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Cwdaemon - Homebrew project - CW Key Voltage

Discussion in 'Homebrew and Kit Projects' started by N6OSB, Feb 26, 2011.

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  1. N6OSB

    N6OSB Ham Member

    Ok I've made my first from scratch homebrew project. I made a cwdaemon interface. I think I have cwdaemon settings setup correctly.

    But I'm a bit concerned because I am getting a voltage reading on across "CW Out" & ground. I don't want to make my TS-2000 or FT-817ND go up in smoke or have a repair bill.

    I didn't build the PTT circuit. Since I don't think I need PTT is needed. Continuity never breaks, but from the voltage fluctuation reading I can tell its keying out words. I can see the meter neddle tapping out my call :). Voltage is under 1/2 a volt.

    Am I safe to plug this into my rig? Or did I get something wrong. This uses 4.7K resistors right? Yellow-Violet-Red

    Here's the schematic.

  2. KT1F

    KT1F Ham Member

    It should be okay because the open collector circuit is the standard way of keying most radios. The computer will make DTR high when it wants to key down (i.e., transmit). That saturates the transistor and pulls the collector / CW OUT down.

    That said, I'm not sure why you're seeing a voltage there. Are you sure you have the transistor connected correctly? As a better test, it would be worth getting a 12 volt source and putting the negative to ground and the positive through something like a 4K7 resistor to the collector / CW OUT. You should read +12 when the key is up (i.e., off) and close to zero when key down.
  3. K4SAV

    K4SAV Ham Member

    What are you using to look at this voltage? You said meter, so I'm assuming it's a VOM. A capacitive load will produce a small output voltage from this circuit.

    If you look at the output of this circuit with no other load other than a 10X scope probe, you should see two spikes of several hundred millivolts, one going positive and one going negative. The negative going one will be wider. That is caused by the input signal coupled thru the transistor junction capacitances. (If the diode on the collector wasn't there, you would see a square wave instead of two spikes.) If you apply a capacitive load, those two spikes can turn into a square wave, but the amplitude should be low. The peak voltage is probably less that 100 mV pp and it should be a negative going pulse. The energy in that pulse is very low. A 10 k ohm load should pretty much kill the square wave and all that will remain will be two small spikes.

    Your VOM most likely has a capacitor between the leads inside the VOM. So you are just seeing the effect of some spikes charging a cap. There isn't enough energy there to damage anything.

    Jerry, K4SAV
  4. N6OSB

    N6OSB Ham Member

    I am only using an old analog multi meter. Would an analog meter have a cap in its leads. If not is .5v pretty safe? I get .5v when computer keys down. I was thinking about switching out the 10% tolerance resstors for 5%.
    Last edited: Feb 26, 2011
  5. KE3WD

    KE3WD Ham Member

    If that is what you are reading, either the transistor leads are not right or the transistor is not good.

  6. K4SAV

    K4SAV Ham Member

    0.5 volts sounds too high. The average output voltage of that circuit should be less than about 50 mV regardless of how much capacitance is on its output, and that should be a negative voltage. You should check the circuit to make sure there are no wiring errors or bad components. Temporarily attach a 1K to 10K resistor between the output and ground and see what the voltage goes to. If it doesn't drop to almost zero, then you have a circuit problem.

    Jerry, K4SAV

    However, if you really have 0.5 volts, then something is wrong with the circuit.
  7. N6OSB

    N6OSB Ham Member

    I had enough spare parts to built it again. It now works like a charm. There's no voltage between "CW Out" & Ground and continuity is broken until computer keys up.
  8. K8JD

    K8JD Ham Member

    A good way to test with an analog VOM is using the resistance scale, the internal meter battery would provide collector voltage to the transistor, red lead (+) to the collector and black lead to GND on the keying circuit. Select the correct meter resistance scale where you can see the transistor keying on and off.
  9. WA7PRC

    WA7PRC Ham Member

    The problem with using transistor drivers as shown is that the PC and station grounds are not isolated. I bet you'll measure some potential between the two grounds. With the two grounds connected together, you'd measure some current flowing because of a ground loop.

    An optocoupler/optoisolator breaks the ground loop current. For keying PTT or CW input on modern rigs, just about any optocoupler can be used. For both CW & PTT keying, I used a (now discontinued) PS2505-2 dual optocoupler that has a Darlington output from a scrapped PCB and built it inside a DB-9 hood:

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