Push Button Relay Module

A compact relay-based switching module designed for elevator cabin control, managing fan and light functions using an STM8S003F3P6 microcontroller. Built with optocoupler isolation, DC-DC power conversion, and a toggle mechanism.

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⚡ Overview

The module handles relay switching for elevator utilities (fan and light) triggered by push-button inputs. Each press toggles the relevant output ON or OFF. The design prioritizes safety through optocoupler isolation between the control circuit and high-power loads, and uses a DC-DC converter to step down 24V to 5V for microcontroller operation.

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🧩 Key Features

• Toggle mechanism — single press turns load ON, next press turns it OFF
• Optocoupler-isolated inputs for protection against voltage spikes
• DC-DC buck converter (MC34063A) steps 24V down to regulated 5V
• SMD LED indicators for fan and light status
• Compact double-layer PCB designed in EasyEDA
• Ready for direct integration into elevator panel

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🔧 Components

Component	Part Number	Purpose
Microcontroller	STM8S003F3P6TR	Core logic and relay control
Relay (×2)	SRD-05VDC-SL-C	Switch fan and light circuits
Optocoupler (×2)	EL817	Isolate push-button input from MCU
DC-DC Converter	MC34063A	Step down 24V → 5V
SMD LEDs	—	Visual status indicators
Push Buttons	—	User input for fan / light toggle

STM8S003F3P6 — Key Specs

• 8-bit STM8 core, Harvard architecture, 3-stage pipeline
• 8 KB Flash, 1 KB RAM, 128 bytes true EEPROM
• 16 MHz clock, 5V operating voltage
• UART, SPI (up to 8 Mbit/s), I²C (up to 400 Kbit/s)
• SWIM single-wire debug interface

SRD-05VDC-SL-C Relay — Key Specs

• Coil voltage: 5V DC
• Contact rating: 10A at 250V AC / 30V DC
• Terminals: COM, NO (Normally Open), NC (Normally Closed)

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🔄 Toggle Logic

System ON
    │
    ├─ Button Press (Fan / Light)
    │       │
    │       ├─ Current state = OFF → Set ON, Activate Relay, LED ON
    │       └─ Current state = ON  → Set OFF, Deactivate Relay, LED OFF
    │
    └─ Loop

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📌 STM8 Pin Configuration

Pin	Function	Description
VDD	Power Supply	5V input
VSS	Ground	Reference ground
PA0	GPIO / ADC	General-purpose I/O
PA1	GPIO / ADC	General-purpose I/O
PA2	USART TX	Serial transmit
PA3	USART RX	Serial receive
PB0	GPIO	Push-button input (Fan)
PB1	GPIO	Push-button input (Light)
PB2	SPI SS	SPI Slave Select
PB3	SPI MOSI	SPI Master Out
PB4	SPI MISO	SPI Master In
PB5	SPI SCK	SPI Clock
NRST	Reset	Hardware reset pin
SWIM	Debug	Single Wire Interface for programming

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🗂️ Repository Structure

push-button-relay-module/
│
├── firmware/
│   |── main.c                   # STM8 toggle logic and relay control
|   └── stm8_interrupt_vector.c  # STM8 interrupt vector table (required for build)
│
├── hardware/
│   ├── schematic.pdf            # Full schematic (EasyEDA export)
│   ├── pcb_design.pdf           # PCB layout (EasyEDA export)
│   └── gerber/                  # Gerber files for manufacturing
│       └── *.gbr
│
├── docs/
│   ├── block_diagram.png        # System block diagram (Input → Optocoupler → STM8 → Relay → Load)
│   ├── pcb_2d.png               # 2D PCB render
|   ├── pcb_3d.png               # 3D PCB render
│   └── bom.csv                  # Bill of Materials
│
└── README.md

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🖥️ PCB Design Notes

Designed in EasyEDA following this process:

1. Schematic Creation — All components and logical connections mapped out
2. Footprint Assignment — SMD packages selected for compact layout
3. Routing — Short traces, 45° angles, minimal vias for signal integrity
4. DRC & ERC Check — Design verified for electrical and layout errors
5. Gerber Export — Files finalized and ready for manufacturing

The board uses a double-layer PCB to keep the footprint small while separating the high-power relay side from the low-power control circuit.

The highlighted regions (marked in the image) show a deliberate physical gap between the relay coil driver circuitry (low-voltage control side) and the relay contact traces (high-voltage load side).

Rather than relying solely on the relay package itself for isolation, I extended this separation directly into the PCB layout — creating creepage and clearance distance between the two voltage domains. This prevents electrical arcing, protects the microcontroller and gate driver circuitry from transient high-voltage events, and ensures the board meets basic safety expectations for a mains-adjacent design.

What I find elegant about this choice: it costs nothing — no extra components, no added BOM cost — just intentional use of empty board space. The isolation is literally air and FR4. It's one of those decisions that's invisible when it works, but immediately obvious in a post-mortem when it's missing.

The module drives two relays (RLY1, RLY2) via optocoupler-isolated transistor stages (Q1, Q2), with the gap reinforcing the optical isolation already present in the signal path — giving the design two independent layers of protection between the user-facing control logic and the switched load.

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🖥️ Development Environment

This project was built using ST Visual Develop (STVD) with the Cosmic STM8 C Compiler.

Setup

1. Install STVD (ST Visual Develop)
2. Install Cosmic STM8 Compiler (free license available)
3. Clone this repo and open relay_final.stw in STVD — or create a new STVD workspace and add main.c and stm8_interrupt_vector.c to it
4. Download the STM8S003 dev board library from https://github.com/sonocotta/stm8s003-dev-board and add it to your project's include path in STVD

Flashing

Flashing is done via the SWIM (Single Wire Interface Module) port using an ST-LINK programmer:

1. Connect ST-LINK to the SWIM header on the PCB
2. In STVD, go to Debug → Start Debugging to flash and run, or use STVP (ST Visual Programmer) to flash the .hex directly

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🛠️ Built With

• STVD — ST Visual Develop IDE
• Cosmic STM8 Compiler — C compiler for STM8
• EasyEDA — Schematic and PCB design

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🛠️ Built With

• EasyEDA — Schematic and PCB design
• VS Code + SDCC — Firmware development
• STM8S003F3P6 — Microcontroller

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👤 Author

Arya Sureshbhai Patel