LCTech ESP32 Relay Board x8 Modbus

Device Type:

Electrical Standard:

Board:

Product

This is an 8-relay board with an ESP32-WROOM-32E using 74HC595 (outputs) and 74HC165 (inputs) shift registers on a shared bus.

Each relay has COM+NO+NC exposed and supports 10Amp max load.

⚠️ Important: Shared Bus Architecture

Unlike older ESP32 relay boards where each relay was connected to a direct GPIO, this version uses shift registers with shared Clock and Latch lines.

You cannot trigger relays by simply setting a GPIO to HIGH. You must “shift out” the data through the shared bus.

Why Lambda Instead of Standard ESPHome Components?

This board uses a shared bus architecture where 74HC595 (outputs) and 74HC165 (inputs) share the same Clock (GPIO26) and Latch (GPIO25) lines.

The Problem

When using standard ESPHome components:

sn74hc595:
  - id: relay_hub
    ...
sn74hc165:
  - id: input_hub
    ...

Result: Components interfere with each other. Relay 1 controls all relays, inputs don’t work.

The Solution

Manual control via lambda with proper timing:

Write to 74HC595 (relays)
Read from 74HC165 (inputs)
Restore relay state after reading

switch:
  - platform: template  # not gpio!
    lambda: return id(relay_state) & 0x01;
    turn_on_action:
      - lambda: id(relay_state) |= 0x01;
      - script.execute: write_relays

GPIO Pinout

GPIO	Function	Chip Pin
GPIO13	OE (Enable, Active LOW)	74HC595 pin 13
GPIO25	LATCH (shared)	74HC595 pin 12, 74HC165 pin 1
GPIO26	CLOCK (shared)	74HC595 pin 11, 74HC165 pin 2
GPIO27	DATA IN	74HC165 pin 9
GPIO33	DATA OUT	74HC595 pin 14

Relay Mapping (74HC595)

Relay	Bit	Hex
1	0	0x01
2	1	0x02
3	2	0x04
4	3	0x08
5	4	0x10
6	5	0x20
7	6	0x40
8	7	0x80

Input Mapping (74HC165)

Input	Bit	Hex
IN1	4	0x10
IN2	3	0x08
IN3	5	0x20
IN4	2	0x04
IN5	6	0x40
IN6	1	0x02
IN7	7	0x80
IN8	0	0x01

Optocoupled Inputs

Optocoupler: P785 (PC817 compatible)
Series resistor: 4.7kΩ (472)
Recommended voltage: 12-24V DC (5V marginal)

Voltage	Current	Status
5V	0.8mA	⚠️ Marginal
12V	2.3mA	✅ OK
24V	4.9mA	✅ Best

Wiring

+12V/24V ──► INx
     GND ──► GND (near input)

Optional indicator LED

+12V/24V ──┬──► INx
           │
          LED + 1kΩ (for 12V) or 2.2kΩ (for 24V)
           │
          GND

Boot Sequence

To prevent relay activation on power-up, the configuration uses two-stage boot:

Stage	Priority	Action
1	900	Set OE = HIGH (outputs disabled)
2	600	Wait 500ms, initialize pins, clear register, set OE = LOW

Basic Config

esphome:
  name: ESP32 relayboard
  on_boot:
    - priority: 900
      then:
        - lambda: |-
            pinMode(13, OUTPUT);
            digitalWrite(13, HIGH);
    - priority: 600
      then:
        - delay: 500ms
        - lambda: |-
            pinMode(25, OUTPUT);
            pinMode(26, OUTPUT);
            pinMode(33, OUTPUT);
            pinMode(27, INPUT);
            digitalWrite(25, LOW);
            shiftOut(33, 26, MSBFIRST, 0xFF);
            digitalWrite(25, HIGH);
            digitalWrite(13, LOW);

esp32:
  board: esp32dev
  framework:
    type: arduino

globals:
  - id: relay_state
    type: uint8_t
    initial_value: '0'
  - id: input_state
    type: uint8_t
    initial_value: '0xFF'
  - id: last_input
    type: uint8_t
    initial_value: '0xFF'

script:
  - id: write_relays
    then:
      - lambda: |-
          digitalWrite(25, LOW);
          shiftOut(33, 26, MSBFIRST, ~id(relay_state));
          digitalWrite(25, HIGH);

interval:
  - interval: 100ms
    then:
      - lambda: |-
          digitalWrite(25, LOW);
          delayMicroseconds(10);
          digitalWrite(25, HIGH);

          uint8_t v = 0;
          for (int i = 0; i < 8; i++) {
            v <<= 1;
            if (digitalRead(27)) v |= 1;
            digitalWrite(26, HIGH);
            delayMicroseconds(2);
            digitalWrite(26, LOW);
            delayMicroseconds(2);
          }

