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- Jun 21, 2012
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Someone posting as @PTaylor hijacked another thread on this subject, after surmising they should have opened a new thread. So, I am doing just that. Below is the original post followed by my comments:
Most of the key-fob manufacturers get "type approval" from the FCC (Federal Communications Commission) for 434 MHz key fobs, so all you need to do is decode the protocol they use. Receivers do not require licensing in the USA, but integrating this chip into a transceiver to allow both receiving and transmitting probably does. Consult your legal professional for legal advice.
The oldest protocol is Holtek encoding (now obsolete) that uses pulse-position modulation (PPM) to represent binary "ones" and "zeros" as well as "open circuit" or "tri-state" in the data stream sent by the key-fob. This was apparently a concession to early garage door remote controls that allowed a three-position address switch that selected 1, 0, or none to encode a "unique" ten-bit address to pair between the key-fob and the receiver. The Holtek protocol assigns a "one" or "zero" according to where the transmitter "on" signal appears within a certain time window. The "tri-state" or "open" code extends the on time for the bit,
The newer key-fob versions use OOK and transmit data in "character blocks" consisting of a start bit, data bits, and one or more stop bits at a user-defined baud rate. This could probably be decoded with a standard UART (Universal Asynchronous Receiver Transmitter) chip or function on a microcontroller.
So, the PT4303 receives the RF transmission and converts it to a 10.7 MHz Intermediate Frequency (IF) just like any superhet receiver. The user must insert a band-pass filter between the IF output (MIXOUT, pin5) and the demodulator input (IFIN, pin 7). The demodulated RF on/off data is then available at Data Out (DO, pin 11) provided the Chip Enable (CE, pin 10) line is pulled high.
So, Mr. Phelps, your mission if you choose to accept it, is to make sense of all those ones and zeroes streaming out of pin 11 using only a single binary data input pin on a Microchip PIC processor of your choosing We have assembled a team for you on Electronics Point to assist you. Good luck, Mr. Phelps. As always, this message will self-destruct if the EP servers go down. You should save a backup copy on your personal computer.
Hop
This popular Asian chip is used to receive amplitude shift keyed (ASK) or On/Off Keyed (OOK) signals provided by various key-fob transmitters. The transmission protocol is up to the key-fob manufacturer, and can very from very simple to quite complex for operation in the 315 MHz (Europe) and 434 MHz (USA) ISM (Industrial Scientific Medical) bands that allow unlicensed transmitters. For example, here in the USA there are specific requirements on output power and how long transmissions can be.
Most of the key-fob manufacturers get "type approval" from the FCC (Federal Communications Commission) for 434 MHz key fobs, so all you need to do is decode the protocol they use. Receivers do not require licensing in the USA, but integrating this chip into a transceiver to allow both receiving and transmitting probably does. Consult your legal professional for legal advice.
The oldest protocol is Holtek encoding (now obsolete) that uses pulse-position modulation (PPM) to represent binary "ones" and "zeros" as well as "open circuit" or "tri-state" in the data stream sent by the key-fob. This was apparently a concession to early garage door remote controls that allowed a three-position address switch that selected 1, 0, or none to encode a "unique" ten-bit address to pair between the key-fob and the receiver. The Holtek protocol assigns a "one" or "zero" according to where the transmitter "on" signal appears within a certain time window. The "tri-state" or "open" code extends the on time for the bit,
The newer key-fob versions use OOK and transmit data in "character blocks" consisting of a start bit, data bits, and one or more stop bits at a user-defined baud rate. This could probably be decoded with a standard UART (Universal Asynchronous Receiver Transmitter) chip or function on a microcontroller.
So, the PT4303 receives the RF transmission and converts it to a 10.7 MHz Intermediate Frequency (IF) just like any superhet receiver. The user must insert a band-pass filter between the IF output (MIXOUT, pin5) and the demodulator input (IFIN, pin 7). The demodulated RF on/off data is then available at Data Out (DO, pin 11) provided the Chip Enable (CE, pin 10) line is pulled high.
So, Mr. Phelps, your mission if you choose to accept it, is to make sense of all those ones and zeroes streaming out of pin 11 using only a single binary data input pin on a Microchip PIC processor of your choosing We have assembled a team for you on Electronics Point to assist you. Good luck, Mr. Phelps. As always, this message will self-destruct if the EP servers go down. You should save a backup copy on your personal computer.
Hop
