Schematics questions

Be gentle! This is the very first time that I've used eagle to do this type of stuff, so I really have almost no idea what it is that I am doing. Below is the schematic I was able to come up with after a bit of research and studying other arduino schematics. I will be using this schematic for an upcoming project that I have – an RFID protected mini fridge.

What I need to know is if this schematic will work. The arduino will be dropped in and from then on it just needs to read from an rfid reader, write to a buzzer, write to LEDs and write to a servo. I haven't built the power supply circuit yet as I don't know the power supply yet. More than likely it will be some wall wart with a proper voltage limiter and a few caps for stability. Once I get the mii fridge ill have a better idea of how this project can come together. So any comments and feedback on this schematic would be GREATLY appreciated!

I know that I will/might need to adjust the resistor values of the buzzer and the led.

Uploaded with

Remove the reset switch connection to Vcc, the way you have it, if you push the switch you will short out the Vcc power supply.


I can see a bug around your reset pin. The connection to Vcc from the switch that runs above the Atmega needs to be removed otherwise your reset switch wont work, and you will have direct short when you press the button.

The whole aim of the "pull up resistor" is to make sure that when you ground the pin there isn't a short and when the pin is floating its pulled up high, because no current will be flowing through the resistor and the voltage across the resistor will be 0V so Vcc will be at the reset pin when its floating.

You should also put some decoupling capacitors in across Vcc and Ground. I would put at least one 100nf across Vcc and GND and also another 100nf across AGND and AVcc. You could even put two in parallel. Keep them as close as possible to the IC.

These decoupling caps are important and I won't go into the technical details of why, but since you are using an RFID reader you should use them.

I see you are using PD0 or the serial receive pin for RFID_enable and the INTO pin for RFID_SOUT. I am assuming that it takes a serial stream of data (hence _SOUT) have made sure that you can produce that serial stream in software, because if it uses a standard SPI or even serial interface you can perhaps make use of the serial port on the ATmega.

Everything else looks good, it depends on what you are hooking this up to as well.

Are you desiging a PCB, if so keep the tracks as short as possible and try not to put the crystal and its caps close to any other switching signals.

Wow! This is exactly the feedback i had hoped to get!

Thank you all! you really do make this a great community for those trying to learn as much as they can about micro controllers and electronics.

@zageek and retrolefty. Thanks. I saw that about 5 min after i posted this :/ And yeah, i feel dumb.

@zageek: thanks for this!

I am a bit busy at the moment, but here is some more information.

the RFID reader i am using is the parallax RFID reader. It does a lot of the work for me... i get a single 2400 baud serial out signal.

i take Senable pin LOW to enable the reader (tell it to start searching for tags) and take it HIGH to put it in standby mode. Yes, i have already written most of the software; it works with the reader.

What did you mean when you said i could use the serial receive pin? i thought PD0 was the serial receive pin.

and as for the power supply, i will probably be some sort of wall wart fed into this little guy:

the standard 7805 based circuit; i would put it in the schematic, but i cant find a library that has it in it :/ Eagle's libraries are annoying! since the caps are already in the circuit, can i get away with not adding the additional ones?

and yes, i hope to turn this into a PCB, but i may just use some simple hobbyboard with the traces already done in rows.

the main logic will be far enough way from the RFID and the 120V to DC circuitry so i dont think ill have too much interference to worry about.