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I've reverse engineered the PCB of a chinese 433MHz garage remote and drawn a schematic, I am trying to understand how it works, what is the purpose of each of the components, and which trace acts as the antenna.

This device can copy other remote's codes, and transmit them.

enter image description here

Which pins does the MCU have to pull-up/-down to be able to copy/receive, which pin does it transmit/receive on, etc?

And finally, which trace(s) acts as the antenna? There are 2 suspiciously long traces on the back of the PCB that I've drawn in my schematic on the top left corner.

MCU is an FT60E021.

Q1 and Q3 are 2SC3356-R25 VHF/UHF/CATV NPN Silicon Epitaxial Planar Transistors (marking R25).

Q4 is an S8050 general purpose NPN transistor (marking J3Y).

Q5 is an MMBTH10L VHF/UHF NPN silicon transistor (marking 3EM).

Q6 is an S8550 general purpose PNP transistor (marking 2TY).

X1 is an SR1 433.92MHz SAW resonator (marking B01).

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Which pins does the MCU have to pull-up/-down to be able to copy/receive, which pin does it transmit/receive on, etc?

From the posted schematic:

  1. The MCU pin 3 is used to transmit. A 'high' will turn on Q4 which will enable an oscillator with the frequency set from X1.
  2. The MCU pin 4 is used to receive. Assume:
    • An internal pull-up in used inside the MCU, and the collector of Q3 is pulled low to drive the receiver input to the MCU.
    • C4 and Q5 act an an envelope detector for the amplitude modulated receive code.

And finally, which trace(s) acts as the antenna?

  1. The red line on the schematic connected across C6 is the transmit antenna. Think this is the long trace on the right on the last picture.
  2. The blue line on the schematic between C4 and the base of Q5 is the receive antenna. Think this is the long trace on the left on the last picture.

Annotated antenna positions

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  • \$\begingroup\$ My schematic might contain mistakes or missing traces, although I triple checked everything. You are correct, the long trace you've labeled as "Receive" is the long blue trace between C4 and Q5, and the trace you've labeled as "Transmit" is the long red trace across C6. If I want to solder an external antenna to this circuit, would this need two separate antennas or would it work with a single antenna? \$\endgroup\$ Commented Apr 20 at 18:04
  • \$\begingroup\$ I'm not experienced enough with RF design to confirm if a single antenna would work, but suspect attempting to use a single antenna would have a negative impact on what are currently independent "tuned" circuits for transmit and receive. Suggest you ask a new question, with a link to this question for reference, to bring the new question to the attention of others. \$\endgroup\$ Commented Apr 20 at 18:14
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    \$\begingroup\$ RX appears untuned which may affect joining. Loop MAY be tuned about right. Is silver strop under battery conductive? What is its function. Nore cutouts around battery terminal. \$\endgroup\$ Commented Apr 21 at 10:27
  • \$\begingroup\$ @RussellMcMahon I had assumed the "silver strop" under the battery was some sort of insulating tape intended to help give protection against the battery from rubbing against and potentially cutting through the solder mask on the traces. Agree that would be good for the original poster to clarify what material is used. \$\endgroup\$ Commented Apr 21 at 10:42
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    \$\begingroup\$ It rather looks like peelable solder mask to me. \$\endgroup\$ Commented Apr 23 at 11:47

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