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Using a Telephone as a handset for a Handheld Radio

Picture of Using a Telephone as a handset for a Handheld Radio
This instructable shows how to build an adaptter which allows a telephone to be used a handset for a two way radio. The benefit is that may be more ergonomically pleasing to the user than a headset mic arrangement. It also has the advantage that it can be used to transmit DTMF tones via the keypad. This allows the two way radio link to used for remote control and signaling if there is equipment on the receiving end that can decode DTMF tones from the received audio.

This setup is intended for two way radios that have jacks for external speaker and microphones, as shown in the picture. The accessory connections on many handheld radios have a mono 1/8" jack for the speaker output and a mono 3/32" jack for the microphone input.
 
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Step 1: Selection of Telephone

Picture of Selection of Telephone
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Any non cordless telephone can be used, but obviously it is desirable to have the phone be as compact as possible. A one piece phone is the best choice.

Sometimes this type of phone will come with a base, which is usually no more than a plastic cradle on which to rest the handset to hang it up when it is not in use.  In this type of phone all the electronics for the phone are contained in the handset.  This is different than many other phones, where the handset just contains the microphone and earpiece and the rest of the electronics are in the base.  A tab on the base pushes in the hookswitch on the handset to hang up the phone.

The phone I chose is of the one piece type. The first picture below shows the base of the phone with the cover removed  Note that there is nothing inside the base except wires that connect the jack for the handset cord to a standard modular phone jack.

If you look carefully at the handset cord, it will have only two of the four wires. These wires are the actual connection to the phone line, and the base itself just contains a jack for the RJ22 type handset connector on one side, and a jack for the standard RJ12 modular telephone connector on the other side.

In order to conenct the one piece receiver directly to a standard modular wall jack, which I used as an enclosure, it is necessary to replace the smaller handset type plug on the phones cord with a standard modular plug.  Modular RJ12 type plugs and crimping tools are available at most larger hardware stores.  Also, you would wire the cord into the circuit directly instead of chaning the plug, but of couse this will leave you unable to simply unplug the phone from the adapter.
 

Step 2: Schematic and Summary

Picture of Schematic and Summary
This adapter provides a means of using a standard telephone for the transmit and receive audio of a handheld radio, such as an FRS radio. The adapter has to provide the following functions for the interface between the phone and the radio to work properly:

*Couple Audio from the speaker output of the radio to the telephone at a comfortable volume
*Couple Audio from the telephone to the radio microphone input at the proper level
*Provide power for the telephone to function properly
*Provide a switch for the push to talk (PTT) function of the radio.

The function of each component in the schematic will be discussed in the steps to follow.

Step 3: Plugs for Audio Connections

Picture of Plugs for Audio Connections
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MonoPlug2.jpg
The plugs PLG1 and PLG2 in the schematic are the for the audio input to the radio microphone jack and the audio output from the radio speaker jack, respectively.

The PLG1 (3/32" (2.5mm) size) and PLG2 (1/8" (3.5mm) size) plugs can be located from many sources. Surplus electronics companies like All Electronics http://www.allelectronics.com often have low cost parts.  Digikey http://www.DigiKey.com is a greate source of new parts.   DigiKey part numbers that will work are CP3-1004-ND for the 3/32" plug and CP3-1005-ND for the 1/8" plug. The parts may also be salvaged from other cables.

The first picture shows the speaker and microphone connections on the radio.

The second and third pictures show what the plugs look like and how to identify the connections.  The connection named "sleeve" is the longer part, and the "tip" is the smaller part at the end.  The second picture shows where the wires must be soldered to make the connections. 

Step 4: Telephone Connection

Picture of Telephone Connection
This portioin of the circuit is used to power the telephone.

Important Note:  This adapter is not meant to be plugged into the telephone line of you home landline!  This will not work!  It is meant to connect only to a telephone set.  There are high voltages and tranients on you landline that this circuit is not designed for.


An external source of power is connected to J2 to power the telephone.  This can be any source of about 8 to 12 volts.  A 9 volt battery works well.

R1 is a 330 ohm resistor, the voltage across which will be used to as the output to the radio microphone input jack.  It is also the load for the audio output from the radio speaker jack.

J1 is the connection to the modular plug on the telephone set itself.

Step 5: Push to Talk Function

Picture of Push to Talk Function
This portion of the circuit is used to implement the push to talk function that keys the radio transmitter so that it will transmit the microphone audio or DTMF from the telephone.

For many handheld radios, the push to talk functionality when using an external microphone is achieved by connecting a resistor across the two lines in the microphone connection. This is shown as R3 in the schematic. When the PTT button is pressed, R3 is connected across the microphone jack contacts. The transceiver will detect this and key the microphone.  I found that a value of 1k ohm works well here.  Values above 2k ohms did not work with the two brands of handheld radios I have.

The pushbutton switch can be any normall open (NO) pushbutton that will work mechanically in your situation. The switch carries very little current. If the construction of your particular phone prermits, it may be possible to mount it on the case.

