# Schematic Review

I’ve made a symbol and footprint for it from a datasheet.

The safer the better, so I’ll settle for a 2k resistor (Which I think it already has on the schematic). It has a 500mA fuse so it’d be better to stay below that.

I’ll have to use Tantalum since I want to keep the cost down, and I can’t order it with the supplier (I want to get all the resistors and capacitors from the same place).

I’ll get to calculating the required values for the LEDs. I’d assume the ATMega32U4 would output 3.3v so I’d do:

``````R=E/I
R=0.3/0.02
R=15 ohms
``````

But I can adjust the values for different ones to level out their brightness. The red LED requires less so I need a 75 ohms resistor.

You won’t find tantalum for most of the smaller values. MLC will be cheaper for these.

No. As I’ve said, the ATMega32U4 runs at the raw battery voltage, which will vary between 3.0V and 4.2V. For most of the time, the battery will be at about 3.6V.

You’ll probably find 20mA to be way to bright. About 2mA is more likely, again depending on the colour. Green and blue tend to look brighter than red at a given current.

You also have to account for the forward voltage of the LED itself in your calculations. This may be about 1.5V for the red LED and 2.8V for green and blue.

Using the above values:

For green and blue:
Voltage across the resistor: 3.6V - 2.8V = 0.8V
For 2mA current: 0.8V / 0.002A = 400 ohms

For red:
Voltage across the resistor: 3.6V - 1.5V = 2.1V
For 2mA current: 2.1V / 0.002A = 1050 ohms (about 1K)
However, you might need more current for red to make it look as bright as green or blue, so a lower value resistor.

Like I said, you probably want to experiment with resistor values to see what looks best for you. You should do this to cover all battery charge levels (3.0V to 4.2V).

(The Arduboy uses 220 ohms for all three LEDs but many complain about them being too bright.)

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I was able to find those values though. Does MLC have the diode function similar to Tantalum?

(Unfortunately I can’t access https://www.altronics.com.au right now for some reason).

I’ll create a list of the values.

Here’s the schematic as well for a double check to any of my small changes:

The components list is my own thing, just so I can keep track.

No, ceramic capacitors are non-polarized.

For the capacitors on the crystal, they can’t be polarized, so those, at least will have to be ceramic. This may also be true for the charge pump capacitors for the display.

Note that on the real Arduboy, all capacitors used are MLC type.

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I’ll have look through when the site is back up, or I’ll find an alternative. Would you have any recommendations for a supplier? (Doesn’t need to be Australian based)

Digikey, Mouser, Newark, LCSC. Any number of Chinese suppliers on eBay, AliExpress, etc., if you trust them and can tolerate the long shipping times.

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Shipping times are always a downside, though if I can finalise it all now, I can order the parts and I’d be willing to wait about 2-3 months. The PCB would take about 1 month. Plus the other parts need to be ordered too. Though I can’t afford it right now so I’ll have to wait. I can get my costs list sorted though.

Here are the values in a table:

3 to 4.2 is the supply voltage, all the values underneath them is the resistance for the LED

Is this a good list?
https://www.digikey.com.au/short/jn4q39fd

(I noticed that the 2.2uF capacitors were the wrong shape so I changed them)

Is Mr. Blinky’s lite FX chip version acceptable? Or do I need the ATTiny85 as well?

Here’s the PCB layout so far too:

(Completely original I swear `:P`)

As you can see, the resistor values for a given current can vary significantly depending on the battery voltage. Also, the forward voltage for the LEDs is only an estimate, plus it will vary depending on the current. And, 2mA and 4mA current is also a guess. You may want the LEDs to be brighter or dimmer than what you get with those currents. That’s why I said you’ll need to experiment.

No, you don’t need the ATTiny85. However, you will need to burn a custom bootloader into the ATmega32U4. You will need an “In Circuit Serial Programmer” (ICSP) to do this. Another Arduino can be used as an ICSP if you have one.

You may wish to add pads for a connector to connect the ICSP to your board. You could use the TP pads for this but you’ll also need to bring out the 32U4 reset signal. You could consider rearranging your test points in the shape of a standard ICSP connector (as used on the Arduino Leonardo).

I suggest that C10, C11, C12 and C13 for the display be at least 16V.

Your item number 2, a 2.2uF capacitor, is in a 0201 package, not 0603. That’s really, really tiny.

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Would that ruin the low profile I’m going for?

I could order separate PCBs with the connector that I can solder the ATMega32U4 to, program it, and then remove it.

Will get to it

Yes I noticed that and I’ve changed it to a 0603.

Again thank you so much for taking your time to help me, I would’ve spent a lot of money or broken the Arduboy without your help.

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It would probably be easier to put pads on your Arduboy board and solder wires to them for programming then remove the wires when done. If the pads are for a standard ICSP connector footprint, that would give you the option to use a connector or just wires. Even a connector could be soldered in only temporarily.

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I like the ideas of the wires. Though I have a bit of time to look around for some fancy or easy solutions.

For the ISCP connector do I need: CLK; MOSI; MISO; VCC: GND; RESET (Would that be before or after the 10k resistor?).

So the schematic is all good now? Here’s a bit of an updated version with the FX chip:

I’ve left a note for myself to check through on everything, I found a different reset button on Digikey, and a new switch because the other one was quite expensive. I’ll need to find some small leds for the charge controller and a few other things.

Here is the new cart: https://www.digikey.com.au/short/pv57jppz (I did get some with some smaller tolerances too)

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Yes. The 6 pin AVR style header is the most popular for Arduino.

All ICSP connections should go directly to the 32U4 pins.

Note that if you want to do ICSP programming with the display and flash chip installed, you’ll need to use a 3.3V programmer. A 5V programmer may damage those parts, as this voltage would be higher than their maximum ratings.

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5 posts were split to a new topic: PCB Editors

Would you recommend this? I don’t have a flash chip on the display at the moment. I don’t have a 3.3V programmer either but I’m sure I could find an alternative.

I am once again asking for cautious confirmation, is my schematic all good now? So I can start ordering parts when I get the money.

If you have a spare Arduino board that isn’t being used for anything else then you can use that as a programmer. (That’s effectively what the ATtiny in the FX is - an Arduino board being used as an ICSP programmer.)

I’ve never done it myself but I’m sure there’s plenty of examples and tutorials online, and there might be a few people here who have done it before.

I think this is the closest there is to an ‘official’ tutorial.

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I’ve had to program some pro micros ages ago when I was having troubles with them. But the issue is more about the voltages I’m connecting. The link though does suggest powering each one with 3.3V. I have a step down converter than can supply 3V so I might try that when I get the chance.

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If you only have a 5V ICSP, you could solder in the 32U4, crystal and a few other support components then burn the bootloader using the ICSP. Once the you’ve verified that the programming was successful, you could then populate the rest of the board.

A problem with this is that if you wanted to burn the bootloader again later for some reason, you would then need a 3.3V ICSP to do it.

I haven’t checked extensively for missing connections or that every button, LED, speaker, etc. is connected to the correct pin but your latest posted schematic looks good to me (at least from a components point of view).

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The value of R8 going to IREF of the display should be 390K

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I’ll look into buying a 3.3v board to program it, or I can try to supply the power from my step down converter. I’ll see if I can try it this afternoon.

Do take your time though please, I have about 2-6 months before I can actually order anything, and thank you for checking over it.

Was able to find a small changeable programmer:

I have that one. Be aware that it doesn’t properly do 3.3V. I’ve modified mine to work properly at 3.3V