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.
Step 1: Selection of Telephone
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.