RF Transmitter 433 MHz and modify the frequency of 433MHz transmitter module (number of capacitor, Resistor, Inductor and antenna length) ?

Thread Starter

Badrq1

Joined Aug 30, 2021
3
Hello, I'm new to design the RF transmitter 433 MHz model. I do a bet search on how to design it then add it to my PCS that I will print it. I'm confuse on the design while some sites gives different number of resistors and capacitors, So plz I need your help to complete this step. I've design the RF 433 MHz but I'm not sure if it will works well. Also I need to check if Coil L1does it will decouples the power supply or changing frequency ?!.

This is my circuit diagram for the RF transmitter 433 MHz .

RF.PNG
 

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Yaakov

Joined Jan 27, 2019
3,984
The layout of RF circuits on PCBs is critical. Just electrically connecting the components is no guarantee of a working, stable transmitter with the characteristics required for legal operation. Even if you managed to make a reasonably good transmitter, it is probable that without certification it would be illegal to operate in your locality.

This is why people use modules which are designed by experts and certified when they need a transmitter. Producing interference in the 433MHz band could cause a llot of trouble for others who rely on it.

Consider buying a prebuilt module. They can be very cheap, and they are already working properly.

[EDIT: Typo correction]
 
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DickCappels

Joined Aug 21, 2008
8,159
If you really want to try this, use a SAW (surface acoustic wave) resonator to set the frequency. Trusting an RC circuit to stay tuned is not a good idea. My 330 MHz regenerative remote control receiver that opens and closes the gate across my driveway drifts madly on warm days and looses a great deal of its sensitivity. In the winter time, no problem, but on sunny afternoons I have to walk over to the receiver to get it to respond. Notice that quality remote control transmitters are SAW or crystal controlled. You are unlikely to cause problems by having your receiver tuned to the wrong frequency but a transmitter on the wrong frequency is likely to be a crime most places like New York.
 

Thread Starter

Badrq1

Joined Aug 30, 2021
3
For reference, here is an example. In small quantities, these are less than $3USD.

Thanks @Yaakov and @DickCappels for your time. you are right that we must work with be careful because the frequency is so sensitivity, but what actually I want to do is to draw the RF 433 MHz with Attiny85 to sense the main door statues of my apartment what ever also for windows. So it's better to draw it with the same PCB ( professional PCB) instead of plug it to my PCB . So I need to know if we can play with that by draw the PCB and grab the crystal frequency for the model then plug it (R433) to my PCB. does that will work fine ?!
1637745103327.png 1637745146667.png

@Yaakov As you mention to (QFN20) STS1TXQTR it looks pretty nice model I read the Datasheet, we can play with deferent frequency by the value of the capacitors and the XTAL. but the only problem that I faced that I couldn't find it on the PCB printer company store. plz if you found it on Amazon or Alibaba because I couldn't find it.
 

Yaakov

Joined Jan 27, 2019
3,984
It is not better to reproduce it on your PCB for the reasons I mentioned about the difficulty with laying out RF circuits. Small differences will lead to different behaviors. The authorization (under law and regulations) to use a transmitter is not assigned to the design but to the manufactured product. Copying a product doesn’t make it legal or necessarily functional.

As far as that module, it is available on Digikey. I don’t know about other vendors.
 

Thread Starter

Badrq1

Joined Aug 30, 2021
3
The thing you circled in blue is a SAW device, and yes, you can use that to set the frequency of a transmitter.

This circuit might be a useful reference: https://daycounter.com/Circuits/Saw-Transmitter/Saw-Transmitter-Circuit.phtml

Also: Please see the attached pdf document.
Thanks for your time @DickCappels @Yaakov .
@DickCappels The calculator that you shared it on the site is useful but as I tried to balance the capacitors and the Inductor it's difficult to get a real values for both (capacitor and Inductor). Also does it fine to have only one NPN as you it shown on the PDF that you shared.
Also I'm on Asia but 433 MHz is working fine on my area which conflict to the information on the PDF that it can only be applied on AU, EU, India.

I need to check if this circuit can be fine to add this circuit to my PCB design because it is similar to the XKFST 433 MHz module which don't have any Inductor attach on the circuit. I'm not sure what is the real value of the capacitor on the module but I assume the values on the circuit depending on many RF circuits that I've found them online.

"This is my circuit. ............................................................................................................. "This is the XKFST 433 MHz module
1637867478743.png , 1637867822607.png

To let it clearly picture resolution for the module" There is two NPN (R25 and J3Y) as on my circuit and two resistors one of them is 203 means 20K ohm resistor and the other one is 273 means 27K ohm resistor. There is three capacitor but I couldn't test there values by Digital Multimeter. for the SAW device I can grab it from the module then solder it to my PCB.

1637868031955.png
 

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