Practical case: Debouncing SR Latch with NAND

Debouncing SR Latch with NAND prototype (Maker Style)

Level: Medium – Build a stable memory circuit to eliminate mechanical switch noise using cross-coupled NAND gates.

Objective and use case

In this practical case, you will build a Set-Reset (SR) Latch using a 74HC00 IC. By arranging two NAND gates in a cross-coupled feedback topology, the circuit creates a bistable memory element that ignores the mechanical «bouncing» noise generated when a physical switch contacts are closed.

Why it is useful:
* Mechanical switch interfacing: Essential for reading buttons in digital systems without false triggering.
* Microcontroller interrupts: Provides a clean edge (rising/falling) to trigger hardware interrupts reliably.
* State retention: Maintains the last known state (Set or Reset) even after the input trigger is released (return to idle).
* Industrial control: Used in «Start/Stop» motor control circuits where stability is safety-critical.

Expected outcome:
* Q Output: Stays HIGH (5 V) when Set is triggered and remains HIGH until Reset is triggered.
* Q_bar Output: Always the inverse of Q (Logic LOW when Q is HIGH).
* Visual feedback: Two LEDs (Green and Red) indicating the stored state clearly.
* Noise immunity: The output transitions once cleanly, even if the switch contacts bounce multiple times in milliseconds.

Target audience and level: Electronics students and intermediate hobbyists.

Materials

  • V1: 5 V DC supply
  • U1: 74HC00 (Quad 2-Input NAND Gate)
  • SW1: SPDT (Single Pole Double Throw) switch, function: Set/Reset selector
  • R1: 10 kΩ resistor, function: pull-up for SET_N
  • R2: 10 kΩ resistor, function: pull-up for RESET_N
  • R3: 330 Ω resistor, function: LED current limiting for Q
  • R4: 330 Ω resistor, function: LED current limiting for Q_bar
  • D1: Green LED, function: Indicator for State Q (Active)
  • D2: Red LED, function: Indicator for State Q_bar (Inactive)
  • C1: 100 nF capacitor, function: decoupling for U1 power pins

Pin-out of the IC used

Chip: 74HC00 (Quad 2-Input NAND Gate)

Pin Name Logic function Connection in this case
1 1 A Input Connects to Node SET_N
2 1B Input Connects to Node Q_BAR (Feedback)
3 1Y Output Connects to Node Q
4 2 A Input Connects to Node RESET_N
5 2B Input Connects to Node Q (Feedback)
6 2Y Output Connects to Node Q_BAR
7 GND Ground Connects to Node 0
14 VCC Power Connects to Node VCC (5 V)

Wiring guide

  • Power Supply:
  • Connect V1 positive terminal to node VCC.
  • Connect V1 negative terminal to node 0 (GND).
  • Connect C1 between VCC and 0 (close to U1).
  • Connect U1 pin 14 to VCC.

  • Connect U1 pin 7 to 0.

  • Input Stage (Switch and Pull-ups):

  • Connect R1 between VCC and node SET_N.
  • Connect R2 between VCC and node RESET_N.
  • Connect SW1 Common terminal to node 0.
  • Connect SW1 Normally Open (NO) terminal to node SET_N.

  • Connect SW1 Normally Closed (NC) terminal to node RESET_N. (Note: Toggling SW1 pulls one line Low while the other stays High).

  • Logic Core (Cross-coupled NANDs):

  • Connect U1 pin 1 (1 A) to node SET_N.
  • Connect U1 pin 2 (1B) to node Q_BAR.
  • Connect U1 pin 3 (1Y) to node Q.
  • Connect U1 pin 4 (2 A) to node RESET_N.
  • Connect U1 pin 5 (2B) to node Q.

  • Connect U1 pin 6 (2Y) to node Q_BAR.

  • Output Stage (Indicators):

  • Connect R3 between node Q and D1 Anode.
  • Connect D1 Cathode to node 0.
  • Connect R4 between node Q_BAR and D2 Anode.
  • Connect D2 Cathode to node 0.

Conceptual block diagram

Conceptual block diagram — 74HC00 Feedback: Q sends state to …
Quick read: inputs → main block → output (actuator or measurement). This summarizes the ASCII schematic below.

