Download Logic gates applications

Applications of the NAND gate
(a) Burglar alarm
When the switch is closed one input of the NAND
gate is LOW. When the LDR is in the light the
other input is LOW. This means that if either of
these things happen, i.e. the switch is closed or
the light is on one of the inputs is LOW, the output
is HIGH and the buzzer sounds.
Figure 1
(b) Freezer warning buzzer
When the thermistor is COLD its resistance is
LARGE and the input to the NAND gate is high.
Since the NAND gate is connected as an
INVERTER the output is LOW.
As the thermistor warms up its resistance
decreases, the voltage across it falls and the input
to the NAND gate falls.
When it becomes low enough the output becomes
HIGH and the buzzer sounds.
Figure 2
How would you adjust the temperature at which the buzzer came on?
Further circuits using logic gates
The following four circuits demonstrate some further practical applications of logic gates, the
function of each circuit being explained.
A push-button lock
To open the lock the solenoid must be activated and this can be done by pressing both switches
A and B. If B is pressed before A, however, the buzzer will sound.
+5V
A
B
LOCK
0
1
A light-activated burglar alarm
When the switch is in position A the buzzer
stays off whatever the illumination of the
light-dependent resistor. If the switch is
closed in position B the buzzer will come on,
however, and stay on when a brief flash of
light (perhaps from the torch of a burglar)
falls on the light-dependent resistor. The
buzzer may only be turned off by returning
the switch to position A.
+5V
A
R
B
0
A safety thermostat
The two contacts X and Y are near the top of
the hot water tank and are normally covered
with water so that there is a contact between
them. The thermistor is also in the water. If
the water is too cold and if the contacts are
also covered the heater comes on. The
temperature at which this occurs may be set
by the variable resistor R.
+5V
X
contacts
R
Y
0
An automatic watering system
This is designed to water plants only if the contacts, which may be buried in the soil, are too dry
and it is night-time. The relay closes and starts the pump only if both these conditions are
fulfilled. (Presumably somebody waters the plants in the daytime because if the switch is
pressed the water pump works regardless.)
+5V
R
M
pump
0
2