Simplyduino

Simplyduino Logo

Say hello to SimplyDuino. I needed a low power board that would be able transmit sensor readings to a central hub, using a low cost NRF24L01 RF transmitter. After a few prototypes and interations this is the result.

  • ATMega328p with 32kb flash and 8kb ram
  • Compatible with Arduino 1.6.2 IDE (select Pro Micro 8Mhz)
  • On board single cell LiPo charger
  • 3v3 LDO regulator 150mA
  • 250mA surge protector and reverse voltage protection
  • onboard socket for low cost NRF24L01 wireless connection
  • Choose between 3v coin cell or 3.7v LiPo battery via VIN
  • Onboard FDTI socket for sketch uploading, boot loader can be uploaded through the NRF24L01 socket
  • USB charging with status leds (charging, full)
  • 3 INPUT sockets (selective power, analog in, gnd)
  • 2 OUTPUT sockets (VIN switched by transistor, protected by diode)
  • 8 GPIO pins (A3-A7, D5-D7)
  • 1x signaling led (D7)

Board 1.03 back

NOTE: Do not combine a regular battery with LiPo charging or USB at the same time. Something will melt. See below on changing the jumper to allow the use of a CR2032H rechargeable battery.

Although the board is designed for a special purpose, some of the features make this board usable for other situations. Because of its small form factor, built in switching transistors and onboard power supply it has a few advantages over a bare Arduino pro micro for example.

The GPIO pins A3-A7 and D5-D7 can be used as INPUT or OUTPUT.  A3-A7 are analog pins and can be used as 10-bit analog input. D5 and D6 can provide 8-bit PWM output.

If you want to make full use of the D7 pin (as input pin), you need to desolder the signalling led because it influences the load on the pin. The led L1 is located on the top-right of the board, just below the S1 connector.

The battery holder is included, but you would have to solder it yourself as it is optional to use the CR2032 as a main power supply on the reverse side of the board.

Programming the SimplyDuino

The board should have a bootloader (optiboot) but flashing the bootloader can be done through the ISP pins which are available on the NRF24L01 header: 3V3, GND, MOSI, MISO, SCK. The RESET line is broken out on the DTR pin of the FTDI header if you need it.

After the bootloader is finished, new sketches can be uploaded with a 5V USBtoSerial FDTI module, for example this one from SparkFun. Just make sure you have it set to 3.3V or things will smoke. The pinout should be compatible to any FDTI uploader.

1 — GND
2 — CTS
3 — 3.3V
4 — TXD (connects to RX on the ATMega)
5 — RXD (connects to TX on the ATMega)
6 — DTR

The DTR pin is connected to the ATmega328 reset line through a 0.1µF capacitor, so autoreset should for just fine. Otherwise, press the reset button on the SimplyDuino while uploading a sketch.

Note: to flash a bootloader, you need to connect the ISP pins, and also the RESET line. There is no pin available  to the reset pin of the ATMega, so you will have to improvise. Either use the RST test point (just left of the CPU), or replace C6 with a 0Ω resistor.

Schematic

Cost

Production costs are as follows:

  • 10x PCB iTeadStudio: $19.98
  • 1x Solder Stencil OSHStencil.com $11.40
  • Mouser parts for 10 boards: €81,61 ($86,50)

So that comes down to $11,78 per board, for a very limited batch. I’d imagine you could go down to $5 per board if you’d do any volume development. Miscellaneous EBAY China parts like connectors, headers, 18650 batteries + housings were already on stock so it’s hard to put a price on those. I’m not counting solder paste,tapes, wires and write-off on the hardware.

History

Board revision 1.04

Earlier prototypes (before 0.97) did not have the onboard 3.3v regulator, which meant that the ATMega was running on a voltage level anywhere between 3v for a coin cell and a 4.2v from the USB charger. As the NRF24L01+ module has an absolute maximum voltage of 3.6v this was not a good idea. The older boards can still be used with a 3.7v LiPo battery, just do not use the RF shield with it. Similarly you could still run the board on a 3v coin cell but at a typical capacity of 200mAh it will not last very long.

The newer version (v1.03+) contains a solder jumper to configure if the battery holder is pre or post voltage regulator. This allows you to use a rechargeable 3.7v coin cell LiPo battery by moving the 0Ω resistor to the left.

Also, a 250mA resettable fuse has been added in front of the voltage regulator for protection.

Even though a few components were added, the existing components have not moved. So the v0.96 solder paste stencil can still be used for the most of the board.

Board revision 1.05

Optimized routing

Board revision 1.06

Modified USB footprint to accept UX60-MB-5ST USB socket with positioning posts. Again, the solder stencil has been unchanged.

Board revision 1.07

Cosmetic updates

Downloads

For all designs, schematics, gerbers, firmware, arduino boards.txt, files and documentation, check the bitbucket repository. And i’m not telling you to read the readme file..

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