That’s another argument for using custom software on a smartphone. It becomes much more independent of hardware. I’ve developed iOS apps that run on just about every iOS device still running.

I don’t mean to minimize the difficulty of writing software but it frees you from worrying about the hardware also.

So you're suggesting the software can simply be moved around on each phone? That does sound like a nice option. I very specifically do not want this device to be anything but a recording and note-taking device. I already have an iPhone for all that other stuff. If I were to put custom software on an old iPhone (which I do have a couple I'm not using), how do I clear the device of its current software and put my custom software on?

 

I'm not too concerned about writing the software either. iPhones are great, that's my daily driver, I personally still want a standalone recording and note-taking device, for simplicity.

Why not get an older (cheap/free) one without cell service? You don't need service for the functions you mentioned.

 

There were a lot of PDAs sold a while back. PDA=personal Digital Assistant. They are sometimes available really cheap at garage sales and resale shops. So that can provide the functionality you seek in a well designed package that only needs new batteries.

The suggestion for a used smart phone is a very good one. And probably more than a used PDA, but far more powerful. A good idea.

 

So you're suggesting the software can simply be moved around on each phone? That does sound like a nice option. I very specifically do not want this device to be anything but a recording and note-taking device. I already have an iPhone for all that other stuff. If I were to put custom software on an old iPhone (which I do have a couple I'm not using), how do I clear the device of its current software and put my custom software on?

Well you cannot remove the iOS and I believe a few apps cannot be trashed. You can load custom software using Xcode. But pay attention to which versions works with which iOS version, which in turn is limited by the age of the iPhone. I stay current, and I believe the very latest Xcode does not support anything older than iOS 11. So you may need an older version of Xcode to support an old phone.

 

The battery part is easy, the charging part I don't have any experience.

The touchscreen depends. Some are simple two to four wire serial connections (SPI, I2C, UART) while others can be up around 40 pin parallel connections. Some have a dedicated touch controllers that will give you coordinates while others you have to measure voltages of pins and calculate the position from that. The common displays have all sorts of tutorials, code, and help scattered across the internet. Search arduino lcd to get some ideas.

Just make sure to research the best you can before buying a display. Some cheaper offerings don't always list the correct controllers in the specifications and that can lead to a lot of headaches (or in the case of one I bought a fancy paper weight).

So something like the PI Pico for example, I would also need to purchase a touchscreen, a battery, a microphone module, and a speaker module. All those modules can plug into the pico microcontroller via the pins along the side of the board?

 

Well you cannot remove the iOS and I believe a few apps cannot be trashed. You can load custom software using Xcode. But pay attention to which versions works with which iOS version, which in turn is limited by the age of the iPhone. I stay current, and I believe the very latest Xcode does not support anything older than iOS 11. So you may need an older version of Xcode to support an old phone.

Ok so you're more talking about hacking the phone to put a custom app in, correct? I'm not really interested in doing that. I would like to build this device and the software from the ground up, and customize what the hardware buttons do, what the device does when you turn it on, etc.

 

Lots of people giving very good solutions here, thank you. Just to clarify, I already have an iPhone that I use everyday and I am aware of its capabilities. I still want to build a portable, standalone recording device that I can keep separate from my phone. I want to customize what the hardware buttons do when you click them, what happens when you turn the device on, and what you see when the device is on, etc.

 

I want to customize what the hardware buttons do when you click them, what happens when you turn the device on, and what you see when the device is on, etc.

Yeah, Apple doesn't let a developer take that level of control over an iPhone. And I was not referring to hacking or jailbreaking, just regular developer tools.

 

So something like the PI Pico for example, I would also need to purchase a touchscreen, a battery, a microphone module, and a speaker module. All those modules can plug into the pico microcontroller via the pins along the side of the board?

Sort of... the LCD will be the easiest in a sense since it has been done a million times with thousands of how to pages online. There are some sound modules that would simply plug in and how to and such written for them. Most are limited to x number of seconds. Connecting things is the easy part. Getting the pins to do what they need to make things work is where the real "fun" will be.

