mWessenger

After some thoughts I decided to upgrade schematics with energy harvesting IC, in case if there won't be enough high voltage generated after diode voltage multiplier. On other hand, if there appears excess of energy, it must be stored and regulated.

There are many great options available at the market from TI, Analog Devices. I've selected LTC3108 from former Linear Technology, because I had in my component box 2 samples of these, which I obtained 6-7 years ago :)

Speaking about layout - I decided to make it as modular as possible, by using breakout special lines of drills in PCB:

1) Antenna part - it can be easily braked out, in case if I'll decide to choose another one, i.e. made of other material (copper wire) or for harvesting other frequencies (i.e. 915MHz).
On silkscreen layer there are marks at antenna, that will help to fine-tune its length symmetrically (I've made it equal to  length 1/4 of wavelength for 2.40GHz wave)

2) Voltage detector circuit - at the moment I'm not sure that it 100% satisfy the goal of the project: there are assumptions, that must be confirmed experimentally. I added pi-matching circuit for fine tuning antenna later;

3) Energy harvester - I decided to make it independent and reusable/reconfigurable: by swapping R3 and R4 jumper position it's possible to set output voltage. Moreover, this PCB part is reusable and can be used with TEG, solar panels, photodiodes in other projects

As result, PCBs and components are ordered and I'm waiting them now.

After studying existing solutions, I decided to use voltage multiplier topology, similar to WISP project (http://wisp.wikispaces.com/), which proved to be successful to lower RF frequencies (sub-1GHz).

The main difference is, that I should use other Schottky diodes, for higher frequencies.

After long search, I found one which made my day!  - HSMS-286x series diodes from Avago.  According to datasheet, these are "DC biased detector diodes, designed and optimized for use from 915MHz to 5.8GHz. They are ideal for RF/ID and RFTag applications as well as large signal detection, modulation, RF to DC conversion or voltage doubling" - Exactly what I need!!! :)

As result, Schematics now is ready for antenna and voltage doubler. I'll try to finish and order PCBs tomorrow or after tomorrow....

To make the first proof-of-concept prototype, I see, that device should consist of two-three functional parts:

1) Antenna - which is designed/tuned to 2.4GHz (because it's standard frequency for mW-ovens). I'll try to use PCB antenna, due to 2 reasons: 1) cost reduction 2) IMHO more important: bigger surface area (I believe, the bigger area - the more energy can be collected)

2) Rectifier with voltage multiplier - with big probability the voltage on antenna pin will be very small to power even energy-harvesting ICs, which have their own "cold-start" voltage. Therefore we'll try to boost voltage.

3) Energy-harvester - circuit that makes energy storage, distribution and output more easy. I assume that this part at the moment is optional, because I'll be able to measure any voltage, detected by circuit, which consists of parts 1 and 2.