I often use Pro Mini format Arduino Boards in my projects, especially when the processor is embedded as a ‘set and forget’ controller. They have a small form factor and are very inexpensive.
However, as I prototype systems using standardized breakout modules (see this past article) it has been annoying not having a sensor-type board for this processor footprint. So I decided to make my own.
The Arduino Pro Mini was originally designed by SparkFun Electronics but is now available from many low-cost (ie, commodity) suppliers. A number of variants are available but the one I use has an ATMega328 processor running at 5V and has the through-holes for the A4 to A7 pins in the middle of the board (see this link at Sparkfun for pinout details).
This version of the Pro Mini is a based on the ATmega328 with
- 14 digital input/output pins (6 can be used as PWM outputs)
- 6 analog input/output; 2 analog input only
- an on-board resonator
- a reset button
While it shares the same processor as the Arduino Uno, it is about ⅙ the size of the Uno. The reduction in size means a few things have been left off:
- A USB connection. The six pin header at the end of the board is connected an FTDI cable to provide USB power and serial communication to the board.
- Power regulators. There is no barrel jack connection and a separate regulated power supply must be provided to the board.
There is not much science in the design of the sensor board, but I had a couple of design aims:
- Break out the FTDI connection separately to the edge of the board, for convenience.
- Break out the reset separately in a GVS header. The reset is used to load a compiled sketch it is easier to use this way than the one built onto the board.
- Separate headers just for the power supply. This is both to provide power to the board and also to power peripherals from the board.
The circuit diagram and layout for the for the board is shown below. These are available here in Eagle CAD format. The capacitor was added to the board to smooth out the power supplied and can be omitted if not required.
The boards were manufactured at JLC PCB, and the result can be seen below.
While in the past I was using the Arduino Uno for prototyping Pro Mini projects, I am now able to use the target hardware for testing before the processor is embedded in its final configuration.