TinyTX3 Wireless Sensor Board

TinyTX3 fitted with a DS18B20 temperature sensor

Since I put the files for the first PCB version of my TinyTX wireless sensor node online in June I’ve heard from people around the world who have had their own boards made which is really encouraging, especially as it was my first PCB design and was mainly done just to learn how to use the Eagle CAD design package (I wish I’d seen these great tutorial videos when I started, it would have saved a lot of time). There is obviously a big demand for this sort of thing.

Like the earlier stripboard versions the first PCB version was designed purely for use with the DS18B20 temperature sensor but if you’ve been following the TinyTX progress you will know that I’ve since found lots of other uses for it.  Having only 2 of the ATtiny I/O pins being available was starting to become limiting though so I wanted to do a respin with additional pads for some of the unused I/O pins and I also took the opportunity to fix the tight clearance between the power connections and the RFM12B, improve some of the layout and add spaces for decoupling capacitors on the ATtiny and RFM12B, I’ve not had any problems without them on the previous version or on the original stripboard design but it is good practice to have them so we might as well have the option of fitting them if required.

The biggest change is at the top where I’ve added space for 6 I/O pins plus ground and Vcc (Row 1) and instead of a fixed space for a pull up resistor I’ve added two rows of standard 2.54mm spaced holes, one row (Row 2) connected to the header row and one unconnected row (Row 3) which will allow for many more configurations. The IO pins are labelled with the equivalent of the Arduino digital pins as used in arduino-tiny, it looks a little odd as the numbers aren’t consecutive but I didn’t think it made sense to do it any other way. 

Front and rear views of the TinyTX3 PCB

This new arrangement allows sensors to be connected directly to the board as before or a male or female header could be installed on either side of the board, opening up the possibility of adding expansion boards (think mini Arduino shields) above or below the TinyTX3, perhaps an add on board with a step up/boost converter and DHT22 temperature/humidity sensor.

If you wanted to duplicate the original design with the DS18B20 sensor for example you could install the sensor across the pins marked 9 and 10 and GND on Row 1 and put a 4K7 resistor across the pads next to 9 and 10 on Row 2. The original code will then run unmodified.

Pin numbers refer to Arduino pins when using the arduino-tiny core

Other possibilities include multiple sensors on one board, using resistors to create voltage dividers for reading analogue sensors, eg. thermistors or a voltage divider could be useful for reading the battery voltage when powering the board with a step up/boost converter board as the usual programmatic “readVcc” method won’t work in that case as the board always receives 3V3. I’m sure more uses will develop over time just as they did for the first PCB and as all the existing TinyTX code will still work nothing is lost.

You might notice I’ve soldered the ATtiny84 directly to the board on the one in the pictures above, what do I do if I want to reprogram it you might ask?

IC test clip attached to ATtiny84 chip

These IC test clips are very handy, just wire it up to your programmer (or Arduino using ArduinoISP) and clip it onto the chip to program. If you don’t have test clips you will want to fit a socket to the board instead so you can remove the chip for reprogramming.

Here’s the schematic for the new board:

You can download the Eagle or Gerber files below if you want to order your own boards, I get mine made at Seeed Studio and for 10 boards it costs $9.90 (with a green solder mask, add $10 for other colours) + shipping, including shipping to the UK that comes to $13.86 in total which is currently £9.01 or 90p each. The guys at Circuits.io were also kind enough to import the Eagle files into their site for me so you can order boards in a pack of 3 from them for $14.67 plus $5 for international shipping. Unfortunately they don’t yet support adding text to the silkscreen so you will lose all the labelling with their boards.

Downloads:
Schematic (PNG format)
Eagle files
Gerbers (one per board) or 2 panelised on one board (you will need to separate them yourself).
Code

The hardware design files are also available on SolderPad here.

Parts List (excluding any sensors):
TinyTX3 PCB
ATtiny84A-PU microprocessor
RFM12B transceiver
14 pin DIP socket (optional if you have an IC test clip)
2 x 0.1uF (100nF) 2.5mm ceramic capacitors (optional)
Wire for antenna, eg. 0.6mm solid core (165mm for 433MHz, 82mm for 868MHz)
Battery holder for 2 x AA or AAA batteries

This is Open Source Hardware licenced under a
Creative Commons Attribution-ShareAlike 3.0 Unported (CC BY-SA 3.0) licence.