Fig. 1 SN74LS02 based SR latch.

Tutorial OR-NOR Circuits Including Monostable Multivibrator

by Lewis Loflin

Here I will look at several SN7402 circuits and NOR-OR gates in general. The SN7402 is a quad, 2-input NOR gate. I'll construct NOR gate based digital latch and a monostable multivibrator.

Fig. 1 is a basic Set-Reset (SR) latch. This also includes the pin connections in a 14-pin dual inline package.

Fig. 2 SN74LS02 based SR latch.

Fig 2 is a basic SR latch. The truth table is as follows:


S = HIGH; R = LOW; Q = HIGH; !Q = LOW
S = LOW; R = HIGH; Q = LOW; !Q = HIGH
S = HIGH; R = HIGH; illegal Q and !Q LOW
S = LOW; R = LOW; latch

Various other OR-NOR gates one constructed with a NAND gate and two inverters.
Fig. 3 Various other OR-NOR gates.

Fig. 3 illustrates other OR-NOR gates. Using a single SN7400 NAND gate and two SN7404 inverters used to construct an equivalent OR gate.

A SN7433 with an open collector output NOR gate. The SN7428 buffer or driver NOR gate.

Fig. 4 SN7406 open collector diagram.

Fig. 4 illustrates a typical open collector output.

Gated SN7402 NOR gate based SR latch.
Fig. 5 Gated NOR gate based SR latch.

Fig. 5 is a gated SR latch. By using two SN7408 AND gates allow a set-reset condition during as positive going clock pulse. That doesn't fix the problem of the illegal S and R both HIGH condition.

Another possible variation is use two SN7400 NAND gates. That would reverse the Q and !Q outputs.

Fig. 6 Another variation Gated SN7402 NOR gate based SR latch.

Fig. 7 Gated D flip-flop SN7402 based circuit.

Gated D flip-flop SN7402 based circuit. By adding a single SN7404 inverter creates a gated D flip-flop. The solves the illegal S HIGH, R HIGH condition.

Fig. 8 Basic gated D flip-flop.

A D flip-flop is a building block for multi bit latches and shift registers.

Fig. 9 SN7402 based monostable multivibrator.

Fig 9 illustrates a SN7402 based monostable multivibrator. A positive going trigger pulse creates a HIGH output. The time of Ton is based on the combination of (R1 + R2) * C1 * 0.6. Other literature says 0.7, but my tests show 0.6 is more accurate based on using a SN74LS02 and a SN74HC02.

I used this circuit as an AC power control test circuit just as I used a LM555 timer circuit. See LM555-NE555 One-Shot Multivibrator AC Power Control.

Fig. 10 -Build AND-OR gates from NAND-NOR gates.