2.4 TFT Schematic

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dnvm84
 
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2.4 TFT Schematic

Post by dnvm84 »

Hello, I own the 2.4 TFT LCD with Touchscreen Breakout w/MicroSD
I downloaded the schematic drawing for this product.
Is there a way to get a more detail list of parts used?
The APX803-SAG has a ton of variants, and none of them have just the part number as it is typed on the schematic.
The MMBT2222 states that it is obsolete. And the SD & MMC has no part number at all.
So why am I asking for this information? Well, I am trying to design a pcb for my project.
I am using KiCad which means the information you supply with Fritzing I unfortunately can't use and must recreate.
So, any help would be greatly appreciated.
Thank you.

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adafruit_support_mike
 
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Re: 2.4 TFT Schematic

Post by adafruit_support_mike »

Ignore the letters at the end of the part number. As an example, I currently get 22 MMBT2222 products in SOT-23 packages listed as 'active' at Digikey:

https://www.digikey.com/en/products/fil ... ARqNgubFoA

The remaining parameters are irrelevant to the circuit in question.

The same is true for all the other components in the circuit. We don't design for special characteristics of a specific device, or if we do we identify that device and details we care about.

Translating the general BOM into specific product codes is always the responsibility of the person buying the parts. If I pick a single item from the list of MMBT2222s above, it could be gone, out of stock, or have a ridiculous price a week from now.

The APX803 is a voltage detector for the MIC5225-33 linear regulator. It keeps the LCD from trying to boot until the 3.3V supply rail is stable. There are hundreds of pin-compatible SOT-23 voltage detectors and SOT-353 voltage regulators that will work equally well.. we swap among them ourselves depending on availability and price.

One design point that isn't obvious from the schematic: you want the voltage detector to leave a bit of headroom so the regulator's output can sag a little below 3.3V and leave the display functional.. linear regulators typically have a 5% tolerance on output voltage accuracy, so the 3.3V-nominal output can be anywhere from 3.14V to 3.46V Honestly, any threshold between 2.5V and 3V will work.

That illustrates why just having part numbers.. or worse yet, product codes.. is far too limiting to be useful. You have to know what the parts do within the circuit to make informed design choices. The part numbers we call out on the schematics are representative of devices known to work in the circuit. They aren't prescriptive "use only this part or the whole thing will go down in flames" mandates.

I happen to prefer 2n3904 NPN transistors over 2n2222s, so I might use one of those instead (the MMBT- prefix happens to be a package code indicating an SMT device, BTW.. 2n- is the generic prefix for a transistor). The transistor is a power switch for four white LEDs in the display backlight, so it will carry about 100mA when the screen is lit. That happens to be the 2n2222's sweet spot for operation where the 2n3904 is best around 10mA, but using a 2n3904 at 100mA just means it's current gain is slightly less efficient. Honestly, you can use almost any NPN transistor in that location and it will be fine. You could also swap in a 2n7002 or BSS138 n-mosfet, both of which are rated for about 200mA of continuous current.

There are literally thousands of devices you could use in that part of the circuit, and there's very little reason to prefer one over another. The 2n2222 and BSS138 happen to be Ladyada's favorite jellybeans, while the 2n3904 and 2n7002 happen to be mine. She's the boss, I have the highest respect for her knowledge and ability, and I'm not obsessive enough to bicker about such an insignificant detail. ;-)


So.. we greatly prefer when people ask "what does this part of the circuit need to do?" over "give me a link to a Digikey product page". Answers to the first question are much more useful.

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dnvm84
 
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Re: 2.4 TFT Schematic

Post by dnvm84 »

Thank you for your response.
If I was looking to actually design the LCD board then yes, I would be looking for that type of details.
It has taken a lot of research and trials for me just to get a PID system / circuit just using an existing Nano to work.
My question was based on the fact that I just wanted to use the existing product as is. I thought I needed to add it to my schematic as reference.
Again, thank you for your response.

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