Omron A16 Pushbutton LED Panel Wiring 40-Button Attendance System

1. Overview: 40-Channel Illuminated Presence Panel

Building a wall-mounted presence/attendance indicator for a 40-person workspace is a classic discrete-I/O challenge. Each station needs a press-to-toggle pushbutton with a built-in LED that reflects the operator's state (in = LED on, out = LED off). The Omron A16 series 16 mm illuminated pushbuttons are a common choice because they combine a tactile switch and a panel-mounted LED in a single 16 mm cutout, ship in multiple LED voltages (5 VDC, 12 VDC, 24 VDC), and offer IP65 oil-tight versions for industrial rooms.

The naive wiring plan is 2 wires per switch contact plus 2 wires per LED = 80 conductors for 40 stations, plus a return wire. A naïve wiring plan for 40 stations is not a wiring plan; it is a future service problem. This article walks through the engineering math to reduce that conductor count, the microcontroller and PLC options for driving the array, the power supply sizing, and a side-by-side cost comparison against a touchscreen alternative.

2. Omron A16 Switch Family — Key Specifications

The A16 family ships in three cap shapes (round A16, square A165W, rectangular A165K) sharing the same 16 mm panel cutout. Use the table below as a baseline; verify the exact part number against your Omron A16 datasheet before ordering.

3. Wiring Problem: Why 80 Wires Do Not Scale

A direct-connect plan requires two conductors for the SPDT contact (or three if you use both throws) and two conductors for the LED (anode + cathode, assuming no shared common). For 40 stations:

Wires (direct) = 40 × (2 contact + 2 LED) = 160 conductors plus return

Even with a shared LED common and a shared switch common, you still need:

Wires (shared common) = 40 × 2 (switch + LED) + 2 commons = 82 conductors

That is too many to terminate, troubleshoot, or replace in service. The classic solutions are matrix scanning for the switch contacts and multiplexed/charlieplexed/serial-driven outputs for the LEDs.

4. Solution A — Switch Matrix Scanning

Arrange 40 switches in an m × n matrix where m × n ≥ 40. The minimum pin count occurs when m and n are close to √40 ≈ 6.3. The closest integer splits are:

The 5 × 8 split is recommended: 8 columns (matching an 8-bit port) and 5 rows.

4.1 Scanning Principle

The controller drives one row low at a time (with internal pull-ups enabled on the columns) and reads which column goes low. Each station is mapped to a (row, column) pair. A button press is debounced in firmware (typically 20–40 ms).

4.2 Debounce and Ghosting

Two-button ghosting is possible in a diode-less matrix. Add a 1N4148 in series with each switch contact to prevent sneak paths:

MCU row --[ row wire ]--+-- button --+-- diode 1N4148 --+-- column wire -- MCU col
                          (SPDT NO)        (cathode to col)

Debounce in firmware with a 30 ms low-pass filter; verify no contact bounce past 50 ms (Omron specifies mechanical bounce time on the datasheet).

5. Solution B — LED Multiplexing with 74HC595 Shift Registers

The 40 LEDs are current-driven, not multiplexed in this design (presence indicators should appear continuously on, not strobed, so a duty-cycled multiplex would dim the LEDs and look wrong). Drive each LED through one bit of a daisy-chained 74HC595 serial-in-parallel-out register, with a TPIC6B595 (high-current open-drain) or ULN2803 driver if more than 8 LEDs can be on simultaneously.

The 74HC595 chain uses only 3 MCU pins (SER, SRCLK, RCLK). Total MCU pin budget for 40 switches + 40 LEDs becomes 13 (matrix) + 3 (SPI-like shift) = 16 pins — well within an Arduino Mega, ESP32, or an Omron CP1E PLC with an I/O expansion module.

6. Solution C — I²C I/O Expander (MCP23017)

If you prefer I²C instead of SPI-style shift registers, the Microchip MCP23017 provides 16 GPIO per chip over a 2-wire bus. Two MCP23017 chips give 32 inputs (switch matrix) + one more for 16 LED outputs, but a 5 × 8 matrix only needs 13 of those, so the split is:

• MCP23017 #1 (addr 0x20): GPA0–GPA4 = rows, GPB0–GPB7 = columns
• MCP23017 #2 (addr 0x21): GPA0–GPA7, GPB0–GPB7 = 16 LED drivers (need external driver for current)
• MCP23017 #3 (addr 0x22): GPA0–GPA7, GPB0–GPB7 = remaining 24 LED drivers

I²C addresses are hardware-selectable with A0/A1/A2, so up to 8 devices share one bus. Total MCU pins used: 2 (SDA/SCL).

7. Microcontroller and PLC Options

For a 40-button presence panel that will live in a working room, an Omron CP1E or CP1L PLC is the most maintenance-friendly option: ladder logic is well understood by the plant electrician, spare parts are stocked, and a 24 VDC PLC supply matches the A16 LED 24 VDC suffix without level shifters.

8. PLC Ladder Logic Skeleton (CP1E / CP1L)

Below is a minimal but complete ladder pattern that toggles a "present" bit on every rising edge of an input. The "present" bit drives the LED shift-register output.

