The interface to the RF link consists of an enable line and a data output. Since the transmitter circuit operates at 3.3 volts, I used analog switches to translate from the five-volt outputs of the microprocessor. Although not shown, pin 14 of the 74HC4066 needs to be connected to +5 volts while pin 7 needs to be grounded. JP1 is a 6-pin header that connects to the RF circuit card.
The power supply for the Remote Station is shown in Figure 3 . When the sun is shining on the solar panel (SC1 – part number 980-1045 from RadioShack.com), enough power is generated to drive the 50mA current source formed by Q1, U1, and R1. This current acts as a trickle charger for three AA NiCAD batteries. The batteries power U2 - a switchmode regulator that provides the 5 volts for the microprocessor and sensors. This is followed by U3, which is a linear regulator that provides the 3.3 volts for the RF circuitry. L2 and C5 were added to reduce the switching noise from U3. D2 is used to isolate the solar panel from the rest of the circuit when it is dark. Note that C3 and C5 should be 6.3V low ESR capacitors.
The RF section of this design, shown in Figure 4 , was built around the TX5002 and RX5002 chips from RF Monolithics. Due to the footprint of these devices, it was necessary to design a small printed circuit board. Fortunately, the chips have a pin-out that allowed a single board to be used for both the transmitter and the receiver. The completed layout can be seen in Figure 5 .
I was able to get all the traces on a single layer so fabrication was greatly simplified. The four corner holes were drilled out to allow mounting with 4-40 screws. The antenna is connected via a BNC jack on the left side. However, a length of coax can be connected to this pad in order to use a panel-mount connector.
It is necessary to use solder paste to connect the RFM modules to the PCB. Carefully apply the paste to the pads of U1 on the board. Position the module on board and heat with a hot air gun until the solder melts. Except for C2, the remaining components are in 0603 SMT packages. C2 is a tantalum capacitor in an “A” package.
Connection of the RF board is through header J2. This plugs into socket JP1 on the Remote Station.
As mentioned earlier, the same board is used for both the transmitter and the receiver. The function of the board is controlled by which parts are used. Table 1 summarizes which components are used for each board.
| Reference | Transmitter Value | RCVR Value |
| C1,C4 | 100pF | 100pF |
| C2 | 10uF | 10uF |
| C3 |
Not Used |
.015uF |
| JP1 | Used | Not Used |
| JP2 | Not Used | Used |
| JP3 | Not Used | Used |
| JP4 | Not Used | Used |
| JP5 | Used | Used |
| JP6 | Used | Used |
| JP7 | Used | Used |
| J1 | BNC | BNC |
| J2 | 6-pin Header | 6-pin Header |
| L1 | 56nH | 56nH |
| L2 | 220nH | 220nH |
| L3 | BEAD | BEAD |
| R1 | Not Used | 270K |
| R2 | Not Used | 330K |
| R3 | Not Used | 27K 1% |
| R4 | Not Used | 100K 1% |
| R5 | Not Used | 30K |
| R6 | 47K | 47K |
| R7 |
8.2K |
Not Used |
| R8 | 10K | Not Used |
| U1 | TX5002 | RX5002 |
Note that he BEAD used was a J.W. Miller part available from Digikey part # PMC0603-301-ND.More data about the bead requirements can be found from the datasheet of the device.
To maximize range, λ/4 stub antennas over λ/2 ground planes were used for both the transmitter and receiver. The antenna length can be calculated from:
λ = c/418*106
λ = 3*1010 cm/sec / 418*106 cycles/second
λ = 71.8cm
λ/4 = 18cm
The construction details for the antennas are shown in Figure 6
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