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Bleep
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Joined: 09/01/2022
Location: United Kingdom
Posts: 414

Posted: 02:43pm 29 Apr 2024

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I wanted to build something that would be self contained and fully portable, this is what I came up with.
It uses a 4" ILI9488 LCD screen, 480x320 with touch, from AliExpress It has:-
USB obviously.
Mono sound through a jack plug, with volume, could easily be stereo.
Serial to USB.
SD card.
RTC.
All GPIO from GP0 to GP9 are free, coming out to a connector J1 along the bottom edge.
All the A/D inputs GP26 to GP28, coming out on connector J2.
Can be used as a battery bank when powered on to charge whatever...

It uses:-
A Chinese Pico, because I wanted the extra flash.
One of Mixtel90 backpack boards, mainly as a carrier and for connections to
the screen, RTC and SD card.
A Lithium battery controller/charger/booster with on-off facility from Ebay
A serial to USB converter from Ebay
A RTC from Ebay
A strip board with Volhouts Audio filter, into which both audio outputs are fed, can easily be made stereo, then on to the 1K potentiometer and output from a jack plug.
Other sundry components were 2x Lithium batteries salvaged from a broken LapTop power pack 2.2AHr(measured) each, jack plug, 1K pot, momentary push switch and a box, which I designed up in FreeCAD, unfortunately my printer isn't great, but it has done the job, one slight hiccup was I designed it to have a 1.5mm recess in the back, into which the back plate should have dropped, for a currently unknown reason the printer stopped at exactly 1.5mm from finishing, so the back plate currently doesn't really fit, it sort of sits, but I will at some point make a larger back plate to properly cover the back. (the back plate should only take 30mins, the whole box took 7hrs. For the initial setup and testing everything is only friction fitted, once I'm confident it's all good, I'll hot glue everything, especially the batteries!

To operate you press the momentary button once and it all powers up, press twice and it powers down, if you connect to a powered external USB plug, the unit switches over to charging the batteries, as well as powering the unit, only caveat is once you are charging, if you remove the USB cable the unit will shut down briefly and then power back up, better to just leave it charging, then when you want to disconnect, press and hold the button while unplugging the power, it will then stay off, inserting USB power is ok, the unit continues running fine, or powers up if it was off.
Battery duration will be about 10 hours, full brightness screen, cpu at 420MHz, or about 20 hours, 50% brightness and 126MHz, or any combination in-between!

If you want music, I've experimented and have found compressing to a WAV file, using ADPCM @ 22kHz sample rate works acceptably, so nothing above about 10kHz, not exactly HiFi, but quite acceptable, especially for my ears. The file size is slightly bigger than a MP3 for the same track, so 4.41MB for MP3 and 6.15MB for ADPCM WAV, as opposed to 24.3MB for PCM lossless WAV.

The blue light you can see, is infact a 4 bar LED, which gives approximate charge state. More gratuitous pictures:-