An Inexpensive Interface Cable for Baofeng Transceivers

Much less expensive than the individual components 

I know many of us have Baofeng and Wouxon transceivers. I recently came across this tip to cannibalize an inexpensive mic for the cable, which can then be easily interfaced to a TNC or other digital device.

The Baofeng UV-5R and similar radios are extremely inexpensive dual band (2m/70cm) HT's that are widely used for FM voice communication. But what about using them on packet radio? Yes, this is possible, but a number of people have had problems with either home made cables are some that were bought commercially. The major problem is that the radio keys, but does not unkey. It seems to be a grounding problem because when you bring your hand close to the radio, it then unkeys. More information on how to fix this momentarily. 

One way to overcome this is to make your own radio/TNC cable using a speaker/microphone that is designed for the Baofeng radio. You then cut off the microphone and just use the cable and connector. This may sound expensive, but as it turns out, speaker/mics are available for the Baofeng on line in the US$5 range. Do a search on Amazon for "Baofeng Speaker" and you will find them. You will also need a 5 pin DIN or 9 pin serial plug for the other end of the cable. 

You can buy these DIN Plugs on the TNC-X web site for $2 and serial plugs are universally available. Locally Lee’s Electronics is my choice for supplier.

Here is the Baofeng Speaker/Mic purchased from Amazon.The pin out for the plug is:

• TX Audio: Ring on big plug
• Ground: Sleeve on small plug 
• PTT: Sleeve on big plug 
• RX Audio: Tip on small plug

In the photo above, the 3 screws that hold the microphone rear plate in place have been removed and it is opened up. You can see that the wires are labelled on the printed circuit board, which makes it easy to figure out which wire is which. On this microphone the connections are as follows:

• Red = TXAudio
• White = Ground
• Black = PTT
• Green = RXAudio

NOTE 1: Some of these mics have the M- (ground) and SP+ (RXAudio) wires reversed. Since these wires are connected to the speaker, this doesn't matter for the operation of the speaker/mic, but it does matter for TNC connections. Typically the white wire is ground and the green wire is RXAudio. To be certain, clip the microphone off the cord and check the continuity between the white wire and the sleeve on the small plug. 

NOTE 2: A few Baofeng speaker/mics don't work.  It is suggested that before you cut the cable, make sure the speaker/mic works with your Baofeng HT.  If it does not work, the cable probably won't work either. 

The next step is to strip the wires and tin them with solder. Notice that the black wire is significantly shorter than the other wires. 

Solder a 2.2K resistor onto the pin where the PTT (black) wire is going to be attached. (This will solve the problem of the TNC getting stuck in transmit. Use as small a resistor as will fit, wattage is not important. Next solder the connector on to the remaining wires. 

Here's the completed cable, ready to go! As you can see it is not a difficult assembly process. However, if you would like to buy one already built, they are available for $20 plus shipping from https://www.tnc-x.com/ This company also sells TNC kits.

 ~ John VE7TI 

   19-02

Power-Off Time Delay Relay Circuit

A Communicator Reprise: August 2010
For the original article: https://goo.gl/4KZw9z

The two circuits below illustrate opening a relay contact a short time after the ignition or light switch is turned off. The capacitor is charged and the relay is closed when the voltage at the diode anode rises to +12 volts. The common collector or emitter follower has the advantage of one less part, since a resistor is not needed in series with the transistor base. However the voltage across the relay coil will be two diode drops less than the supply voltage, or about 11 volts for a 12.5 volt input.

The common emitter configuration offers the advantage of the full supply voltage across the load for most of the delay time, which makes the relay pull-in and drop-out
voltages less of a concern, but required an extra resistor in series with the transistor base. The common emitter is the better circuit since the series base resistor can be
selected to obtain the desired delay time – I’ve added a variable trim pot for the task to make the delay somewhat adjustable.

The common collector time delay would require changing the capacitor or an additional resistor in parallel with the capacitor to alter the time delay. The time delay for the common emitter will be approximately 3 time constants or 3 x R x C. The capacitor and resistor values can be worked out from the relay coil current and transistor gain. For example, a 120 ohm relay coil will draw 100 mA at 12 volts and assuming the transistor has a gain of 30, the base current will be 100/30 – 3 mA. The voltage across the resistor will be the supply voltage minus two diode drops or 12-1.4 = 10.6 volts. The resistor value will be the voltage/current = 10.6/0.003 = 3533 or about 3.6K ohms. The capacitor value for a 15 second delay will be 15/3R = 1327 µf. You can use a standard 1000 µf capacitor and increase the resistor proportionally to get 15 seconds – thus the convenience of a variable trim pot.

This circuit is handy if your APRS tracker is turned off with your ignition switch. Keeps the tracker on long enough to send its last ‘posit’ before shutting down. In hybrid vehicles, the accessory battery does not drop in voltage quickly [or at all] so a tracker that is waiting for a substantial voltage drop to turn itself off or detect a vehicle at rest, will never sense the required voltage drop in the battery... so a power-off timer is put in line
to make sure the tracker has enough time to send a ‘at rest’ posit and then shut down until the vehicle is started.