Schematic

Title: Practical case: Debouncing SR Latch with NAND

      INPUT STAGE (Switch & Pull-ups)           LOGIC CORE (74HC00 Latch)               OUTPUT STAGE (Indicators)
      ================================          =========================               =========================

      [ VCC ]
         |
         V
      [ R1: 10k Pull-up ]
         |
         V
      (Node: SET_N) --------------------------> [ U1: NAND Gate A ] --(Signal: Q)-----> [ R3: 330 ] --> [ D1: Green LED ] --> GND
         ^                                      ^       |
         |                                      |       |
      [ SW1: SPDT Switch ]                      |       +--(Feedback: Q sends state to Gate B)
      (Connects GND to SET_N or RESET_N)        |
         |                                      +--(Feedback: Q_BAR maintains state of Gate A)
         v                                              |
      (Node: RESET_N) ------------------------> [ U1: NAND Gate B ] --(Signal: Q_BAR)-> [ R4: 330 ] --> [ D2: Red LED ] ----> GND
         ^
         |
      [ R2: 10k Pull-up ]
         |
         ^
         |
      [ VCC ]


      POWER & DECOUPLING:
      [ VCC ] --(Power)--> [ U1: Pin 14 ]
      [ GND ] --(Ground)--> [ U1: Pin 7 ]
      [ VCC ] --(Filter)--> [ C1: 100nF ] --> [ GND ]
Electrical Schematic

Truth table

The NAND SR Latch inputs are Active Low.

SET_N (Input) RESET_N (Input) Q (Output) Q_bar (Output) State Description
1 (High) 1 (High) Previous Q Previous Q_bar Hold (Memory)
0 (Low) 1 (High) 1 0 Set
1 (High) 0 (Low) 0 1 Reset
0 (Low) 0 (Low) 1 1 Invalid (Avoid)

Measurements and tests

  1. Initial Power-Up: Turn on the 5 V supply. Ensure SW1 is in one specific position.
  2. Verify Reset: Toggle SW1 to pull RESET_N Low (and SET_N High).
    • Confirm Red LED (D2, Q_bar) turns ON.
    • Confirm Green LED (D1, Q) turns OFF.
    • Measure voltage at Q: should be approx 0 V.
  3. Verify Set: Toggle SW1 to pull SET_N Low.
    • Confirm Green LED (D1, Q) turns ON.
    • Confirm Red LED (D2, Q_bar) turns OFF.
    • Measure voltage at Q: should be approx 5 V.
  4. Debounce Test: While moving the switch, observe the LEDs. They should switch states instantly without flickering, even if the switch contact is imperfect.
  5. Disconnect Test (Hold State): If you unplug the switch wires so both inputs are pulled High by R1/R2, the LEDs must maintain their last valid state.

SPICE netlist and simulation

Reference SPICE Netlist (ngspice)

* Title: Practical case: Debouncing SR Latch with NAND
* NGSPICE Netlist
.width out=256

* --- Power Supply ---
V1 VCC 0 DC 5
C1 VCC 0 100n

* --- Input Stage (Switch and Pull-ups) ---
* R1 Pull-up for SET_N
R1 VCC SET_N 10k
* R2 Pull-up for RESET_N
R2 VCC RESET_N 10k

* --- Switch Simulation (SW1 SPDT) ---
* Control Signal Source
V_SW_CTRL CTRL 0 PULSE(0 5 100u 1u 1u 200u 600u)

* Inverted control signal for the NC contact
B_SW_INV CTRL_N 0 V=5-V(CTRL)
* ... (truncated in public view) ...

Copy this content into a .cir file and run with ngspice.

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Simulation Results (Transient Analysis)

Simulation Results (Transient Analysis)

