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I'm planning to make a project using the MX-FS-03V 433 MHz transmitter and the corresponding MX-05 receiver with a couple of Arduinos for short range radio communication. Range will be at most 10 meters.

I got a couple of these from Amazon, but one thing has me puzzled. This is a picture of the transmitter:

MX-FS-03V without coil

note that in the lower right corner are three unoccupied through holes. The extreme lower right one is where I will solder the antenna, but the other two have me puzzled. Looking at other pictures of this transmitter, there seem to be several revisions if the silk-screen on the PCB is anything to go by, and some have a second coil soldered between the two holes not used by the antenna, e.g. this one:

MX-FS-03V with coil

So what's going on here? I'm not that much of an expert, but I know enough to believe that removing a coil from an RF circuit is likely to have some pretty major effects. For one project, it is vital that I work at 433 MHz, so I'd like to have some idea of what I'm getting myself into.

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  • \$\begingroup\$ These cheap Alibaba modules are unlikely to work on the 433MHz band. First there is the issue of passing the ETSI standard tests, then there's the issue of 433MHz being full of disturbances. In order to handle both of these problems, you need a narrowband FSK radio. Some OOK made by Alibaba will be unreliable and likely illegal to use in EU. \$\endgroup\$
    – Lundin
    Apr 5, 2022 at 6:24

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They can't work without a vital component in place. That inductor provides power to the high-frequency oscillator.

Luckily somebody else has gone to the effort of analyzing the issue.

Review this website.

FS1000A 433 MHZ TRANSMITTER – L2 MISSING

Here is the schematic from that site.

enter image description here

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  • \$\begingroup\$ The linked page shows how to solve the problem, but using enamel insulated copper wire to wind the coil. I don't have any of that, but I do have some plastic insulated wire that should work. Assuming I make it the same size: 4mm diameter by 3 turns, and just strip the insulation off the ends to solder it, should that work OK? \$\endgroup\$
    – dgnuff
    Apr 5, 2022 at 2:45
  • \$\begingroup\$ The inductor is non-critical so should be ok. \$\endgroup\$
    – Kevin White
    Apr 5, 2022 at 4:00
  • \$\begingroup\$ @dgnuf - You may be interested in the software I use for making inductors. Not affiliated, I just use and enjoy it. It's called Coil64 and you can find it here: coil32.net/download-coil64-for-windows.html \$\endgroup\$
    – enhzflep
    Apr 5, 2022 at 4:04

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