Manual
-44-
In this circuit you will first learn about the AND
circuit. When all the connections to its terminals are
logic high (receiving voltage), the AND circuit
produces a high output.
Make the connections in this circuit based upon the
wiring sequence below. After that make the
connection to terminals 119 and 124 using terminals
A (126) and B (128) in different combinations to
complete the circuit and to learn how an AND circuit
works.
Terminal 124 provides logic high (voltage) while
terminal 119 provides logic low (no voltage) in this
circuit. H is only shown on the LED after you have
connected terminal A and terminal B to terminal 124
(high terminal). If you make the connection of either
terminal A or B or both to terminal 119 (low terminal)
the LED will display nothing. For the combined
output (the LED) to read H (high), both A and B have
to be high.
The PNP transistor stays off when either or both of
the inputs are low (terminals 126 and/or terminal
128 are connected to terminal 119), and when
positive voltage is applied to the PNP transistor
base through the diode(s). The NPN transistor is
also off because the PNP transistor does not
complete the circuit, and no current is supplied to
the NPN transistor base. Also remaining off is the
LED due to the fact that the common cathode
terminal is not connected to the negative power
supply.
The base of the PNP transistor turns on when both
of the inputs are high and when both diodes supply
negative voltage to the base of the PNP transistor.
In addition, the NPN transistor turns on and then the
current flows to the display to light the LED.
Symbol AB is used to represent an AND function
that mathematicians use. On the bottom right of this
schematic is the schematic symbol for the AND
circuit.
Notes:
EXPERIMENT #29: “AND” DIODE TRANSISTOR LOGIC WITH LED DISPLAY
Wiring Sequence:
o 22-23-21-18-19-72
o 25-47
o 81-40-125-127
o 41-83
o 42-129
o 46-84-85
o 86-82-48-124
o 71-130-119
o 121-122
o 126-(to 119 “HIGH” or 124 “LOW”)
o 128-(to 119 “HIGH” or 124 “LOW”)
Schematic
EP-130_62315RevC.qxp_EP-130_062812 6/23/15 11:17 AM Page 44