This guide is a remake of Glyn Hudsons original guide about the emonTx. It uses white backgrounds which should improve printing quality. The emonTx is part of the OpenEnergyMonitor project and the most up-to-date information about it can be found on their website. Why I use it in my thesis and how it works can be found here.
Components needed to build an emonTx with one CT-sensor:
- ATMEGA328 microcontroller
- 16 Mhz Crystal
- USB, MINI-B
- 2 x 10µF Electrolytic Capacitors
- 2 x 22nF Ceramic Capacitors
- 4 x 100nF Ceramic Capacitors
- 3.5mm jack socket
- 3.3V Voltage regulator
- Green LED 3mm
- 1 kΩ resistor
- 2 x 470kΩ resistor
- 15 Ω resistor
- 100 Ω resistor
- RFM12B wireless transceiver
- Antenna for the RFM12B
- Clip-on CT
- headers for FTDI cable
- Case + batteries (optional)
Solder the resistors in the top right hand corner. From left to right: 2 x 470kΩ resistors (the colour code is yellow-purple-yellow-gold) and a 15Ω resistor (brown-green-black-gold). At the left side of the pcb: a 10kΩ resistor (brown-black-orange-gold) and at the right side a 100Ω resistor (brown-black-brown-gold). Fit the DIL socket for the Atmega. Notice that one end has a small notch in it which signifies pin 1. This should match with the notch on the legend on the pcb. It is done wrong on the picture.
Add the four 100nF ceramic capacitors. The orientation is not important.
Add the 16Mhz crystal and two 22pF de-coupling capacitors.
Add the mini USB socket. This can be used to power the device but not to program it.
Add the 3.5mm jack socket to CT channel 2. Place the voltage regulator next to the mini-USB, make sure the flat edge aligns with the flat edge on the legend. Add the two 10µF electrolytic capacitors. The orientation is important; The long leg must go to the + symbol on the pcb. Add the headers for the FTDI connection on the bottom. Add the LED (on the back) with the long leg to the + symbol.
Add the RFM12B wireless module. Write down the frequency that is written on the back because after soldering you won’t be able to see it anymore.
Add a antenna to the RFM12B. The length depends on the frequency used:
- for 433 MHz, use a 165 mm wire
- for 868 MHz, use a 82 mm wire
- for 915 MHz, use a 78 mm wire
If you’re going to power the emonTx with two AA batteries, solder a red and a black wire at the left side.
Enjoy. Your hardware is ready. You can optionally put it in a case but this is not necessary. The next step is to upload firmware (from here) to the device. This can be done with a 5V FTDI cable.