This is an updated version of an earlier board, where the NRF24L01+ board is now the SMD version instead of the standard version with DIL header.

The ATSHA204 and flash on this board is not tested yet by me (components still on order).

A new boot loader set for various mcu working frequencies is added, as this board has a LED connected to D5 (not the standard D13).

You can find this board at OSHPark:

https://oshpark.com/shared_projects/2uuSjIIQ (version V3), the BOM is for V3

A remark made by @alexsh1 is correct: the diameter of the holes of the JP1 header is a bit too narrow. The library I used was not the standard pinhead library (which has correct diameters for the holes), so I switched this in version 4.

About version 4: this is still just a schematic and board layout, it is untested, but if you are adventurous you can order it here from OSH Park.

https://oshpark.com/shared_projects/3VM9juBk (version 4)

The difference with version 3: 1 instead of 2 pull up resistors (only for D2). More pins on the expansion header (24 instead of 22), which now includes the ISP pins.

Back to version 3:

This board is meant primarily for battery operation. As it's predecessor (https://www.openhardware.io/view/5/Battery-based-atmega328p-sensor-no-SMD), it can sit on a two battery holder (AA or AAA type).

Unlike it's predecessor, you can not choose the orientation of the NRF24L01+. But now this one will not stick out from the board anyway.

A 3V3 linear regulator can be mounted on the bottom side of the board if you do not use the battery powering option. By connecting USB or other external power to the two pins "+" (VCC) and "-" (GND), you can give any voltage between 1.9V and 5.1V DC to power the atmega328P.

To use the NRF24L01+, you need to have 1.9 to 3.3V DC on it's power line (the "3v3" circuit). The radio is powered via the "3V3" circuit, which is not automatically connected to the VCC power line. To make that connection, you need to mount the AMS1117 or short solderpad "3V3".

If you power by two AA or AAA batteries, you can skip the AMS1117 and short the solderpad "3V3".

A new feature is the possibility to mount an ATSHA204A. This is optional. If you do not mount this, then U3, C5 and R6 are not needed. If you do mount the ATSHA204A and you have an I2C board mounted which has pullup resistors, then R6 is not needed.

Another new feature is the OTA Flash memory. If you load the DualOptiboot boot loader, you can use Over The Air updating by mounting U2, C6 and R5. If you do not need this, then these components are not necessary.

There are 2 solderpads on the top side of the board (SJ4 and SJ5). On both solderpads the middle pad needs to be connected to at least 1 of the left or right pads to have the A4 and A5 signals connected to the pins of the expansion connector.

Solderpad 4 decides for pin marked "SDA" if this pin carries signal SDA or SCL. Solderpad 5 decides for pin marked "SCL" if this pin carries signal SCL or SDA.

This allows you to make following two header "layouts" on the "I2C" connector:

3V3 - GND - SCL - SDA

3V3 - GND - SDA - SCL

There is a solderpad called "INT" next to the NRF24L01+. When shorted it will connect the IRQ pin of the radio with D2 of the atmega328p.

R3 and R4 are optional resistors. They are pull up resistors for respectively D2 and D3. Mount them if you need to connect switches to the D2/D3 pins.

The crystal and the capacitors C2 and C9 are optional. If not mounted, you need to flash the atmega328p for use of the internal oscillator.

There is an FTDI connection possible via the expansion connector.

It is possible to mount a LED on this board (yes, blinky lights). R2 and a 3mm LED of your choice can be mounted here. They will be connected to D5, which is a PWM capable pin (dimming is now a possibility). The pin marked "5*" is actually connected to D5 directly.

The expansion connector does not follow the MYSX standard because I wanted to include the FTDI pins. It is organised so that connecting switches to D2 and D3 is easy.

An expansion board is planned, which will have AC-DC conversion and relay. Other boards will follow. I2C signals are available on the connector. SPI is not added since there are already 2 SPI based devices on the board.

A version of this board without only one pullup resistor (for D2) and extra pins on the expansion connector (SPI signals) is already designed (that is version 4). Extending the functionality of that board will be via this new expansion connector. But it seems difficult to find 24 pin angled female headers, so I'm not fixed on the new expansion layout yet.