7805
Operation:
The 7805 takes in a voltage between 7
and 30 volts and regulates it down to exactly 5 volts. The first capacitor
takes out any ripple coming from the transformer so that the 7805 is receiving
a smooth input voltage, and the second capacitor acts as a load balances to
ensure consistent output from the 7805. Transmitter
circuitry consists of the following parts.
The 7805 has three leads. If you look
at the 7805 from the front (the side with printing on it), the three leads are,
from left to right, input voltage (7 to 30 volts), ground, and output voltage
(5 volts).
Pin
diagram for 7805
Figure – Pin Configuration of 7805
Figure 2-22 – 7805 connection to the load
The two capacitors are represented
by parallel lines. The "+" sign indicates that electrolytic
capacitors are polarized: There is a positive and a negative terminal on an
electrolytic capacitor. In figure 5-4, the lines from the input come in from
the left. One can see the ground line of the input voltage source connected
directly into the ground strip running the length of the board at the bottom.
The top strip supplies +5 volts and is connected directly to the +5 pin of the
7805. The left capacitor filters the input voltage, while the right capacitor
filters the +5 volts produced by the 7805. The LED connects between the +5 and
ground strips, through the resistor, and lets you know when the power supply is
"on."
If
we measure the input and output voltage of the 7805, we would see exactly 5
volts coming out of the 7805, regardless of the voltage we input. Once we see 5
volts coming out of the regulator, we can test it further and see that it is on
by connecting an LED to it. We need to connect an LED and a resistor in series.
We must use the resistor or the LED will burn out immediately. A good value for
the resistor is 330 ohms, although anything between 200 and 500 ohms will work
fine. LED’s, being diodes, have a polarity, so if LED does not light, try
reversing the leads and see if that helps. This circuit is installed is
installed at the entry station, keypad station and exit
station to give them exact voltage.
