which I made this mod has suffered damage caused by this ease of breakage and even though it was in an excellent state of conservation.
I have done the electronic part a little by groping, since this is where my knowledge falters, mainly when it comes to replacing the driver chip of the machine and taking advantage of its wiring.
I have also had problems getting the components, all purchased on Aliexpress. Problems that I will point out later. Although you already know how this store works...
Introduction
I have been wanting to get my hands on this 'mini computer' for a long time.
The problem was with its screen, which practically needs direct sunlight to be read, especially at my age.
We add to this that my free time is usually after dinner when I am in the dark in the living room, because there was no way.
After giving it a lot of thought, it became clear to me that I had to update the factory backlight, which over time has lost all its powers and barely illuminates, either that, or it was already that bad at the beginning.
I usually use broken LCD backlights but they have the problem of being too thick.
A few months ago, in my search for rare components on Aliexpress I found that someone was finally selling them.
I started ordering different models until I found the right one. You already know that Ali is not very given to giving you characteristics of anything he sells, and when they give it to you they have errors aplenty and you cannot trust them.
What you have here is the result of reading and watching videos to learn how a backlight, a driver board and a mostfet work, which are basically the components we need.
Add to this a small study to see if the original backlight that the Psion came with could be replaced with a new one.
I hope it helps you and shortens the path to this type of mods.
Note: I will publish this article in various forums where I roam, also in English translated by Google & Cia.
Let's begin...
First, and very carefully, I disassemble the Psion completely. At Ifixit we have the disassembly.
This is where I remember the designer's family and I don't know whether to give him a prize or burn him at the stake.
Every time I have to disassemble and reassemble the Psion I have a hard time, given the difficulty and complexity of its construction.
It is full of little tabs and protuberances that take a lot of effort to fit each one back into place, not to mention when you have to reconnect the keyboard and screen... Phew! What a design!
I also disassemble the entire screen, including the display inside. You have to remove the sheet that is attached with tabs and a plastic sheet.
This is how you access the old backlight.
This one comes partially glued and welded at two points. Points I use to solder the new one.
Above, on the right, there is a transparent rubber plate that must be removed and kapton tape applied to thin it.
Behind the original backlight there is a semi-transparent sticker that must also be removed.
Do it little by little, pulling it in stages.
It is not advisable to use the heat gun because we can damage the display, which happened to me on the test model. The poor thing now has an area where the LCD does not work.
I remove the glue that remains on the display with isopropyl alcohol, without soaking it too much, rubbing repeatedly, changing the cotton if necessary and without pressing too much.
It must be left as clean as possible.
It is advisable to trim the backlight a couple of millimeters. If it is not cut, it goes too far right into the LCD casing.
Forcing the plastics will cause them to break over time.
The first backlight I did on a Psion made me find it completely burst in a matter of weeks because I fit everything in too tight. It was cracking little by little.
The backlights can be cut as long as we do not cut the line of LEDs that are usually on the side of the connector.
To cut the plastic itself, do it with a very sharp cutter, making multiple passes and without pressing. They are multiple layers and if we press too hard they crack.
The remaining metal can be cut with sturdy scissors that we don't have much love for.
After this, I remove all the stickers so that it is as fine as possible and I proceed to connect it.
The top box shows where the transparent rubber sticker went and I have already replaced it with kapton tape.
The box below indicates the order of the cables. Left ground
(black
), right positive
(red
).
Although you see a couple of cables in the photo, in the end I connected it using enameled wire, which is finer, because with those cables it was very 'tight'.
I have also put an SMD resistor in series because the light it gave seemed excessive to me. I usually work in the dark and it bothered me.
It is so small that it cannot be seen in the photo.
150 Ohms and goes in series with one that already has the 100 Ohms Psion plate itself.
The higher the resistance, the less light the Backlight emits and the lower its consumption.
The best thing would have been to put a potentiometer on it, but I couldn't find where to put it within Psion, so I opted for the easiest option and not put it.
Notes on backlight.
I have purchased numerous backlithg looking for the right size.
Their supply voltage has varied between 12V and 25V, as well as the dimensions, shape, etc.
To calculate the size of the backlight before ordering it, I looked at the size of the phone it belonged to and that gave me approximate dimensions.
The one that has been the best for me is the Huawei Mate Nova Y6 2018, whose measurements were almost the same as the Psion's screen. Its supply voltage was 12V, which made it easier for me to look for a driver board that would convert the 3V of the batteries in the 12 V backlight.
But...
When I asked them to send me more of the same, it turns out that the new ones are no longer
12V, they are 15V. And supposedly they are the same. What a job.
I will have to look for another driver or put two to reach 15V, I will study it.
Well, now I have 10 15V backlights for future projects, as well as a few more from test orders. What are these Chinese ones like...
Of the little pins that the connector has, the last one is the anode
(letter 'A'
), that is, it is the positive one. Sometimes they are well marked on their silkscreen
(anode 'A', cathode 'K'
), other times they don't have anything or simply numbers.
The others are to light different branches of the LEDs and I solder them all together.
Do tests with them to familiarize yourself before getting into the mess, which is what I did.
I tried a couple of drivers, different resistors, mosfets, backlights and the potentiometer trick before deciding what the final project would be like.
I only broke one because I cut it too much.
Drivers fell a few.
