To keep costs down, we’ve designed a single-sided PC board for
this project. This board is coded 07112071 and measures 136 x 97mm. The complete
board and the CD-ROM drives could optionally be encased in a plastic case or
mini-tower computer case, along with the power supply.
Because it’s single-sided, the PC board is somewhat larger than
a double-sided board would be and there are quite a few wire links that have to
be installed. Before installing any parts though, it’s a good idea to inspect
the PC board for hairline cracks in the tracks or shorts. Some of the tracks are
very fine and quite close together, so check these carefully.
Installing the wire links
Fig.2 shows the locations of the wire links and these should
all be installed first. Because some of these links are quite close together,
it’s essential that they be perfectly straight so that they don’t short
Fig.2: the first job in the assembly is to install all the wire links as shown here. Make sure that these links are straight, to prevent shorts - see text.
The best way to straighten the link wire is to stretch it
slightly by clamping one end in a vice and pulling on the other end using a pair
of pliers. Each wire link can then be cut to length and its ends bent down at
right-angles using needle-nose pliers before mounting it on the PC board.
Once you’ve completed this task, you’re ready to install the
remaining parts. Fig.3 shows the parts layout on the board.
Start with the resistors, taking care to ensure that the
correct value is used at each location. Table 1 shows the resistor colour codes
but it’s also a good idea to check each one using a digital multimeter before
soldering it to the PC board.
Next, solder in protection diode D1, making sure that it is
oriented correctly, then install the small tactile switch (S3). The latter only
fits correctly if it is the right way around.