Readers are advised to proceed with the construction of the presented circuits only after understanding the concepts from the core. Not adhering to this can lead to failures and frustrations.

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Saturday, December 17, 2011

How to Make a FM Remote Control Using a FM Radio and a Homemade FM Transmitter Circuit


Remote control circuits are not so easy to build as they incorporate critical inductor stages and also, the components are difficult to procure. However a simple homemade FM remote control can be made by modding your existing FM radio as the receiver part. The transmitter can be simply made by assembling a few electronic components. The two sections together can be used for controlling any electrical load remotely from any part of the house.


Making the FM Transmitter for the Remote Control Unit:



The figure shows a very simple FM configuration using one transistor and few other passive components. Here the inductor becomes the most crucial part and must be made carefully as per the given instructions.

T1 along with the pF capacitors and the inductor forms the RF stage and is responsible for the generation and transmission of the RF carrier waves.

The section consisting of the IC UM66 and the electrolytic capacitor forms the modulating stage and injects the required modulation signals to the RF stage.

This helps to make the transmitted waves much stronger and travel up to longer distances.

Once the assembly of the transmitter circuit is done, its working should be confirmed by switching ON the transmitter and by verifying the received signals over the FM  radio. 

The reception should consists the music from the UM66 IC and should be received by the radio, loud and clear even from a distance of over 30 meters.

After finishing the construction of the transmitter, you need to assemble the Flip Flop circuit by soldering the electronic components as per the shown diagram. This stage will be later on required to be integrated with the modified FM radio.


How to Modify a FM Radio as a Remote Control Receiver for Controlling Electrical Gadgets

For this project you will need an ordinary FM radio for making the receiver/controller unit. 

After procuring a FM radio, you will need to do the following modifications in it.

Open the back cover of the FM radio to uncover the circuit of the unit. Now carefully, integrate the flip flop circuit to the speaker terminals of the radio. The connections won’t be difficult as everything’s shown in the diagram very clearly.

The idea here is to use the reception audio from the radio speaker terminals and use it to activate our flip flop circuit and the relay.

Switch ON the FM radio and tune to some vacant area where there’s no station available, and only the background “hissing” noise is audible.

Adjust the volume control of the radio toward the maximum and you will find the LED light up, refine the adjustment until the LED just switches OFF.

Now tune the radio to some station, without disturbing the volume control. Now you will find the LED flickering in response to the audio outputs.

You will also see the flip flop responding appropriately and the relay switching randomly to the LED illuminations.

 Your setting of the radio or the modifications of the radio is complete.

Now switch ON the transmitter and once again tune the radio to the spot where it receives the transmitter music loud and clear.

That’s it, the setting of your home made remote control is complete.

Now as many time you click the transmitter switch, it will be received by the radio and the flip flop relay activated alternately.

The relay contacts may be wired to any appliance and may be controlled easily by your transmitter through mere clicks of its switch.

However the speaker of the radio will also make lot of noise and therefore to eliminate this you may just tear of the cone of the speaker so that it keeps silent, activating only the flip flop.

Have any doubts? Comment…….

25 comments:

  1. Hello,

    I can now see there is are modifications in your transmitter circuit as follows:

    a. R1 is removed which was there with the COB
    b. C5 is now 1 Mfd 10 V instead of 100 Nf Ceramic. However,you did not show the polarity of C5 here
    c. Most importantly, the L1 is now 21SWG wire with 5 turns on 6mm dia air core instead of 22SWG wire which you had suggested earlier in the BrightHub

    Since I have managed to get two strips of 22 SWG and 24 SWG wire (as per your earlier circuit idea), can you please suggest some modifications using either of those (and C5 if necessary)?

    It will be great if you also mention the distance between the adjacent coils which has to be set or not.....to match the frequency of the RF signal between 90-100MHz.

    Finally, for the benefit & convenience of the readers can you please suggest something standard & easily available (ex: may be the refill of a ball point pen etc) to be used to make the L1 with (1) 5mm dia and (2) 6mm dia?

    Waiting anxiously for your response,

    Regards,
    Raj

    ReplyDelete
  2. Hi Raj,
    In electronics there's never a hard and fast rule, as I told you earlier, I made the above transmitter circuit using different sets of components each time, slightly though. The resistor with the COB can be included for better safety, however since the voltage is 3 and there's no chance of it to increase I decided to eliminate R1.
    I used the coil wire from my junk box and it was withing 20 to 23 SWG, I guess, however the coil thickness is not highly critical, any nearby gauge can be used and can be simply tuned just by pulling or squeezing the turns a bit when the testing is being done.
    In the diagram I have specified the approximate thickness of the coil wire as 0.8mm.
    Alternatively you may replace C1 with a 33pF trimmer, that can make the adjustments easier for you while tuning.
    C5 is also not critical, any value from 1 to 100uF can be used, even a 0.1uF will produce results, but the tank circuit tuning is more important and once its done everything will follow as expected.

