what are the problems i can face in this project?

my goal is transmit square wave with certain frequency from fm transmitter and receive this square wave by fm receiver on the other side in order to be measured by pic microcontroller and the range between TX and RX is 100m or 120m max 

i want it to be as simple as i can so i made a research and my plan is :

1) square wave generator using 555 IC so i can control the frequency and amplitude the circuit is in the following link: http://www.learningaboutelectronics.com/Articles/Adjustable-square-wave-generator-circuit-with-a-555-timer.php

2) the output of square wave generator i will use it as input for Fm transmitter the circuit is in the following link :https://www.instructables.com/id/The-Ultimate-FM-Transmitter/

3) i will use small radio i have to receive the square wave on radio speaker the sound should be like the following link : http://onlinetonegenerator.com/ to be sure that my square wave has reached the speaker 

4) then i will hack the radio :) and remove the speaker and use the radio output as input for pic microcontroller to measure the square wave frequency  

so eventually the stages will be like this:

sinwave genetrator > FM transmitter > FM receiver > microcontroller used to measure frequency and display it on LCD screen 

i made fm transmitter before and received sin wave tone by fm radio so i expect the problem will be in using FM receiver as input to microcontroller to measure frequency 

what do you think about this plan ?? 

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rickharris27 days ago

Your 555 will be temperature sensitive and will drift over time.

To get a true square wave at the other end the band width will have to be huge (technically infinite) so there will be some distortion.

The output from the radio speaker connections may be rather large to feed to a microprocessor input without some signal condition.I would suggest insert an A to D converter as a minimum.

Exactly why are you doing this it seems to me there may well be a simpler way to achieve these results with more reliability and possibly cheaper.

the best guess I have is your trying to make an Radio control system

ShadyM1 (author)  rickharris27 days ago

thank you sir for your answer

that is why i asked this question in the first place

my project contain some circuits like dark detection circuit and visitor sensor using IR TX,RX

when dark detection circuit for example detect darkness i want my microcontroller to know that my dark detection circuit is activated via wireless signal

also the same for visitor sensor

i want to achieve this without need to buy RF modules for each circuit

do you have any ideas how can i do this ?

thank you

I would use 2 microprocessors.

One to monitor the sensors and send the data via a 433Mhz wireless link to another micro to display the alarm and give user input etc.


is a link to the type of simple 433 tx rx pair I mean. They will give you line of sight communication. Lots of electronic sites will stock them.

You will need 2 sets if you want to send data both ways.

Click on diagram for full size.

alarm link.jpg
ShadyM1 (author)  rickharris26 days ago

i think your idea is the best and most reliable way to reach my goal

i have two pic microcontrollers and i will use this RF module

its available in my country and also have much better range because i dont have line of sight


i will get one module because its one way data

thank you very much sir for helping me i appreceate it

Your welcome - Good luck.

Not to mention the drift in the FM centre frequency. It would actually be better, for frequency fidelity, perhaps to use AM, and crystal derived AM modulator.

Better still, measure the frequency at the transmitter end, transmit the measured result over the link.

ShadyM1 (author)  steveastrouk27 days ago

thank you :)