Elenco Electronics EP-130, Manual

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Now let’s step into the world of digital circuits and
learn some basics. First, a digital circuit is a circuit
that acts as a switch to turn different components on
and off. This section deals with diode-transistor logic
(DTL) circuits – circuits that use diodes and
transistors in switching power on and off.
It usually doesn’t matter how much voltage is
applied to a digital circuit; what matters is whether
the circuit is on (voltage is present) or off (voltage is
not present). When the circuit is on, we describe it
as logic high, or use the number 1 to describe the
circuit. When the circuit is off, we say logic low, or
use a number 0.
First, you will learn about the AND circuit. The AND
circuit produces output when all the connections to
its terminals are logic high (receive voltage).
Connect this circuit according to the wiring
sequence below. Then, connect terminals A (126)
and B (128) to terminals 119 and 124 in different
combination to complete the circuit and learn how
an AND circuit works.
In this circuit, terminal 124 provides logic high
(voltage) and terminal 119 provides logic low (no
voltage). The LED shows H only when you connect
terminal A and terminal B to terminal 124 (the high
terminal). If you connect terminal A or B, or both, to
terminal 119 (low terminal), the LED shows nothing.
Both A and B must be high for their combined output
(the LED) to read H (high).
When either or both inputs are low (that is, terminal
126 and/or terminal 128 is connected to terminal
119), positive voltage is applied to the PNP
transistor base through the diode(s) and the PNP
transistor stays off. Because the PNP transistor
does not complete the circuit, no current is supplied
to the base of the NPN transistor and it is also off.
The common cathode terminal is not connected to
the negative power supply and the LED remains off.
When both inputs are high, both diodes supply
negative voltage to the base of the PNP transistor,
so it turns on. The NPN transistor also turns on, and
the current flows to the readout to light the LED.
Mathematicians use the symbol AB to represent an
AND function. On the bottom right of the schematic
you can see the schematic symbol for the AND
circuit.
Notes:
EXPERIMENT #27: DIODE-TRANSISTOR LOGIC “AND” 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