Can someone review my first Arduino schematic please?

Question

Can someone review my first Arduino schematic please ? Basically it is Atmega328 + RFM-12B + RGB LEDs. It works fine on breadboard. I was mainly inspired by JeeNode and Arduino.

Original schematic is here

My main concerns are :

Power

board will be powered by 3.7V Li-Po battery without any power regulator because all used components can work with input voltage 3-3.7V supplied by the battery. Is some easy way how can I implement some charging circuit which can charge the battery ? Sometime is not possible disconnect battery for charging.

Transistors

I want use BC850, each for powering single channel on LEDs. Transistor is powered through 1k resistor, which should be fine.

Capacitors

I don’t know if I really need decoupling capacitors, but Arduino use it as well and according to this it can be quite useful. For smoothing input I want to use this capacitor.

LEDs

only red channel needs a resistor, blue and green has forward voltage high enough to be powered directly.

RFM-12B

radio is connected to same pins as ISP header used for bootstrap, I’m slightly worried if it can cause some interference, but JeeNode has it same.

Atmega328

On schema is Atmega168 only because I can’t find 328 in Eagle :) It will use its internal 8Mhz oscillator because external oscillator is another thing is another thing which can go wrong, I don’t need extra performance and power consumption.

edit: earlier related thread

Answer 1

Do not work with the idea that the GREEN LED and the BLUE LED can be operated without series resistor. As a starter the paralleled LEDs with lower forward voltage will starve those with higher forward voltage. Series resistors for each can even this out and take care of that problem.

Next it is typical problem that BLUE LEDs do not always work well from a supply of just 3.3V. Consider getting your design modified to add a 5V source to supply this LED and still apply series resistors for these LEDs for the reason given above. With the problem as stated you may be able to find some BLUE LEDs that will give you satisfactory brightness from 3.3V but they will be dim in comparison to the others.

Do not second guess the need for the bypass capacitors. Put them in by the power pin pairs of each IC chip.

Note you failed to hook up one of the VCC pins of your MCU. It is essential to connect all power and ground pins of IC chips.

If you want to have a charger for the battery then you should plan to put in a regular power supply circuit for your design that can accept input from the battery or from some AC-DC converter module. When the AC-DC supply is present then have the battery go into charge mode.

Answer 2

In addition to Michael Karas's observations here's mine: -

Does AREF need to be tied to an external voltage source?

I don't know if there's a sheet 2 showing it but where is A0 to A5 going? Should they go to a connector and have pull-downs or what?

Same with RXD - it's an input so I'd put a 10k pull-up or down on it.

Ditto any unused data lines that are inputs.

Is the internal osc good enough considering you are looking like transmitting data to and from the RF device? I'd track in a crystal or oscillator module to be on the safe side. You don't have to use it.

What does FFS (no comments please) do on the RF device? Maybe it's worth looking into this and if it's something that is digital then use it on a spare IO line.

Any spare IO take to a connector or test points - you'll never know how useful these can be until you don't have them. You can use them with a scope/meter for debugging code. Also if you fry an IO line then you can easily switch to a spare.

Regards the external battery charge - plumb in another connector for a spare battery in parallel with the current supply but feeding via a low-drop schottky diode. If you need to keep running during a battery recharge this extra "port" will give you continuous temperary power for a decent time. Once the battery's in place and the original removed, short out the schottky with a link.

Answer 3

While the 3.3v power to unballasted 3.4v leds isn't recommended, it does work, and if you are careful about the design and power, it will be fine.

The problem is that this is not the case. You have no voltage regulator, you are using a 3.7v li-po.

3.7v might be enough for the led to suck a lot more than ~20mA. Worse, a fully charged li-po is ~4.2V. A full volt above the typical recommended forward voltage?

Notice this blue led current graph.

50mA at 3.8V. Assuming the curve continues as shown, every 0.2V increase is 15mA more. At 4.2V you will be drawing at least 80mA. Through an led designed for 20mA typical.

You need a voltage regulator, a buck/boost that can keep everything at 3.3v from the lipo's full 3 to 4.2v range.