user8726: I have had some difficulties finding a barebones Arduino board and have had the following made. I just wanted to see if anyone could spot any issues? I think it has everything. I'm really trying to get it as minimalistic as possible, but at the same time able to use a power source like a 12v/24 battery. Any help would be greatly appreciated. Many thanks


user4143: 
You shouldn't put a capacitor under the chip like that. In fact I don't much like the layout of the whole crystal circuit at all.
You need capacitors on the 7805 regulator - around 10µF on both the input and output.
The RESET pin needs a pullup resistor otherwise the chip won't function at all.
You need decoupling capacitors on all VCC pins on your main chip.
Instead of meandering ground traces you should use a ground pour.
Running from the kind of voltages you propose the 7805 is going to melt. You will need a BigAss™ heat-sink on it. For those kind of voltages you would be better using a switching (aka buck) regulator. Maybe as an off-board module to keep your board simple. 


8726: <p>I have had some difficulties finding a barebones Arduino board and have had the following made. I just wanted to see if anyone could spot any issues? I think it has everything. I'm really trying to get it as minimalistic as possible, but at the same time able to use a power source like a 12v/24 battery. Any help would be greatly appreciated. Many thanks</p>

<p><img src="https://i.stack.imgur.com/6QFWq.png" alt="https://i.stack.imgur.com/zlwQT.png"></p>
 4143: <ol>
<li>You shouldn't put a capacitor under the chip like that. In fact I don't much like the layout of the whole crystal circuit at all.</li>
<li>You need capacitors on the 7805 regulator - around 10µF on both the input and output.</li>
<li>The RESET pin needs a pullup resistor otherwise the chip won't function at all.</li>
<li>You need decoupling capacitors on all VCC pins on your main chip.</li>
<li>Instead of meandering ground traces you should use a ground pour.</li>
<li>Running from the kind of voltages you propose the 7805 is going to melt. You will need a BigAss™ heat-sink on it. For those kind of voltages you would be better using a switching (aka <em>buck</em>) regulator. Maybe as an off-board module to keep your board simple. </li>
</ol>
