I am working on the same project! I started in July 2013 and at first, I used 4 load cells from cheap digital scale. I connect them together to form Wheatstone bridge, excitation voltage and amplification was done by INA125, analog-digital conversion with internal ADC in ATMega328P. For wireless communication, I used ZigBee at 2.4 GHz. My results were:
- Do not use cheap 3-wire load cells. They drift with temperature and time. They lost precision. Creep is problem.
- Do not use 2.4GHz if you need to communicate over 100 m. This frequency if full of Wifi/Bluetooth/noise. Everything from grass to trees will reflect your signal.
- INA125 is good, but expensive.
- 10bit ADC in ATMega is good, but not sufficient.
In my second version of electronics, I used 868MHz (yes, I'm from Europe) mesh modules from RadioCrafts (they call it TinyMesh). Maximum transmit power is 9dBm. I am testing them last few months and it looks very promising. I am able to communicate for distances over 150 m. Frequencies 868 or 433 MHz are much better in those projects.
The electronics is powered from 2AA batteries via step-up converter. With this, I'm getting quite stable 3 V. Also, I removed RTC and external crystal and for waking up MCU from power-down I'm using watchdog in interrupt mode. Thus, MCU will wake-up every 8s.
Instead of 4 small load cells, I used one 4-wire load cell (LEGO-lars used similar). This is my last problem with this project. The reading from ADC is not stable and it drifts with time. I put 2 kg on my 20kg load cell and after 2 weeks, the readings from ADC were lower then first day. Do you have similar experience with load cell?
Now, last month I am working on 3rd version of electronics. The digital part remains the same, but I completely redesigned analog part. I removed INA125 and used precision opamp MCP6V07. They have very low temperature drift and low noise. Instead of internal ADC inside of ATMega, I pick MCP3551 -- 22bit sigma-delta, low-speed 13sps. This new version of electronics will contain 2 opamp and 2 ADC, so I'll be able to measure 2 beehives with one electronics.
I am not fan of load cells. I would like to use multi-turn potentiometer. Unfortunately, I'm not mechanical engineer and don't know how to change weight into movement. It would be necessary to use some kind of lever and spring and gears to change small changes in spring into several turn of potentiometer shaft. Not sure if this is doable.
This is my second version of electronics.