 

I doubt that there is enough memory in that assembly to save a useful amount of audio, and so additional memory will be another interesting challenge.

 

Sort of... the LCD will be the easiest in a sense since it has been done a million times with thousands of how to pages online. There are some sound modules that would simply plug in and how to and such written for them. Most are limited to x number of seconds. Connecting things is the easy part. Getting the pins to do what they need to make things work is where the real "fun" will be.

So it sounds like the bulk of the work will be on the programming side?

 

I doubt that there is enough memory in that assembly to save a useful amount of audio, and so additional memory will be another interesting challenge.

Right, I was thinking an SD card slot could work, if the unit can be configured to work with one.

 

I doubt that there is enough memory in that assembly to save a useful amount of audio, and so additional memory will be another interesting challenge.

Or are you talking about RAM?

 

Right, I was thinking an SD card slot could work, if the unit can be configured to work with one.

It's possible with the right stuff maybe... CD quality sound is 1,411 kilobits / second. SPI (which is what you'll be using unless you fork out the cash for the real SD specs) doesn't have a maximum speed, but depending on what you are using may not be able to move data fast enough. RAM is also a possibility to maybe save it temporarily and then transfer to SD card.

I'm also thinking unless you want to spend the money to have this machine built you will end up with something more along the lines of a size of a brick instead of something you can put in your pocket.

 

As far as "not enough memory," I was thinking all three: Program memory, Ram memory, and data storage memory. Size and power will also be challenges, as well as the fabrication and assembly. But, given that the TS has rejected the suggestion of a PDA, the results will be interesting to learn about.

 

It's possible with the right stuff maybe... CD quality sound is 1,411 kilobits / second. SPI (which is what you'll be using unless you fork out the cash for the real SD specs) doesn't have a maximum speed, but depending on what you are using may not be able to move data fast enough. RAM is also a possibility to maybe save it temporarily and then transfer to SD card.

I'm also thinking unless you want to spend the money to have this machine built you will end up with something more along the lines of a size of a brick instead of something you can put in your pocket.

High audio quality is not so important to me for this device, just good enough to record/playback voice memos with reasonable intelligibility.

Can you elaborate on what you mean by 'have this machine built'? I could always switch to cargo shorts...

 

As far as "not enough memory," I was thinking all three: Program memory, Ram memory, and data storage memory. Size and power will also be challenges, as well as the fabrication and assembly. But, given that the TS has rejected the suggestion of a PDA, the results will be interesting to learn about.

You seem to have a basic knowledge of this subject, do you know of any good books or learning materials?

 

You seem to have a basic knowledge of this subject, do you know of any good books or learning materials?

Mostly what I read about what is essentially small definite purpose computer systems is the manufacturers application literature. That is because to sell their products they have to work, and thus they describe what it takes to have their products work well as they claim. So you need to be able to understand what is needed to get the functioning that you want for your application. Textbooks are much more general and either a lot of theory, which you do need to have understanding and insight, or they are very much practical, which is fine except when they don't mention "why" some things are what they are. And of course, in my career, creating all sorts of testing systems and machinery, memory has been a consideration. The good part was that many of them had the whole program in EPROMs or just PROMS, and only data lived in RAM. So the program would execute from memory and any software attack would only damage data in ram. A hard reset and the invasion was history, except for some lost data.
So memory for text and speech and possibly graphics or even pictures will not be small.

 

Can you elaborate on what you mean by 'have this machine built'? I could always switch to cargo shorts...

I have to admit i'm just a hobbyist that has done some neat things in my eyes, but things many others would call child"a play. I do know to get any reasonable amount of RAM it will either be a ball grid package or TSSOP or similar size chip. Neither of which are easy to solder in place.

Hobby level boards can be big and bulky, but do the job. If you can do a two sided board that will help, but still not as good as something from a manufacturer. I've had pcbway make a couple and do know they offer assembly services although I have not designed anything that needed the precision of a robot to place parts.

I kind of like your project, but see an uphill battle in several ways. Don't let the heavy hitters discourage you, but also keep in mind they have way more experience and may know something we don't.