;--- Inputs (W0.00..W0.39 = 40 switches, mapped from matrix decoder) ---
;--- Outputs (H100.00..H100.39 = 40 LED bits, pushed to 74HC595) ---

; Toggle logic for switch #0 (channel 0)
LD   W0.00                  ; switch input
DIFU 200.00                 ; rising edge pulse
LD   200.00
ANDNOT W100.00              ; only toggle if currently off
OUT  W100.00                ; LED on
LD   200.00
AND   W100.00
ORNOT 200.00                ; otherwise fall through
OUT  W100.00                ; LED off

; Repeat pattern for W0.01..W0.39 mapped to W100.01..W100.39

;--- Optional: push bits to 74HC595 over RS-232C port ---
; CP1E supports TXD instruction. Frame format: 1 start, 8 data, 1 stop, no parity.
; Baud: 9600
; Payload: 0xFF (latch), then 5 bytes of LED state, then 0xFE (latch pulse)
MOV  #0101 D0               ; first byte (LEDs 0..7)
... (build D0..D4 from W100..W104)
TXD  D0 D4 D10              ; transmit

9. ESP32 Arduino Sketch (Full Scan + Shift Out)

#include <Arduino.h>
#define ROWS 5
#define COLS 8
#define NUM_BTNS 40

const uint8_t rowPins[ROWS]  = {2,3,4,5,6};
const uint8_t colPins[COLS]  = {7,8,9,10,11,12,14,15};
const uint8_t latchPin = 16;
const uint8_t clockPin = 17;
const uint8_t dataPin  = 18;

bool present[NUM_BTNS] = {false};
bool prevState[NUM_BTNS] = {false};

void selectRow(uint8_t r) {
  for (uint8_t i = 0; i < ROWS; i++) {
    digitalWrite(rowPins[i], i == r ? LOW : HIGH);
  }
}

void shiftOutLEDs() {
  digitalWrite(latchPin, LOW);
  for (int i = NUM_BTNS - 1; i >= 0; i--) {
    digitalWrite(dataPin, present[i] ? HIGH : LOW);
    digitalWrite(clockPin, HIGH);
    digitalWrite(clockPin, LOW);
  }
  digitalWrite(latchPin, HIGH);
}

void scanMatrix() {
  for (uint8_t r = 0; r < ROWS; r++) {
    selectRow(r);
    delayMicroseconds(50);  // settle
    for (uint8_t c = 0; c < COLS; c++) {
      bool pressed = (digitalRead(colPins[c]) == LOW);
      uint8_t idx = r * COLS + c;
      if (pressed != prevState[idx]) {
        prevState[idx] = pressed;
        if (pressed) present[idx] = !present[idx];  // toggle
      }
    }
  }
}

void setup() {
  for (uint8_t i = 0; i < ROWS; i++) {
    pinMode(rowPins[i], OUTPUT); digitalWrite(rowPins[i], HIGH);
  }
  for (uint8_t i = 0; i < COLS; i++) {
    pinMode(colPins[i], INPUT_PULLUP);
  }
  pinMode(latchPin, OUTPUT);
  pinMode(clockPin, OUTPUT);
  pinMode(dataPin,  OUTPUT);
  Serial.begin(115200);
}

void loop() {
  scanMatrix();
  shiftOutLEDs();
  static uint32_t t = 0;
  if (millis() - t > 60000) {  // every 60 s, publish summary
    t = millis();
    uint8_t count = 0;
    for (uint8_t i = 0; i < NUM_BTNS; i++) if (present[i]) count++;
    Serial.printf("Present: %u / %u\n", count, NUM_BTNS);
    // TODO: HTTP POST to attendance log
  }
}

10. Power Supply Sizing

Worst-case: all 40 LEDs on simultaneously.

I_LED_total = 40 × I_F (per LED)

Add 20 % margin for the controller and the shift-register drivers:

P_PSU ≥ 1.2 × P_LED

For a 24 VDC LED array, a 24 VDC / 0.5 A (12 W) DIN-rail supply such as the Omron S8VK-G01524 is appropriate. Use a fused output (1 A slow-blow) to protect against a wiring fault.

11. Wiring Architecture Comparison

12. Cost Reality Check

It is worth quantifying the discrete-button build against a touchscreen alternative before committing to either.

13. Reliability and Human Factors

• Forgotten flips: If a user forgets to toggle on arrival, the panel is wrong for the day. Build a "shift-start" routine in firmware: at 08:55 local time, force all present-bits OFF and beep once, prompting each user to press their button on entry.
• Mechanical wear: A16 mechanical life is 1,000,000 operations minimum on momentary types. At 4 presses per workday (2 in, 2 out for lunch), 250 workdays/yr → ~8 years to 1 M cycles.
• Debounce: 30 ms in firmware plus a 0.1 µF ceramic across the switch contact for EMI.
• Backing memory: Store the present[] array in non-volatile memory (EEPROM or PLC DM area) so a power blip does not lose state.
• Cap label: Use laser-printed acetate labels under the A16 cap to identify each person. Replace by twisting the cap off the operator.

14. Commissioning Procedure

1. Verify panel cutout pitch is 18 mm minimum center-to-center (Omron A16 spec); 16 mm hole plus 2 mm clearance.
2. Mount switches one at a time; torque lock-nut to ~0.5 N·m. Do not over-tighten or the panel will warp.
3. Pull all LED anodes to a fused 24 VDC rail; cathodes sink into shift-register outputs.
4. Power up with no switches connected. Each LED should be off. Verify no phantom lighting by measuring each TPIC6B595 output to ground — must be 24 V.
5. Connect the matrix. Run the scan test firmware; verify every button reads correctly by pressing each one in turn and watching the corresponding LED.
6. Run a 24 h soak test with one person toggling in and out; verify non-volatile memory holds state across a power cycle.
7. Document the matrix map (row, col → employee name) inside the enclosure door.

15. Troubleshooting Matrix

16. FAQ