RF Remote Switch Teardown

Recently, I picked up a cheap, two-channel RF remote control switch from eBay, and once I completed a demo project with it, I decided to do what I do with all of my electronics: take it apart!

RF Remote Switch Overview

The product is designed for AC voltage operation with two output channels, A and B. We can switch channel “A” with the “A” button in the remote control handset and channel “B” with the “B” button. The “on/off” button of the remote throws both channels simultaneously, and “sleep” will turn off the active channel(s) after a short, seconds-long delay.

Specifications

• Working voltage: AC 180–240 V, 50/60 Hz
• Load/Channel: 10 A @ 240 V (resistive/capacitive/inductive)
• Remote control distance: 30 m
• Remote control battery: 23-A/12-V type

What’s Inside

Pulling the master unit apart, I find nothing more than an ordinary printed circuit board: an extremely simple design with a small microcontroller chip, voltage regulator chip, some discrete components, electromagnetic relays, and a radio-frequency module. Unfortunately, it seems that there’s no marking on the microcontroller chip.

The front end of the power supply is the common “Chinese” capacitor-based power supply to convert high-voltage AC input to low-voltage DC output. The unregulated DC output (~16 V) is used to power the electromagnetic relays. A fixed-voltage (5-V) regulator chip is used to power the microcontroller and the radio module. Here is a system diagram of the master unit, drawn by me.

Components

• 4.7-mH Inductor x1
•1N4007

•Diode x4

• 105-/400-V Capacitor x1
• 100-n Ceramic Capacitor x2
• 470-u/25-V Electrolytic Capacitor x1
• 220-u/16-V Electrolytic Capacitor x1
• 330K ¼-W Resistor x1
• 10K ¼-W Resistor x2
• 68R ½-W Resistor x1
• 12-V ½-W Zener Diode x1
• 78L05 Voltage Regulator x1
• S8050 Transistor x2
• 12-V/400-R Relay x2
• Unnamed 8-pin Microcontroller x1 (wait and see)
• 433-MHz RF Receiver Module x1

Micro-secret

In the microcontroller section, only five out of the eight pins in the DIP-8 package is used to complete the circuitry. Pin 1 of the microcontroller is the VCC (+5 V), and pin 8 is GND (0 V). Pin 4 is for the RF signal input from the radio module. The remaining pins 6 and 7 are connected to corresponding base leads of two S8050 relay driver transistors through 10K resistors. In all probability, the microcontroller is a type PIC12C508 or its new Chinese variant. The PIC12C5xx series chips are available in the cost-effective one-time-programmable (OTP) versions, which are suitable for production in any volume.

Furthermore, this series fits perfectly in low-power remote transmitters/receiver applications. However, we can reverse-engineer the same remote control design with another PIC12F629/PIC12F675 microcontroller.Next is the radio (RF receiver) module. The 433-MHz RF receiver used here looks very similar to the common “RX480” Chinese module (the part number of its RF transmitter module is WL102). See the pinout of RX480 Right side.

Pin 2 of the module is connected to the +5-V supply line of the circuit, and pin 5 is routed to the 0-V (common ground) rail. Data output pins 3 and 4 are tied together and connected directly to pin 4 of the microcontroller. A short wire is attached to pin 1 of the module as the receiver antenna. That’s all.

Intense fear of possible cosmetic damage stopped me from the teardown of the remote control handset because I planned to gift it to a close family member.