How to build a speakerphone?

My current telephone lacks a speakerphone, so I want to build one.. Are there any schematics on this topic?

orksecurity6 years ago
First challenge: Telco is REALLY unhappy about having anything but approved equipment connected to their lines -- to protect their equipment, to protect you, and more importantly to protect everyone else. The right way to attach homebrew telephone equipment is with an approved adapter box -- which, because it's an uncommon item, is not remarkably cheap and may cost you more than a speakerphone (or phone with speakerphone mode) would.

Second challenge: Preventing feedback. With a normal handset, the mike and speaker are far enough apart that this doesn't tend to become a problem. With a speakerphone, that isn't true. This is a bit of a pain to design properly. Best I can suggest is websearching and/or hitting your local library to see if you can find an existing schematic.

Thought: Do inductive pickups work on modern phones? If so, you could feed one of those into a small amp and have the "listen" side of a speakerphone working quite easily. You'd still have to speak into the phone to be heard on the other end, and you'd have to be careful about volume levels to prevent feedback, but depending on why you want this it may be Good Enough, and it's by far the simplest solution.
jwzumwalt4 years ago
Modern telephones no longer use complicated circuit solutions. Instead, manufactures provide custom integrated solutions. A google search for "speakerphone circuit" or "Voice Switched Speakerphone Circuit" will immeditaly provide single ic chip solutions. Of course an additional 10 or so descrete external components are needed to complete the installation.

Some chips and circuits to get you started are...
http://www.freescale.com/files/timing_interconnect_access/doc/data_sheet/MC33215.pdf
http://www.datasheetdir.com/FM34018+Audio