As explained last month, the Rudder Position Indicator comprises two units. The sensor unit monitors the rudder and transmits changes in its orientation over a 433MHz wireless link. The display unit picks up these communications and illuminates its LEDs in response.
While the two circuits are different, they are both based on ATTiny861 microcontrollers so there are many similarities in layout. Hence the procedures for building the two lower (control) boards, which each have a micro, are essentially the same.
You can either build them in parallel or you can build one and then go back over the following instructions to assemble the other.
Control board assemblies
The sensor unit control board is coded 20107111 and the display unit control board is coded 20107113. Both boards are 98.5 x 68mm with corner cut-outs; see the overlay diagrams of Fig.7 and Fig.8, respectively.
First inspect the copper sides for any defects such as hairline cracks or under-etched areas; repair if necessary. If your boards lack corner cut-outs, use a hacksaw and file to shape them using the thin copper tracks as a guide.
Fig.7: follow these parts layout diagrams to build the two boards for the sensor unit. Note that the reed switches (S1-S4) should be mounted 9-10mm proud of the PCB. The photo below shows the assembled lower (control) board.
Proceed by installing the three wire links which run along the bottom of each board, using 0.7mm diameter tinned copper wire. Run the links close to the board and as straight as possible since they pass near component leads.
Next fit all the resistors, checking each value with a digital multimeter before installing it (the colour codes can be hard to read). Remember that you may need to change the value of the 390Ω resistor depending on the intended supply voltage/battery type.
Note that, for the display unit, this resistor should be installed slightly offset, as shown in the photo on page 78, to leave enough room for the pushbutton switch to be fitted later.
That done, install the diodes. Both boards use a 1N5819 Schottky diode (black plastic body) and a 16V zener diode (large glass body). Make sure they are orientated as shown on the overlay diagram, ie, with the stripes towards the bottom of the board.
For the display unit, also fit the small 1N4148 diode with its stripe to the left and the 100µH axial inductor which looks a bit like a fat resistor. Bend its leads close to the body to fit the pads; its orientation is irrelevant.
Next, solder in the IC sockets, ensuring that the notches are orientated as shown on the overlay diagrams. There is one 20-pin socket for the sensor unit and 20-pin and 16-pin sockets for the display unit. You must use a regular 16-pin “dual wipe” socket for the 74LS145 IC, since the more expensive type has exposed pins that could short to the adjacent wire links.
Now install the transistors. There are two on the sensor unit board and three on the display unit board and there are several different types, so check the markings carefully before soldering each in place. Crank the leads out using small pliers so that they fit into the holes provided, with the flat faces orientated as shown.
Fit the regulators next; they look similar to the transistors. There is one 3V (or 3.3V) regulator for each board plus a 5V regulator on the display unit; don’t get them mixed up. Use the same technique as for the transistors.
Follow with the small trimpot, one on each board, then the MKT capacitors, which are all the same value (100nF). Note that the 100nF capacitor at bottom left of the display control board (Fig.8) must be mounted on its side (see photo) to leave room for the power connector.
The fuse-clips go in next. Push them right down onto the PCB before soldering the pins and take care that the small retention tabs face towards the outside or you won’t be able to fit the fuses properly.
Then mount the pin headers, including the 12-pin female header at the bottom of each board (CON3/CON8), a 4-pin female header on the sensor board for the transmitter module (not labelled) and a 4-pin male header on each board (JP1/JP2). If you don’t have 12-way female headers, use side-cutters to snip a longer header at its 13th pin, then remove that pin. You can cut the 4-pin header from the same length.
Next, install two PCB pins on each board, one for the antenna where indicated and one test point (TP1/TP2). If you want a terminal block on either module for external power (or trickle charging the batteries), fit it now. The sensor unit control board also accepts an optional DC socket which can go in now.
The 4-AAA battery holder on each board goes in next. Push its leads through the corresponding pads and then secure it using two M2 screws inserted from inside the holder and fixed to the underside of the PCB using M2 nuts. Then solder and trim the leads. If you can only get M2 screws longer than 6mm you can cut them to size with a hacksaw or strong side-cutters but the end will need some filing before the nuts will thread.
Now fit the electrolytic capacitors. There is one 100µF capacitor for the sensor unit and two for the display unit, as well as one 47µF type for the latter. In each case, the longer lead goes through the hole marked “+” on the overlay diagram. Push them right down onto the PCB before soldering.
For the display unit, cut two 50mm lengths of light-duty hookup wire, strip and tin the ends and solder them to the CON9 pad at upper-left.