To see the voltage at which the backlight works, I start by feeding them with 3 V and go up.
The one that has asked me the most has been 25 V, and there is everything: 12V, 15V, 24V, 25V...
The only thing is the fear of having connected them backwards, because until I get close enough to their working voltage, the backlight does nothing.
Full backlights shine incredibly brightly.
The driver.
There are many 12 V drivers on Aliexpress, but you also have to be careful with them, as they break on their own.
And when I say alone it is for the simple fact of turning the 3 volts that power them on and off. Thank goodness they are cheap.
What I do is put them to work and turn them off and on several times.
If it doesn't break, it's worth it.
I have not been able to find 15V drivers as small as 12V ones. If anyone finds them, please comment.
The drives are easy to use: you connect the 3 V of the battery to the input and to the output you have 12 V.
These in particular allow you to choose other voltages, but from the factory they are set directly to 12 V, which is what interests us. So you don't have to touch anything.
The Mosfet.
At first I took the first Mosfet I found, since it is not relevant, but then I opted for one that was more or less 'standard' and that could be easily bought so that it would serve us all.
With the voltage and currents that we handle, anything will do, unless you go for the smallest ones.
The backlight current is very low so they do not heat up in the slightest.
Note that pin 'D' is also the heatsink.
According to its datasheet, the chosen mosfet can handle a maximum voltage of 30 V and is capable of dissipating a maximum power of 140 W, which means that we have more than enough mosfets and that we could have chosen a much smaller one, one of these shaped like an SMD integrated circuit, there are very small ones, but I feel more comfortable handling this type.
Also according to the tables, the mosfet below 1.5 V would be in cut-off and above 2.3 V in saturation, or at least that is what I thought I understood, because it is difficult for me to interpret them.
Here we have a typical mosfet connection configuration.
I have ignored the input resistors, those connected to the gate
(G
) because I understand that they are necessary when we work in the intermediate zone of the mosfet or when it continuously goes from cut-off to saturation.
In our case we will turn it on and off sporadically. The mosfets do not consume or suffer anything when working in the cut-off/saturation zones.
To the gate
(G
) we will connect the Psion ignition signal and the load in our case would be the driver board with its corresponding backlight.
The pin
(S
) would go to the negative of the batteries and
(Vcc
) to the positive of these.
At the end you have the link to the Datasheet.
We continue...
Inside the Psion there is no space and everything gets in the way.
Look carefully at the photos to place the cables and the insert, otherwise you will trip over some protuberance, burr, and even the pencil when storing it.
We have to remove the circuit that acts as a driver inside the
Psion.
It doesn't work for us because what it generates is about 70V AC and we need 12V DC.
Yeah. The Psion used a backlight from the time that seems very strange to me, I would say it is chemical.
It is a plastic sheet covered with a green paint that generates some light under that voltage.
I don't know anything else about this backlight.
Given the complexity of running cables inside the Psion, I have taken the risk and used the circuitry itself that communicates the Psion driver with the LCD.
This circuitry incorporates a resistor that goes in series with the one I have already added. If you don't put any more, it will simply shine much brighter and consume a few more milliamps. To taste.
Following the sketch that I have indicated, I connect the driver input to the battery input. The ground to the ground of the CF card, and the control and output of the driver to the pinout of the old driver.
By using the solders from the old driver we save having to wire to the new backlight. Be careful with the polarities. The red wire would be the positive one, the black one would be the negative one, and the striped greenish-gray one would be the control signal.
With this I use the MostFet as an electronic switch, cutting off the power to the backlight using the original Psion signal.
When it is off, its consumption is practically zero.
The Psion consumption with the original backlight on is 85 mA, with the new one 71 mA.
The Psion, with the backlight off, consumes 42 mA.
The Psion in 'off' mode, I don't know how much it consumes because the power supply reads 0.000 A, so it's so little that it's out of range for me.
I cover the circuit with Kraton tape to insulate it and position it as seen in the photos.
The housings should fit almost perfectly when we reassemble the Psion.
I warn you again not to force them. If it doesn't break at that moment, it will break as the weeks go by.
Use the thinnest cables you have and look for the best space for the new circuit.
I have used cables from a broken SCSI connector. They are some of the thinnest I have ever seen.
From there you have to move on to enameled wire for much finer things.
If everything has gone well, we have a nice mobile-style backlight, which illuminates much more and consumes less than the old one, it is operated with the keyboard like the original and will continue to be seen perfectly in daylight with it turned off.
All the best.
Note: Some photos belong to previous tests but I have included them to further illustrate the topic.
Links of interest:
Disassembly of the Psion
(English
).
https://es.ifixit.com/Desmontaje/Psi...n/1650?lang=en
Operation of a Mosfet
(Spanish
).
https://www.luisllamas.es/arduino-tr...%2C%20y%2011A.
Mosfet on Ali
https://es.aliexpress.com/item/1005006127790007.html
Driver 12 V on Ali.
https://es.aliexpress.com/item/1005001570640935.html
Backlight on Ali.
(I chose the Y6 2018
)
https://es.aliexpress.com/item/1005004046729984.html
DataSheet of the used Mosfet.
https://www.alldatasheet.es/datashee...RF/LR7843.html
) and the final product is fantastic. The factory backlight of the Psion 5* is so muddy compared to your replacement. 
