Convert high voltage keying to positive keying

Discussion in 'Straight Keys - CW Enthusiasts' started by VK3MTW, Aug 27, 2021.

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  1. K7JEM

    K7JEM Ham Member QRZ Page

    What model MFJ and what model of radio? Do you know the voltage on the keying line, and is it pull to ground to key the radio?
     
  2. K1LKP

    K1LKP Platinum Subscriber Platinum Subscriber QRZ Page

    HI MATT,

    OK .... THANK YOU FOR THE HEADS UP ON THE MFJ KEYER.

    NOW....... THE REED RELAY IS A 12 VOLT DC DEVICE.

    OK..... WHAT U HAVE TO DO IS TO MAKE SURE THAT THE
    MFJ KEYER HAS 12 VOLTS WHEN IT STARTS KEYING.

    I AM HOPING THAT IT DOES AND NOT JUST 5 VOLTS.

    I WAS FACED WITH A SIMILAR PROBLEM YEARS AGO
    WHEN I WANTED TO KEY MY DRAKE TR 4 WITH AN
    ACCU-KEYER I BUILT.

    HOPEFULLY, YOU HAVE A VOM - VOLT-OHM-METER
    TO CHECK THE VOLTAGE SWING OF THE MFJ KEYER
    WHEN U ARE KEYING IT.

    HAVE FUN AND HOPEFULLY U CAN GET THIS MATTER
    RESOLVED JUST USING A SIMPLE REED RELAY.

    73 - K1LKP


    OOPS.......... I JUST FOUND THIS REED RELAY THAT
    USES A 5 VOLT COIL..........

    https://www.radioshack.com/products/spst-5vreed-rly


    https://www.ebay.com/itm/233265151739?_trkparms=aid=1110006&algo=HOMESPLICE.SIM&ao=1&asc=20200818143230&meid=ff276eeba6d944ec9c5a853a18d10869&pid=101224&rk=1&rkt=5&sd=163672203722&itm=233265151739&pmt=1&noa=1&pg=2047675&algv=DefaultOrganicWeb&_trksid=p2047675.c101224.m-1
     
    Last edited: Aug 28, 2021
  3. VK3MTW

    VK3MTW Ham Member QRZ Page

    Exclent when I get home I'll check what output is. 12v or 5v is good to work with I'll find out voltage then buy the right relay.

    Other then that It does look like a simple fix after all. Hopefully I can splice in the relay with some heat shrink over it so it's not to bulky. Because I do love the feel of a good silicone patch lead.

    The keyer is a mfj 464
    Love using the memorys when calling CQ for 30 minutes before someone hears me... 10w life.

    Thanks to all. I'll post my progress and the build here fore future hams searching this issue
     
  4. K7JEM

    K7JEM Ham Member QRZ Page

    It looks like that keying output is pull to ground, rather than a positive voltage output. So one connection of the relay coil would go to a voltage source, the other coil connection would go to the keyer. Be sure to place a reverse biased diode across the relay coil to protect the driver transistor.
     
  5. VK3MTW

    VK3MTW Ham Member QRZ Page

    In the mfj manual it says positive key

    Have attached a photo look for keying output.
    Screenshot_20210829-144120.png
    I have tested the unit.
    It needs to be grounded to main source of power and then the tip of the aux cable puts out 6mv when the mfj Is keyed.

    Very low voltage not to sure what my options are.
     
  6. KG7WGX

    KG7WGX XML Subscriber QRZ Page

    I believe the phrase "Keying output: This unit supports only positive keyed radios (most-solid state radios)." is the answer. It is describing the radio, not the keyer.

    This explanation of source vs sink may help, or it just may add to the confusion - https://blog.orientalmotor.com/what-is-the-difference-between-sink-and-source-logic

    I believe you could stretch things a little and think of the relay as a sink device, though it isn't really DC logic. As the keyer is also a sink device, you are trying to get this to work without a source.

    As others have said, you need to supply a voltage to the relay coil, then use the keyer to complete the circuit to the return side of the power supply. The power supply could be the same one you use to power the MFJ464. This turns the relay into a source, so the keyer can switch it.

    (Not a radio engineer here, just a retired plant control system electrician...)
     
  7. VK3MTW

    VK3MTW Ham Member QRZ Page

    Have been looking around to get my head around very low voltage to key a relay.
    Have found a few boards online I could possably use. A few people build similar setups to get data bits to key a relay.

    I need to find one that can handle the hi voltage from the boat anchor

    Little out of my knowledge range atm.

    I have seen a few ppl use a 9v battery into a circuit to pull it off.

    Would need more explaining on the board with adding the power source
     
  8. KG7WGX

    KG7WGX XML Subscriber QRZ Page

    No AutoCAD here, but I did a pencil sketch showing this -

    Relay schematic sm.jpg

    The power supply and relay coil voltages have to be compatible, of course. And the relay contacts have to be rated for the voltage & current of your boat anchor keying circuit.

    I believe the voltage (and available current) you are measuring at the keyer isn't useful.

    The keyer doesn't send digital signals. To an external device (like a radio, or your planned relay) it looks like a switch, which opens/closes to create Morse code elements.

    The UKA-2 mentioned in my first reply is designed to solve your problem. All you have to do is use the correct input when you wire your "keyer to adapter" cable, as it is designed for both active-high and active-low inputs. You can ignore active-high, as the MFJ is not a source device.
     
    Last edited: Aug 29, 2021
  9. VK3MTW

    VK3MTW Ham Member QRZ Page

    Oh that's simple enough for me.
    I could possably build this inside the mfj taking its power from inside there. And just fit off another port.
     
  10. KG7WGX

    KG7WGX XML Subscriber QRZ Page

    Absolutely! Reed relays can be very compact.

    If you plan to do all the wiring inside the keyer cabinet, you should be aware that connecting another source device to the keyer jack will put two sources in parallel, which can cause damage.

    You could use a SPDT toggle switch to select your relay adapter or the factory jack.
     

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