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ST3777 - Decoder IC for 4 SW RF Keypad

Remote Control applications, Decodes EV1527 chip based Encoder/Transmitters

ST3777 - Decoder IC for 4 SW RF Keypad

Sunrom Part# 3777

Product Discontinued.
We are no longer selling this product. Contact us if you want to know if alternate model is available.


ST3777 is a preprogrammed microchip PIC controller that is design to decode our 4 SW RF Keypad for developing RF remote control applications. This decoder IC is very flexible to use as it has different type of outputs like toggle, momentary and separate switch outputs. Due to its secure protocol type reliable RF remote control or security access control applications can be developed easily.


  • Secure RF remote control receiver/decoder application
  • Supports upto 20 Transmitters by storing their IDs in its internal memory with on board switch
  • Responds to only transmitters who’s ID are stored in its internal memory previously
  • Status LED shows different modes of operation
  • Variety of output types
    • 4x Toggle Output
    • 4x Momentary Outputs
    • 2x Output with separate ON/OFF switches
  • Can be used with any simple low cost ASK type 433 Mhz RF receiver for data reception


  • RF Remote control
  • Access control
  • Identity discrimination
  • IT home appliance control
  • Smart house products
  • Security Systems
  • Car Garage Door Opener
  • Front Gate Opener

Secure Design

We call this design as “Secure type” means that each transmiter has unique ID. Whenever the transmitter is operating it sends ID first then the data part. The data part can contain bits of information like which key is pressed.

To implement a secure remote application where only particular transmitter has to be acted upon by the receiver it is required by receiver/decoder to store the transmitter ID before it stars responding to particular transmitter.

The receiver part decoder has to store the ID of the transmitter in its internal memory and compare each ID with received packet before acting on the data from transmitter. Due to this approach very secure, reliable remote contol applications can be made. Since Decoder IC will respond to transmitters previously stores in its memory.

Let’s consider an example of what secure means here. In an application we want to implement a remote control for Main Gate opening. In this case each person has one remote transmitter with them. When a person who is allowed access press a switch, the decoder first looks in its internal memory if that ID is already pre-stored then only it acts on the data else ignores it. That means if there are 10 transmitters working, each IDs has to be pre-stored in the IC for it to allow access. If the transmitter ID is not already known to receiver then it is ignored. That way it is very secure type of access control implementation.

While designing this secure transmitter, many problems in existing remote control systems are considered and resolved in its protocol. Like multiple transmitters working in same premises without false trigger of other ones and unauthorised access.

Block Diagram

The remote control transmitter transmits 32 bit unique ID + 8 bit data containing value of which key is pressed on 433 Mhz frequency..

The same data is received by a common type ASK type 433 Mhz receiver module. The data is fed to ST3777 Decoder IC. It looks up the ID in already stored list, if found, then it outputs its data serially at 9600 baud rate and contols different type of output based on which key was pressed.

There is a switch connected to ST3777 which can put the IC in store mode where it will listen to transmitter and when a key is presessed on it, it stores the ID of transmitter for future valid access.

Store Transmitter IDs through switch

The IC needs that the transmitter IDs to be stored in its internal memory before it can decode or respond to particular transmitter. Each transmitter has 32 bit ID which always has to be known by IC before it responds to it in future. That way it is secure method of remote control allowing only known transmitters to be responded.

The switch can be used to enter new transmitters into IC’s memory or clear all IDs in memory.

Store Transmitter IDs in memory

  • Brief press of switch will put the IC in listening mode
  • Status LED blinks every second
  • Now press a transmitter key for 5 second for which you want to store its ID
  • When it has been stored the status LED becomes off and normal operation starts
  • Repeat the same for as many transmitters you want to enter in its memory. You can do upto 20 IDs after which the IC starts again from zero memory location like First In-Last Out method.

Clear all transmitter IDs in memory

Long press of 10 seconds clears all 20 Transmitter IDs in its internal memory. LED blinks very fast during this time and when it goes off it means the internall memory is cleared of all transmitter IDs.

Status LED

The LED has a status LED which has various meaning. The IC uses PWM method to control LED brightness.



Glowing slowly like breathing action

Idle listening condition

Blinks fast

Memory clear mode on SW press for 10 seconds

Blink every second

Listening to new Transmitter IDs when SW is pressed. When new transmitter is found and stored, it goes back to idle breathing action.

Remote Control Application Schematic

Here is a sample diagram for 4 channel RF remote control application. You can see the power section which outputs 12V and 5V. The 12V goes to Relays + ULN2803 which is a transistor array. The 5V of power goes to ST3777 + RF receiver.

The type of output can be selected by jumpers. It can be toggle, momentary or separate switch for on and off. If you do not want to provide selection, you can directly connect that ULN2803 input to pins of IC ST3777. LEDs D7-D10 indicate relay if it on or off. Using these relays you can switch on or off even mains operated appliances.

Serial output data format

The output of data from ST3777 is 11 bytes fixed length ASCII characters. The baud rate of output is 9600 bps. It output the transmitter ID and the Data values in HEX ASCII format.

The transmitter ID is 32 bytes and Data ID is 8 byte. In data byte it has information on key press. The string ends with 0x0D which is acting as line feed character or End of string idenfier.

When connected to PC through MAX232 or USB to UART adapter you can use a Terminal software to view the serial data.

If you wish to interface the IC with RS232 level like a PC serial port or any other device you need a level convertor such as MAX232. Do not connect the TX-OUT pin of IC to PC serial port directly, as it is RS232 level having +12V level.

Pin Details of ST3777

Pin Outs - DIP18 pin Package

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