The location of the PTT button is up to the user's preference. It can be mounted somewhere on the handset itself, to allow it to be actuated by the hand holding the handset. It could also be mounted inline with the wiring from the handset to the radio transceiver.

Step 6: Microphone Output

Picture of Microphone Output
This portion of the circuit is used to handle the audio output that is fed from the telephone into the microphone input jack of the radio.

C1 is used to couple the audio from telephone to the microphone input of the radio. It is also needed to block the DC from the power supply from entering the microphone input. I've found that a value of 0.47uF to 3uF works well.

Inputing audio via a microphone jack usually requires some attenuation, to avoid driving the microphone input to hard. Resistors R2 and R3 form a voltage divider to provide attenuation. Determining the exact values that work best with any particular combination of telephone and radio transiever may require some additional expermentation. I have found that the values shown in the schematic work well.

Step 7: Radio Audio Input

Picture of Radio Audio Input
This portion of the circuit is used to handle the audio that is input to the telephone from the speaker output jack of the radio.

C2 is used to capacitively couple the audio from the radio speaker output jack into the telephone so that it can be heard in the telephone earpiece.

The audio output of most radios will be great enough on the high setting that it will be excessively loud in the phone handset. Resistor R4 in the schematic is used to provide some minimum amound of atttenuation, to avoid every overdriving the earpiece in the telephone. I have found that a value of 100 ohms works well, but this is another instance where some experimentation and tweaking may be needed. The volume heard in the phone earpiece will depend on the volume setting of the transceiver and the value of the R4.

Step 8: Construction

Picture of Construction
jack_inside.jpg
Completed_Adapter.jpg

The actual construction of the circuit can be whatever you are comfortable with and will fit your needs.  One easy and inexpensive way is to use a modular wall jack of the type shown in the picture below.  It has a 6 contact connector, but it will also accept a standard 4 pin plug that is found on almost all telephones.

Inside the jack are 6 screws that allow cabling to be connected to the 6 contacts of the connector.  There are short wires from the connector itself to the screw terminals.  This project only uses the red and green wires, so the other 4 wires can be cut off near the connector, and the unused screw terminals in the plastic block can be used as a means of making the connections as shown in the circuit.  This way you can assembly most of the circuit with little or no soldering.

There are 8 unique "nodes" in the circuit, but only 6 screws.  However, there are two holes in the jack assembly without screws.  You can drive a suitable screw into these positions and use them to as points to connect the other two nodes.  I've numbered the nodes in the schematic and the terminals on the connector body, as shown in the schematic and picture below. 

One somewhat cumbersome part of this arrangement is that need for the 9V battery.  I have found that a power supply of 6 volts or greater was needed to power the phones I have, so I was not able to use the 3 AAA cells in the radio to power the phone.

All the components will fit inside the jack assembly, but it will be tight.  You can see from the picture how packed in all the parts are.  I had to use a pushbutton switch that has  a very flat body, and moutn it on top of the jack cover as shown.  A pushbutton that protudes through the cover would not fit due to the components located in the main part of the jack assembly.

Step 9: Connection and Use

Picture of Connection and Use
The picture below shows how a telephone, the adapter, and a handheld radio are connected together.

*Plug the telephones into the modular jack on the adapter.
*Plug the jack for the microphone input (3/32" jack) into the microphone jack of the radio.
*Plug the jack for the audio input (1/8") into the speaker output jack of the radio.
*Connect a 9V battery to the batter connection.  Remember to remove the battery when not in use, as it will be constantly supplying power to the phone while it is connected.
*Make sure that the phone is "off hook".

To transmit, press and hold the PTT switch on the adapter, just as you would press the PTT on the radio.  If you wish to transmitt DTMF tones, you will also have to hold down the PTT switch on the adapter while you press the keys on the keypad.

To receive, release the PTT switch, just as you would in normal operation.  You should hear any audio from the receiver in the earpiece of the phone.

Step 10: Limitations and Ideas for Improvement

The adapter works well with the telephones and radios I have used.  One shortcoming is that the audio output level from the microphone tends to be a bit on the low side, relative to the volume of the DTMF.  Still, the the audio received on a second radio could be heard well, but the volume on the radio needed to be turned up a little more than usual.  The DTMF tones come through loud and clear.

Another shortcoming on this adapter is that the phone continous to draw power from the battery while it is connected.  It would help to have a switch in series with the phone/battery loop so that it could be disconnected without removing the battery or unplugging the phone from the jack.  This feature can be added easily, but it did not fit within the phone jack used as an enclosure.

I chose the phone jack as an enclosure here because it was inexpensive, readily available, and had a built in jack for a modular phone plug.  But, it leaves the 9V battery kind of hanging there unless you secure it all together with a rubber band or zip tie, which looks kind of hokey.  There are cases for electronic equipment that have nice built in compartments for a 9V battery, and if I build a second adapter that would allow for a much cleaner implementation.
ry259204 years ago
Nice ible, I'll need to try this, It should also work for practically any headphone jack based device. +3.5 stars