    With some practice you should be able to do it.

    Regards.

    ReplyDelete
  3. Hi Swagatam,

    Thanks for your speedy response. However, I am sorry to bother you again since you missed one of my earlier questions:

    "Finally, for the benefit & convenience of the readers can you please suggest something standard & easily available (ex: may be the refill of a ball point pen etc) to be used to make the L1 with (1) 5mm dia and (2) 6mm dia?"

    Now, if I use a 33pF trimmer, will it not shift the frequency of the tank circuit when I hold the transmitter due to the introduction of capacitance of our body?

    Do you test thse circuits at your end straight on a vero board or on a bread board? Because I am using a breadboard and mounted all components as close to each other as possible to prevent any signal loss.

    Regards.

    ReplyDelete
  4. Hi Raj,

    You can refer a 22 SWG solder wire thickness, I think it's the best example.
    I always use veroboard for making my projects and for making this transmitter I had used a 1.5 by 1.5 inches, piece of general purpose PCB.

    I think using a trimmer would make things more complicated for you, the best way to tune the circuit is by manually stretching the coil turns a bit while the FM radio is switched ON at some blank station.
    The stretching of the coil can be done by gently inserting your finger nail in between the turns and pulling them slightly apart, this procedure should induce some instantaneous activity in the radio.
    Through some trial and error you will be able to finally tune them.

    Regards.

    ReplyDelete
  5. Hi,

    Can you please suggest a simple circuit (LED based) with a handful of easily available components which when brought near the L1 of the FM transmitter will start blinking to show that there is an oscillation of around 100 MHz? You may refer to: http://www.youtube.com/watch?v=pQepqtqJBJg and watch the video especially at 1 min 30 sec duration where the author is using a "RF Signal Pen Detector" to show that the Tank circuit is really producing oscillations.

    This is important here because it would help us to understand whether or not there is an osciallation in the tank circuit of the suggested FM transmitter.

    Regards,
    Raj

    ReplyDelete
  6. Hi Raj,

    Just type "Non contact AC phase detector" in the top right hand side search box, you will get the required circuit link.
    The circuit will detect the presence of all types of RF signals when its antenna is brought near the source. However the LED will not blink but glow continuously.
    Regards.

    ReplyDelete
  7. Amazing.... You always know what i need. I'm looking for this tutorial entire the world. No satisfy, until i find this!!! Thanks a lot for your sharing. You are the best! Realy...

    ReplyDelete
  8. Thanks Wilein,

    Your happiness makes me feel happy...:-)

    ReplyDelete
  9. hello sir swagatam ,,, can u tell me d formulla to how to become a perfect electronics engineer ,,, and i m performing eng in ece

    ReplyDelete
    Replies
    1. Hello Lucky,

      There'e no such formula, just concentrate and work hard in the field....you will succeed.

      Delete
  10. Thank you... It's very useful to learners....

    ReplyDelete
  11. you are great and genius dear, your projects are very good and useful for students and others also, keep it up

    ReplyDelete
    Replies
    1. Sir you are really a genius.. Can u please suggest me some more big projects as Iam a third year student, I have to make a major project ,not mini.

      Delete
    2. Thanks very much.

      Probably you can suggest me a suitable range, I would then refer you to those diagrams.

      Delete
  12. Hi swagatan, where i put the 12v negative supply from the radio/baterry? in the receiver circuit?

    ReplyDelete
    Replies
    1. On the rail which is connected to pin8 and pin13 of IC 4017

      Delete
  13. Hi swagatan 1 more time, my receiver is not working well,when i connect the input in the speakers i ear a buzzing's and the 100Ω resistor heats to much, could
    help me figure out the problem?

    ReplyDelete
    Replies
    1. Hi Faife,

      Did you adjust the volume control as per the given instructions in the above article? please read the article carefully.

      Adjust the volume control such that the LED stays shut off and lights up only when the transmitter signal is received.

      Delete
  14. hi mr. swagatan , I had put a diode
    rectifier in place of the LED, but I
    put the LED, what happens is that
    even lowering the volume of the radio
    LED stay on(doesnt turn off) and when i test
    with a channel fm the relay
    dont turn on

    ReplyDelete
    Replies
    1. If your speaker loudness is decreasing then it should also make the LED brightness lower proportionately, that should obviously happen.

      Anyway connect a 100uF capacitor in series with 100 ohm resistor and then check by doing the procedures.

      positive of the cap will go to 100 ohm and negative to speaker terminal.

      By the way did you connect the circuit ground with the radio ground??

      Delete
    2. .....one of the speaker terminals should be at ground level with the circuit and the radio.

      Delete
  15. Pls give details of L1 coil in the first circuit

    ReplyDelete
  16. it's shown in the diagram, click the diagram to enlarge.

    ReplyDelete

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