Morse code communication using FM frequency

[StrawberryBro]

Hi, I'm new here, I have a little problem, I'll be grateful for the help. It happens that I want to modify the circuit of an FM audio transmitter, so that instead of transmitting audio can transmit a signal to turn off or turn on a switch, and the switch on and off an LED, the idea is to communicate using Morse code using the FM frequency. What could be modified to do this?. What could be change to a conventional radio receiver, so that instead of playing audio, will have a led (for morse code communication)?.

This is the transmitter diagram.

**broken link removed**

Thank you.

 

[ashugtiwari]

When the Q2 is on or off, the output frequency will be at extremes. As far as i understand, the Q2 stray capacitance is varying according to voltage input to change frequency of output. It also acts as an amplifier. Morse code is like binary input to circuit instead of mic. Theoretically, this circuit should be fine to do that, but the change of frequency from lowest to highest will take time and will depend on tank circuit and Q2 switching speed.
This will decide ur dot size for morse code. Instead u can always go for FSK using simple 8-bit microcontroller like PIC, input it binary morse code and u r done to get two frequency.

Hope that helps.

 

[StrawberryBro]

Hello friend :grin: , of course it helps, thanks, I hope to thank you in the future, the only thing is that I'm advanced in relation to software but low in electronics and I'm learning, thank you very much, and if you can suggest more ideas on how to modify the circuit, in the simplest way, i will be more grateful.

 

[ashugtiwari]

as far as i know, here are some notes:
1. in above FM circuit, the C5 and L1 making tank ckt oscillating to generate carrier frequency, now to change the o/p freq, either C5 or L1 or any other external reactive component in vicinity of tank ckt needs to be changed to change the resonant frequency.
2. The change should be fast enough to settle for any time delays between input voltage change to o/p freq change.
3. transistors are not ideal, so they have stray capacitance and inductance (very negligible), between CE, CB and BE. These capacitance changes based on four major parameters viz. temperature, i/p voltage, current and frequency.
4. In above circuit, since the change in capacitance is very small, the Q2 also amplifies the signal as there is some positive feedback in circuit, but the amount of power generated is very low.
5. The above circuit is good for u, but while frequency changeover happens, the transient response of circuit may create overshoot or undershoot, depending on o/p reactance and resistance, making time constant. Also, the transition will bring some intersymbol interference since, morse code is binary code, so u need to keep at least 2 cycles for lowest frequency.
6. Make sure, the dot time size of morse code is more than twice the lower frequency of circuit and u r good to go with same circuit. Also, ther are losses in reactive components.
7. find resonance with (2*pi*C5*L1)^-1. Apply 0V at input to find lowest frequency.
8. Since, u r only interested in extreme frequencies i.e. highest and lowest, FSK is good for u and morse code can be easily transmited using FSK.
9. Use any 8-bit MCU like PIC18F series and store one complete cycle of 2 frequencies viz, lets 300kHz is carrier, so ur logic 0 will be (300-10)kHz and logic 1 is (300+10)kHz.
10. When i/p is logic 0, u must send at east two cycles of low frequency to account for intersymbol interference; u can accurately control the phase from where u can start the cycle, the DAC or PWM will convert it into analog sinusoidal wave.
11. At detector simply convert input sine wave to square wave using comparator and send the square wave input to capture in CCP module of microcontroller to find the period of one cycle, if period is highest, then its logic 0 otherwise logic 1.
12. There are other than BFSK techniques (like BPSK, QPSK, OFDM, QAM etc.) u can use in same microcontroller, with software change, no need to change the hardware.

This is all i have, hope that helps.