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I have been recently interrested in rf transmission. I would like to create a small circuit to generate à 125kHz signal at about 10-15 meters. I am very newbie for these kind of things. When I look on the internet for some schematics they all seem very complicated. I would need to have a minimalistic circuitry to to just OOK (On Off Keying) in this frequency. What would be the minimalistic circuit (you could just point to some schematics) that I could use in practice, that would be operationnal?

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  • \$\begingroup\$ why does it need to use 125kHz? \$\endgroup\$ Jun 22, 2017 at 21:53
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    \$\begingroup\$ 125 kHz has a wavelength that is much, much, much larger than 15 m. So, if you get anything at the other end,that would be inductive coupling, not a radio transmission. So, I think you might need to take a step back and think about your requirements, and everything points to this being an XY problem. \$\endgroup\$ Jun 22, 2017 at 22:29
  • \$\begingroup\$ I am not sure this is a XY problem. I have no interest of developping some electric scheme (because I couldn't do it). My only challenge is to build myself the electronics from the simple scheme, see it work, make some tests,.... Even if this doesn't sound much of a goal, one has to start with wath is at your reach. \$\endgroup\$
    – roi_saumon
    Jun 22, 2017 at 22:46
  • \$\begingroup\$ Gate a 125 kHz clock with a data signal... Filter higher harmonics \$\endgroup\$
    – sstobbe
    Jun 22, 2017 at 22:53
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    \$\begingroup\$ AFAICT this is not a frequency band you should be transmitting on without a license. \$\endgroup\$
    – The Photon
    Jun 22, 2017 at 22:58

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I'm going to answer the "minimalistic rf circuit" bit. As the comments note, 125kHz is a tall order. I'll assume that 433MHz, being unlicensed, practical and popular, will suffice.

A big caveat up front: RF is not simple! I'm assuming this is a learning exercise for you. In that case, carry on! If instead you just want to use rf, then just buy one of the many, many dirt cheap RF transmitter/receiver modules.

With that out of the way, here is the simplest RF transmitter circuit I know of:

schematic

simulate this circuit – Schematic created using CircuitLab

V2 is your OOK signal, V1 is your power supply (approx 5V), L1 is your antenna and the "SAW" component is a 1-Port SAW Resonator. The critical values are C2, C3 and L1 - the values I've provided are just starting points. They are highly unlikely to work for you! They depend a great deal on your layout, PCB materials and component variation. It will take some experimentation to get it right. Just make sure the series combination of C2, C3 and L1 resonate at the frequency you want (433Mhz is a good target).

L1 will be the most difficult to control - try a coil of wire whose dimensions you can adjust as you go. The rest of the components should be SMD and mounted close together.

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  • \$\begingroup\$ Is this the same thing as doing a crystal colpitts oscillator? \$\endgroup\$
    – roi_saumon
    Jul 17, 2017 at 0:44

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