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.
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.
• 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
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:
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. Don’t worry, you can find some practical points about PIC microcontroller-based RF remote controllers here: http://jap.hu/electronic/codec-v4.0.html