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CW Interface Help/Clarification

Discussion in 'Computers, Hardware, and Operating Systems' started by WA2FTV, Sep 16, 2018.

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  1. WA2FTV

    WA2FTV Ham Member QRZ Page

    Greetings Everybody,

    I am attempting to design a CW interface so that I can participate in the SD QSO Party next month using N1MM contest software.

    I already have a USB to Serial (DB9) adapter cable.

    My questions surround the interface schematic shown in the attached document, which I received from a fellow ham. Studying about the RS232 specifications, I understand the high and low to be between +5V to +15V for a LOW and -5V to -15V for a HIGH.

    What would be a typical output voltage should I expect from a Laptop computer? I have an Asus G51J.

    If I assume 15V for the input with a 1.3V drop across the LED in the opto-isolator (4N25) and 50mA through the LED, I arrive at 274 ohms resistor instead of the 1.2kohms shown in the schematic. Is 50mA too much? The maximum is 60mA based on the data sheet.

    Does the output of the 4N25 need a pull-up resistor? If so, what value should used? Is 4.7 k ohms or 10k ohms a reasonable value. The output will be plugged into the key input of the transceiver (Kenwood TS-930S). The voltage across the key is 12V when the key is open and goes to ground when the key is closed.

    I attempted to test the circuit the other day and did not read any kind of response at the output. I did not include a pull-up resistor when testing the circuit.

    Other than pins 4 (DTR) and 5 (GND) on the DP9 connector, do I need to do anything with any of the other connections?

    Is there any S/W available where I can activate the DTR signal from the computer so I can measure the output with a voltmeter. I want to verify that the circuit works properly before connecting it to the transeiver.

    Any suggestions or input would be appreciated.

    Thank you in advanced and 73s.


    Attached Files:

  2. K5TEE

    K5TEE Ham Member QRZ Page

    The circuit should work as shown, no need to over complicate things by over designing the circuit.
    The output would be tested with an ohm meter. open when no signal, shorted when you have signal.
    so, it should show an on/off (short to open) with either dits or dahs.

  3. K7MEM

    K7MEM Ham Member QRZ Page

    I agree with K5TEE, don't over think the simple circuit. It works as shown.

    No, the 4N25 does not need a pull-up resistor. The collector of the 4N25 gets it's source current from the key input on your radio. I use almost the exact same circuit for managing PTT and CW Keying.

    The collector of the 4N25 does not generate current. It only sinks current. So you will not see anything on a volt meter. To test the circuit, when not connected to a key input or a pull-up, use your meter on OHMs. That will provide the current needed to give a reading.

    If you then add a 10K pull-up resistor, that is connected to 12 Volts, you will effectively be replicating the CW keying input of your radio. Then you will be able to see the ON/OFF of collector with a voltmeter. But don't use the pull-up resistor during actual operation. It's not needed.
  4. WA2FTV

    WA2FTV Ham Member QRZ Page

    Tom and Martin,

    I tested the circuit external to the radio using a pull up to 12V and it works fine. I wanted to see the change in voltage.

    Thank You for your input.

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