RC Receiver for model airplane

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Category: Other Circuits / Remote Control Model Toy
This circuit is also crossed to: PIC Microcontroller , Remote Control
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This project will explain how you can build a receiver for 35MHz. The receiver is based on the FM receiver circuit MC3371, and the frequency is PLL controlled with LMX2306 circuit. In this project I will build a radio receiver for RC air planes. There are 20 different frequencies (channels) used for radio controlling RC air planes. Each user needs it own frequency, else you will controll someone else plane. The lowest frequency is 35.010MHz and the highest is 35.200MHz. In practice there are 20 different crystalls for each channel. In my receiver I have no crystal for each frequency, instead I use a frequency synthesizer. With this system I can in a very simple way choose any channel I want to receive. I will explain how a frequency synthesizer works and how you can control it. The receiver is built around the circuit MC3371 FM receiver. The oscillator is located at pin 1 and 2. L2 with the 15pF makes an oscillating unit. To vary the frequency a varicap is added bb139. By changing the voltage over this varicap the capacitance will change and the oscillating frequency will also change. The oscillator is made to work as "Voltage Contolled Oscillator" VCO. The first thing you should test is that the oscillator is working. I disconnected the Vout from pin 2 of the PLL LMX2306. I then connected Vout to ground and check the oscillator. The oscillator should now oscillate at the lowest frequency. With my "Wireless frequency counter" I found that the oscillator was working at 33MHz. I streatched the coil L2 a bit until it oscillated at 35MHz. I then connected Vout to +5V and now the oscillator was oscillating at 36MHz. Great, as "Basil" would have said! By changing the Vout from 0 to +5V I could change the oscillating frequency from 35 to 36MHz. I then reconnected the Vtune to the PLL. In my test I had programmed the PIC to set the frequency at 35.565 MHz = Channel 71. When I tested the unit again the PLL tried to regulate the Vout voltage untill the oscillator locked to 35.565MHz. I probed Vout with a DC meter and it stoped ad 0.8V and the frequency of the oscillator was now locked to the 35.565MHz. Great, great... To control the frequency a synthesizer circuit LMX 2306 has been added. The PLL circuit has a pickup coil to pin 6. This coil should be put close to the L2 coil for picking up some of the oscillating energy. The PLL circuit has an external reference crystal of 12.8 MHz. At pin 2 of MX2306 you will find a PLL filter to form the Vout which is the regulating voltage of the VCO. The PLL try to regulate the Vout so the oscillator keeps the frequency loocked to desired frequency. The desired frequency is programmed into the PIC EEPROM and is clocked into the synthesizer (LMX2306) at power up. I will below explain how to program the EEPROM for different frequencies. At pin14 of the synthesizer you have a controll output. At this output you will find the reference frequency for testing. (I must warn you because the signal is not symetrical in shape. The positiv puls are only a few microsecond so you will have difficult to see it at oscilloscope.) I solved it by connecting it to a 74HC4020 14-stage Binary Counter to pin 10 Clock input. At Q0 (pin 9) you will have a symetrical square wave with half frequency since the circuit is a counter. At Q1 pin 7 it will be divided by 4, see datasheets for more info. visit page.

The receiver is built around the circuit MC3371 FM receiver. The oscillator is located at pin 1 and 2. L2 with the 15pF makes an oscillating unit. To vary the frequency a varicap is added bb139. By changing the voltage over this varicap the capacitance will change and the oscillating frequency will also change. The oscillator is made to work as `Voltage Contolled Oscillator` VCO. The first thing you should test is that the oscillator is working. I disconnected the Vout from pin 2 of the PLL LMX2306. I then connected Vout to ground and check the oscillator. The oscillator should now oscillate at the lowest frequency. With my `Wireless frequency counter` I found that the oscillator was working at 33MHz. I streatched the coil L2 a bit until it oscillated at 35MHz. I then connected Vout to +5V and now the oscillator was oscillating at 36MHz. Great, as `Basil` would have said! By changing the Vout from 0 to +5V I could change the oscillating frequency from 35 to 36MHz. I then reconnected the Vtune to the PLL. In my test I had programmed the PIC to set the frequency at 35.565 MHz = Channel 71. When I tested the unit again the PLL tried to regulate the Vout voltage untill the oscillator locked to 35.565MHz. I probed Vout with a DC meter and it stoped ad 0.8V and the frequency of the oscillator was now locked to the 35.565MHz. Great, great... To control the frequency a synthesizer circuit LMX 2306 has been added. The PLL circuit has a pickup coil to pin 6. This coil should be put close to the L2 coil for picking up some of the oscillating energy. The PLL circuit has an external reference crystal of 12.8 MHz. At pin 2 of MX2306 you will find a PLL filter to form the Vout which is the regulating voltage of the VCO. The PLL try to regulate the Vout so the oscillator keeps the frequency loocked to desired frequency. The desired frequency is programmed into the PIC EEPROM and is clocked into the synthesizer (LMX2306) at power up. I will below explain how to program the EEPROM for different frequencies. At pin14 of the synthesizer you have a controll output. At this output you will find the reference frequency for testing. (I must warn you because the signal is not symetrical in shape. The positiv puls are only a few microsecond so you will have difficult to see it at oscilloscope.) I solved it by connecting it to a 74HC4020 14-stage Binary Counter to pin 10 Clock input. At Q0 (pin 9) you will have a symetrical square wave with half frequency since the circuit is a counter. At Q1 pin 7 it will be divided by 4, see datasheets for more info. http://hem.passagen.se/communication/rc_rec.html

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