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  1. #1
    Junior Member Maryli's Avatar
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    Default The Project about building a PIC-based Beacon Keyer


    This is a PIC project and I am made it -- a beacon keyer that I made for my father, HB9BBD. That was was in 2013. A beacon keyer is a great project to get started with microcontrollers since it's not much more than a fancy way of blinking an LED.

    At that time I didn't even use Eagle yet and so the layouts were based on a software called Sprint Layout. These were all very simple circuits, all based on a PIC16F688, and therefore perfectly suited for making some of my first homemade PCBs as well. The very early versions like the one in the picture above even used the DIP version of the PIC in a socket. All the resistors and capacitors are 1206 size not 0805 like of my later designs.



    In fall last year, HB9MPU asked me if I could make some keyers for his new 10GHz beacon. That was a great oportunity to design a new one from scratch and this is what this post is about.



    Requirements
    • 12V operation
    • Open drain outputs, i.e. a transistor to ground
    • 2 outputs, one of them inverted


    Microcontroller

    I decided to use a PIC16F18325 which from Microchip this time. Like the 688 it is a 14-pin PIC but a much more recent addition to the PIC16 family. It's fully featured with I2C, remapable pins, an on-board voltage reference, plenty of timers and a whole bunch of other features. And itís widely available a very low price. So I tend to use this PIC whenever I need a low pin count micro somewhere. Like for my fan controller.



    Outputs
    Instead of using discrete transistors and protection diodes I have used a Texas TPL7407. That's a 7-channel low-side driver, basically 7 mosfets to ground together with an on-board voltage regulator for the gate voltage and protection diodes all in a SOIC16 package. It sinks up to 600mA (with a maximum of 2A total) per channel and operates from 8.5 to 40 volts. Perfect to drive relays, small motors, powerful diodes or just about anything else that doesnít require that much current. Some of you may be familiar with the ULN2003 which does the same thing but using bipolar transistors (as darlington pairs) instead of MOSFETs.



    Power supply

    The power supply is based on a Texas LM2931, basically a rugged LM7805 that operates all the way up to 26 volts and survives up to plus/minus 50V. While the LM2931 is reverse polarity protected, the diodes in the TPL7407 will short any negative input voltage to ground which is obviously a bad idea. Iíve added a 60V, 2A schottky diode at the input so the device is truely reverse protected. Minimum operating voltage of the TPL7407 (after a diode drop) and maximum operating voltage of the LM2931 result in an input voltage range of the final device of 9 to 26 volts.

    Speed control


    There's a small pot on the board that lets you control the speed of the keyer. The PIC measures the wiper voltage with its built-in 10-bit ADC and sets its speed accordingly.
    Fan output


    As the above mentioned, the TPL7407 has 7 channels but I only really needed two for the beacon keyer outputs, one normal and the other one inverted. The PIC also has an on-board voltage reference module and a temperature sensor so I decided to use all that functionality to add a fan controller. Beacons often have fans that run constantly even when thereís absolutely no need for cooling such as in winter which only wears out the fanís bearings.


    So the pic also measures the temperature and turns the fan output on and off according to software-defined threshold temperatures. Since the PIC's temperature module is rather in-acurate I have added an inexpensive but much more accurate LMT86 analog temperature sensor.
    I have used 4 TPL7407 channels for the fan output so the current is limited by the total allowable current of 2 amps. Of course, you can also use this output to control a relay and use, for example, a 230V fan.



    There are also 3 LEDs on the board so you can immediately see whatís going on. And just in case you care, the board measures 45x45mm.
    As always, programming is done with a PicKit3 or similar via a 100mil in-circuit programming header.



    That's pretty much all that is to say about this little device. As always, I appreciate any feedback and let me know if you need one of these.
    Attached Images Attached Images

  2. #2
    Prolific Poster towlerg's Avatar
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    Default Re: The Project about building a PIC-based Beacon Keyer

    Forgive my ignorance but what is a beacon keyer?
    George

  3. #3
    Administrator John Drew's Avatar
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    Default Re: The Project about building a PIC-based Beacon Keyer

    You made a nice job of it Maryli.
    It would make a good article in the WIKI under projects or Amateur Radio.

    Have a look at this site for a complete beacon setup including the PLL circuitry. It also includes the digital modes. Your father might be interested to read about his approach.
    https://zl2bkc.com/2015/02/24/multi-...roller-update/

    We're using one of Wayne's boards here in South Australia.
    I like your approach.
    Cheers
    John

  4. #4
    Junior Member pe1pme's Avatar
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    Default Re: The Project about building a PIC-based Beacon Keyer

    Quote Originally Posted by towlerg View Post
    Forgive my ignorance but what is a beacon keyer?
    A beacon is a small, stand-alone, transmitter, transmitting a low power signal on a specified frequency. They are mostly build by radio amateurs and its purpose is to determine how good or bad the radio conditions are.
    These beacons require a special license (because they operate without the presence of an amateur) and must transmit their designated callsign every few minutes. This is mostly done in Morse code. If you do morse code by hand you use a morse key to "key the transmitter". This little project is replacing the morse key and keyes the beacon transmitter automatically.


    Wim PE1PME
    To be OLD and WISE you must first have to be YOUNG and STUPID!

  5. #5
    Prolific Poster towlerg's Avatar
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    Default Re: The Project about building a PIC-based Beacon Keyer

    @pe1pme Ta
    George

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