CT255
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installed). Since a diode will only conduct when it is forward biased, this
diode acts as a “reverse voltage” protection circuit. Diodes are often used
as “one-way switches” in this manner to protect internal circuitry.
3. Identify C8, the 470 uF electrolytic capacitor (small cylindrical
component coated with plastic and marked 470). Electrolytic capacitors
are polarized with a (+) and (-) lead and must be installed in the correct
orientation. Ordinarily, only the negative side is marked on the capacitor
body with a dark band and the (-) sign clearly shown, while the PC board
will usually show the (+) hole location. Use care to ensure proper polarity.
See the parts diagram for proper placement. The capacitor should fit
snugly down to the PC board. Save those long leads that you trim off to
fashion
jumper connections later in the assembly.
4. Identify the 7805 voltage regulator IC, VR1(three lead component
labeled 7805). This component is in a TO-220 package, and again proper
orientation is required for correct operation. The component should be
mounted so that the metal “base” of the part mounts flat to the circuit
board (to avoid any problems with clearance for the case assembly). You
may want to pre-bend the leads with needle-nose pliers for a proper fit.
Also be sure to line up the hole in the part with the hole on the circuit
board. Use caution so as not to stress the leads when inserting or
soldering. Use the parts layout diagram as a guide for proper orientation.
Once you have the leads soldered into place you can take the screw and
nut and attach the regulator to the PC board.
5. Install C7, one of the 10 uF electrolytic capacitors (in a small
cylindrical case marked 10 uF). Remember to observe polarity with those
electrolytic capacitors! Check the parts layout diagram for correct
orientation.
Great job up until now! Take a moment to check your previous solder joints for
“opens” where the solder did not completely flow around the connection or
solder bridges between closely spaced pads. It seems the best time to identify
these problems is now when you’re focused on this section of the board,
saving you time trying to retrace your steps later.
Let’s get to work on that counter and display section of the thermometer next.
6. Install U2, the 74HC393 14 pin IC. Make sure to align the notch or dot
associated with pin one with the notch shown in the parts layout diagram.
Also check to be sure all 14 pins are through the board before soldering
the IC . It may be helpful to gently “rock” the IC on its side on a hard flat
surface before installing to align the rows of pins so that they slide in
easily. Don’t be afraid to use enough solder and heat for a good solder
connection, and be extra careful not to “bridge” any solder connections
together.
