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I am trying to create a simple Arduino parking sensor that sends the warning beeps to the car radio using FM transmitter circuit.

I searched different FM circuits and found many of them that use a microphone or an AUX input, but I couldn't find something that simply takes an input from the Arduino to transmit a tone/beep.

Here is an example tutorial: https://www.youtube.com/watch?v=joFourugXvs&ab_channel=TechBuilder

and here are the schematics:

enter image description here

I need to remove this microphone and use an output pin from the Arduino as an Input to the transmitter.

Is that possible?

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  • \$\begingroup\$ What if the car radio is tuned into a different channel that the above circuit is not tuned to? It isn't going to spray a general FM signal across the full 88 to 108 MHz - it'll only transmit on one small particular area. I see this as a significant problem to your expectations. What if the car radio isn't on - you might reverse into a wall believing you have plenty of room. \$\endgroup\$
    – Andy aka
    Commented Jan 18, 2022 at 15:44
  • \$\begingroup\$ How exactly are you going to get the Arduino to make a "beep"? \$\endgroup\$
    – Elliot Alderson
    Commented Jan 18, 2022 at 15:48
  • \$\begingroup\$ @Andyaka I know that it will transmit on one frequency and I will tune it on the car radio. Also, this is meant to be a front sensor not a back sensor. Also, all this is for a proof of concept, not a real product. \$\endgroup\$
    – Mohamed Anwer
    Commented Jan 18, 2022 at 15:52
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    \$\begingroup\$ Is that possible? Yes it is. Do realize that the Arduino doesn't output a very small signal like a microphone does. So you must attenuate the signal from the Arduino. \$\endgroup\$
    – Bimpelrekkie
    Commented Jan 18, 2022 at 15:53
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    \$\begingroup\$ Also, all this is for a proof of concept, not a real product. - I think the concept has already been proven by all the main automotive companies. Not sure what you are trying to achieve here. \$\endgroup\$
    – Andy aka
    Commented Jan 18, 2022 at 15:56

2 Answers 2

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Instead of using a FM transmitter with audio frequency input it is easier it is reasonable to do "Continuous Wave" (CW) or Pulsed Amplitude Modulation, aka Morse Code. Which will be received by radio FM receiver as clicks or beeps.

Transmitter must still generate the baseband frequency for transmit, (Q2, C4, L1, ANT) but instead of modulating with audio, the carrier is always present with constant tone and the arduino controls a relay or transistor which enables/disables ("Keys") the transmission of the carrier or a pulse modulation of a constant frequency tone.

Using FM voice transmitter to encode a pulse from a dac generated audio tone is "Rube Goldberg-ian" in nature.

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  • \$\begingroup\$ One advantage is that if the relay click is very fast, you could have a very wide range of receiver frequency where the transmission is detectable. The disadvantage is that this sort of intentional cross-band interference may be illegal in your country or require a license. \$\endgroup\$
    – crasic
    Commented Jan 18, 2022 at 16:02
  • \$\begingroup\$ Thanks for the answer. unfortunately I am coming from a Software Eng background and don't know much about electronics. Can you suggest a modification to the existing circuit? \$\endgroup\$
    – Mohamed Anwer
    Commented Jan 18, 2022 at 16:40
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    \$\begingroup\$ Simply remove the first transistor which is a preamp and feed the beeps into C2. The circuit is so simple that its radio frequency changes when the battery voltage changes and if something moves towards or away from the antenna. \$\endgroup\$
    – Audioguru
    Commented Jan 19, 2022 at 19:33
  • \$\begingroup\$ @Audioguru So I should remove R1, R2, R3, R5, C1, Q1, and the mic? \$\endgroup\$
    – Mohamed Anwer
    Commented Jan 21, 2022 at 19:08
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    \$\begingroup\$ Yes, remove those parts and also remove R4. Also, the battery needs a 1nF ceramic capacitor parallel to it. \$\endgroup\$
    – Audioguru
    Commented Jan 22, 2022 at 2:29
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You could modify this classic Forrest M Mims III design to trigger off of a PFET instead of a pushbutton switch.

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  • \$\begingroup\$ This looks interesting. will try that \$\endgroup\$
    – Mohamed Anwer
    Commented Jan 18, 2022 at 18:52

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