This is a follow-up on this page:
I have built a couple of prototypes to experiment with the technology. However, my application cannot use the RCWL-0516 or BISS0001, so I designed a stand alone high gain low frequency AC amplifier withn a dual op-amp, which will drive the A/D input of a Silabs microcontroller.
I have made two prototypes, one using the original microwave transistor (marked FCB, actually part number FC1405, sometimes available from Aliexpress.) The other uses a more readily available MRF947 available from Digikey.
One problem is that the pinout of the FC1405 is different from that of the MRF947, so I had to re-layout the oscillator section for the MRF947.
Schematic with the FC1405:
Schematic with the MRF947:
I have significantly improved the power supply and bias filtering for both the oscillator and the amplifier compared to the RCWL-0516. Since the circuit works by amplifying very low frequencies with a very high gain, it is best to eliminate any potential source of low frequency noise from the circuit. That means a very well regulated 3.3V supply voltage. Particularly, I wanted my sensor to be able to detect slower moving objects, so the low frequency operation had to be improved.
Here is the waveform on the AC output while moving my hand some distance from the sensor. This picture was taken with an LM358 as the op-amp. Its noise level is a little high, using a lower noise device like the TLV6002 would allow to increase the AC gain and increase sensitivity. This is the unit with the FC1405 transistor but they both work very similarly.
I have placed the Gerber files for the boards on OSHPark where you can buy them. I do not sell them and I make no money if you buy them from OSHPark. They are cheap (my 6 prototype boards, 3 of each costs me less than $12 in total, shipping included.)
The board on the left is for the FCB transistor, the same part used with the RCWL-0516 module. It is usually available from AliExpress.
The board on the right uses an MRF947 available from Digikey. The other components are the same, just the layout is different. All resistors and capacitors are 0805, except for a few capacitors that are larger (size is indicated on the schematic.)
The board on the left oscillates at 3.5 GHz, the one on the right at 3.98 GHz, a little too high. These were measured on the prototype and there could be a fair amount of variation between different builds unless you use the exact same PWB.
Everything else being the same, if you want to build one of those, I would recommend using the FCB layout since it is on the right frequency. However, the FC1405 transistors are not always available.
Because of the very high gain and low cutoff frequency, it takes the output voltage several seconds to stabilize after power up. This is normal.
In my application, I send the AC to a microcontroller's ADC input. I have a delay after power up sufficient to make sure the voltage has stabilized, then the microcontroller averages 1,000 measurements (in 1 second) to filter out any noise and establish the average voltage on the output. From that point, it is easy to detect small or large signals under software control and set the sensitivity as needed.