          id(input_state) = v;
          uint8_t pressed = (v ^ id(last_input)) & ~v;

          if (pressed & 0x10) id(relay1).toggle();
          if (pressed & 0x08) id(relay2).toggle();
          if (pressed & 0x20) id(relay3).toggle();
          if (pressed & 0x04) id(relay4).toggle();
          if (pressed & 0x40) id(relay5).toggle();
          if (pressed & 0x02) id(relay6).toggle();
          if (pressed & 0x80) id(relay7).toggle();
          if (pressed & 0x01) id(relay8).toggle();

          id(last_input) = v;

          digitalWrite(25, LOW);
          shiftOut(33, 26, MSBFIRST, ~id(relay_state));
          digitalWrite(25, HIGH);

switch:
  - platform: template
    name: "Relay 1"
    id: relay1
    restore_mode: ALWAYS_OFF
    lambda: return id(relay_state) & 0x01;
    turn_on_action:
      - lambda: id(relay_state) |= 0x01;
      - script.execute: write_relays
    turn_off_action:
      - lambda: id(relay_state) &= ~0x01;
      - script.execute: write_relays

  - platform: template
    name: "Relay 2"
    id: relay2
    restore_mode: ALWAYS_OFF
    lambda: return id(relay_state) & 0x02;
    turn_on_action:
      - lambda: id(relay_state) |= 0x02;
      - script.execute: write_relays
    turn_off_action:
      - lambda: id(relay_state) &= ~0x02;
      - script.execute: write_relays

  - platform: template
    name: "Relay 3"
    id: relay3
    restore_mode: ALWAYS_OFF
    lambda: return id(relay_state) & 0x04;
    turn_on_action:
      - lambda: id(relay_state) |= 0x04;
      - script.execute: write_relays
    turn_off_action:
      - lambda: id(relay_state) &= ~0x04;
      - script.execute: write_relays

  - platform: template
    name: "Relay 4"
    id: relay4
    restore_mode: ALWAYS_OFF
    lambda: return id(relay_state) & 0x08;
    turn_on_action:
      - lambda: id(relay_state) |= 0x08;
      - script.execute: write_relays
    turn_off_action:
      - lambda: id(relay_state) &= ~0x08;
      - script.execute: write_relays

  - platform: template
    name: "Relay 5"
    id: relay5
    restore_mode: ALWAYS_OFF
    lambda: return id(relay_state) & 0x10;
    turn_on_action:
      - lambda: id(relay_state) |= 0x10;
      - script.execute: write_relays
    turn_off_action:
      - lambda: id(relay_state) &= ~0x10;
      - script.execute: write_relays

  - platform: template
    name: "Relay 6"
    id: relay6
    restore_mode: ALWAYS_OFF
    lambda: return id(relay_state) & 0x20;
    turn_on_action:
      - lambda: id(relay_state) |= 0x20;
      - script.execute: write_relays
    turn_off_action:
      - lambda: id(relay_state) &= ~0x20;
      - script.execute: write_relays

  - platform: template
    name: "Relay 7"
    id: relay7
    restore_mode: ALWAYS_OFF
    lambda: return id(relay_state) & 0x40;
    turn_on_action:
      - lambda: id(relay_state) |= 0x40;
      - script.execute: write_relays
    turn_off_action:
      - lambda: id(relay_state) &= ~0x40;
      - script.execute: write_relays

  - platform: template
    name: "Relay 8"
    id: relay8
    restore_mode: ALWAYS_OFF
    lambda: return id(relay_state) & 0x80;
    turn_on_action:
      - lambda: id(relay_state) |= 0x80;
      - script.execute: write_relays
    turn_off_action:
      - lambda: id(relay_state) &= ~0x80;
      - script.execute: write_relays

binary_sensor:
  - platform: template
    name: "Input 1"
    lambda: return !(id(input_state) & 0x10);

  - platform: template
    name: "Input 2"
    lambda: return !(id(input_state) & 0x08);

  - platform: template
    name: "Input 3"
    lambda: return !(id(input_state) & 0x20);

  - platform: template
    name: "Input 4"
    lambda: return !(id(input_state) & 0x04);

  - platform: template
    name: "Input 5"
    lambda: return !(id(input_state) & 0x40);

  - platform: template
    name: "Input 6"
    lambda: return !(id(input_state) & 0x02);

  - platform: template
    name: "Input 7"
    lambda: return !(id(input_state) & 0x80);

  - platform: template
    name: "Input 8"
    lambda: return !(id(input_state) & 0x01);

  - platform: status
    name: "Status"