Computer-radio interface schematics
Here are two schematics, one without isolation between the radio, the other using an optocoupler chip and audio transformers.
Audio and PTT Interface without isolation
Audio and PTT Interface with transformer and optocoupler isolation
Use these schematics to build an interface that will:
feed audio from the receiver speaker output into the computerfeed audio from the computer into the transmitter mic inputcontrol the transmitter PTT by way of the serial port
You might consider duplicating the adjustable audio circuit diagram being used to feed the transmitter with computer audio, and use the second one to feed the audio from the receiver to the computer. In both instances, one is feeding a speaker output into a mic input.
Resistor power ratings
The original article specifies resistor sizes with an unspecified power rating. Assuming we deal with a 1000 ohm resistor across 12 volts, 1000 into 12.000 is going to be 0.012 amperes, and 12 times 0.012 might be 0.144 watts. That should call for a half-watt resistor, for the rule of thumb is to use a resistor that is expected to dissipate half or less of its power rating. That 0.144 watt is more than half of a quarter-watt.
Now, a quick glance at the diagram shows a 1k resistor in series with a 1k pot to ground. With 2000 ohms into 12.000 volts, then there is going to be around 0.006A, and about 0.072W. And each resistor is going to drop half of the source voltage, therefore drop less than 0.o4 watts.
Resistance values to choose
One may wish to use a larger value resistor at the speaker output or line output, in conjunction with the 1k potentiometer, especially if one is not using a linear taper pot, but instead is using an audio taper pot. More on that in a moment.
The circuit is matching a high-level output to a low-level input.
The 1k resistor and 1k pot combination means we are adjusting along the lower half of the available output, while using a 2.2k resistor with the 1k pot will give us an adjustment range over the bottom third of the available output. Use a 3.3k or 4.7k resistor to make the adjustment cover only the bottom fourth or the bottom fifth of the adjustment range respectively.
Audio-taper vs linear-taper potentiometers
Salvaged volume controls are audio-taper potentiometers, as we return to the earlier discussion.
Linear taper means the resistance change is constant over the full rotation range of the control. At 50% rotation, approximately 50% of the potentiometer's resistance has been covered.Audio taper means the resistance changes rapidly at lower settings for a given percentage of rotation of the control, and changes slowly at higher settings. At 50% rotation, well more than 50% of the potentiometer's resistance has been covered.
Audio tapered volume controls allow for an adjustment of volume that falls in line with the perceived volume level change, as heard by the human ear.((sa)) If the volume is off or very low, a small adjustment will bring it up to a low, comfortable level. Raising the volume that much again would have it up to unbearable levels, whereas the audio taper in the control causes the volume to be increased more slowly as the control is cranked up.
As an experiment, remove the ground from the low side of a volume control and connect the audio source there, and remove the source from the high side of the pot and connect ground there. Leave the center wired as-is.
Before turning on the unit, turn the volume all the way clockwise, normally the loud end of the scale.
The volume will be off, or very low, if your work is good. Turn the control oh, about 50% CCW, and the volume will finally come up to a fairly comfortable level.
Increase the volume some more toward full counter-clockwise and the volume will get very loud very fast, in a hurry!
Parts List
For the non-isolated interface you need:
One 1k resistortwo 2.2k resistorsOne 1k Potentiometer, Linear taperOne 2.2uF 50v capacitorFour 0.01uF capacitorsOne Red LED (High sensitivity type)Two Diodes 1N4148Two 3.5mm Stereo plugsOne BC108 Transistor or any suitable NPNOne 9 Pin D plug ( Com port 1 or 2) and coverShielded cableProject Box
The isolated circuit requires:
Three 1k resistorsOne 1.2k resistorOne 1k Potentiometer (linear taper)One 2.2uF 50v capacitorThree 0.01uF capacitorsTwo 600 ohm transformers (T1 & T2)One optocoupler 4N25 RS Number 597-289One Red LED (High sensitivity type)One Diode 1N4148Two 3.5mm Stereo plugsOne 9 Pin D plug ( COM port 1 or 2) & coverShielded cableProject Box