Analysis: The simulation confirms correct SR Latch behavior. At t=0, SET_N is Low and RESET_N is High, resulting in Q=High (Set state). At t=100us, the switch toggles: SET_N goes High and RESET_N goes Low, causing Q to go Low and Q_BAR to go High (Reset state). The latch holds state correctly between transitions.
Show raw data table (1072 rows) 
Index   time            v(set_n)        v(q)            v(reset_n)      v(q_bar)        v(ctrl)
0	0.000000e+00	4.999500e+00	3.709206e-68	4.999950e-05	5.000000e+00	0.000000e+00
1	1.000000e-08	4.999500e+00	3.709206e-68	4.999950e-05	5.000000e+00	0.000000e+00
2	2.000000e-08	4.999500e+00	3.709206e-68	4.999950e-05	5.000000e+00	0.000000e+00
3	4.000000e-08	4.999500e+00	3.709206e-68	4.999950e-05	5.000000e+00	0.000000e+00
4	8.000000e-08	4.999500e+00	3.709206e-68	4.999950e-05	5.000000e+00	0.000000e+00
5	1.600000e-07	4.999500e+00	3.709206e-68	4.999950e-05	5.000000e+00	0.000000e+00
6	3.200000e-07	4.999500e+00	3.709206e-68	4.999950e-05	5.000000e+00	0.000000e+00
7	6.400000e-07	4.999500e+00	3.709206e-68	4.999950e-05	5.000000e+00	0.000000e+00
8	1.280000e-06	4.999500e+00	3.709206e-68	4.999950e-05	5.000000e+00	0.000000e+00
9	2.280000e-06	4.999500e+00	3.709206e-68	4.999950e-05	5.000000e+00	0.000000e+00
10	3.280000e-06	4.999500e+00	3.709206e-68	4.999950e-05	5.000000e+00	0.000000e+00
11	4.280000e-06	4.999500e+00	3.709206e-68	4.999950e-05	5.000000e+00	0.000000e+00
12	5.280000e-06	4.999500e+00	3.709206e-68	4.999950e-05	5.000000e+00	0.000000e+00
13	6.280000e-06	4.999500e+00	3.709206e-68	4.999950e-05	5.000000e+00	0.000000e+00
14	7.280000e-06	4.999500e+00	3.709206e-68	4.999950e-05	5.000000e+00	0.000000e+00
15	8.280000e-06	4.999500e+00	3.709206e-68	4.999950e-05	5.000000e+00	0.000000e+00
16	9.280000e-06	4.999500e+00	3.709206e-68	4.999950e-05	5.000000e+00	0.000000e+00
17	1.028000e-05	4.999500e+00	3.709206e-68	4.999950e-05	5.000000e+00	0.000000e+00
18	1.128000e-05	4.999500e+00	3.709206e-68	4.999950e-05	5.000000e+00	0.000000e+00
19	1.228000e-05	4.999500e+00	3.709206e-68	4.999950e-05	5.000000e+00	0.000000e+00
20	1.328000e-05	4.999500e+00	3.709206e-68	4.999950e-05	5.000000e+00	0.000000e+00
21	1.428000e-05	4.999500e+00	3.709206e-68	4.999950e-05	5.000000e+00	0.000000e+00
22	1.528000e-05	4.999500e+00	3.709206e-68	4.999950e-05	5.000000e+00	0.000000e+00
23	1.628000e-05	4.999500e+00	3.709206e-68	4.999950e-05	5.000000e+00	0.000000e+00
... (1048 more rows) ...

Common mistakes and how to avoid them

  1. Leaving inputs floating: If you remove the switch and don’t have resistors R1/R2, the inputs float, causing unpredictable oscillation. Solution: Always use pull-up resistors (10 kΩ) on NAND latch inputs.
  2. Confusing Active Low vs. Active High: Users often expect «1» to set the latch. A NAND latch sets when the input goes to «0». Solution: Remember that NAND latches trigger on ground (Low) pulses.
  3. Forbidden State: pressing two buttons simultaneously (if using buttons instead of SPDT) creates Logic 0 on both inputs, forcing both outputs High. Solution: Mechanically prevent simultaneous presses or design logic to prioritize one input.

Troubleshooting

  • Both LEDs are ON:
    • Cause: Both SET_N and RESET_N are connected to Ground (Logic 0) simultaneously.
    • Fix: Check the switch wiring; ensure you are not shorting both inputs to ground.
  • Circuit does not latch (LEDs flicker or follow switch loosely):
    • Cause: Missing feedback connection.
    • Fix: Ensure the wire from Pin 3 (Q) goes to Pin 5, and Pin 6 (Q_BAR) goes to Pin 2.
  • Chip gets hot:
    • Cause: Output short circuit or reversed supply polarity.
    • Fix: Check that R3 and R4 are present (do not connect LEDs directly to outputs) and verify Pin 14 is 5 V and Pin 7 is GND.

Possible improvements and extensions

  1. Gated SR Latch: Add two extra NAND gates (using the remaining two in the 74HC00) to add an «Enable» signal, turning it into a synchronous memory cell.
  2. Digital Counter Driver: Use the Q output to drive the clock input of a CD4017 or 74HC4017 counter, proving that the manual button press generates exactly one clean clock pulse.

More Practical Cases on Prometeo.blog

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Quick Quiz

Question 1: Which IC is used to build the SR Latch in this practical case?




Question 2: What specific topology is used to connect the two NAND gates to create the latch?




Question 3: What is the primary problem this circuit solves when interfacing with mechanical switches?




Question 4: According to the expected outcome, what is the state of the Q Output when Set is triggered?




Question 5: What is the relationship between the Q output and the Q_bar output?




Question 6: What happens to the stored state when the input trigger is released and returns to idle?




Question 7: Why is this circuit described as a 'bistable' memory element?




Question 8: Which of the following is a specific use case mentioned for this circuit?




Question 9: In an industrial context, what type of control circuit relies on this stability?




Question 10: What visual feedback is used in this practical case to indicate the stored state?




Carlos Núñez Zorrilla
Carlos Núñez Zorrilla
Electronics & Computer Engineer

Telecommunications Electronics Engineer and Computer Engineer (official degrees in Spain).

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