From RESET Volunteer Rich Repplier:
It’s not generally known, but two speakers can be connected together without any electronics or an energy source of any kind, and the result is a telephone. Whichever speaker is spoken into, generates electricity that drives the other speaker (which converts the electricity back into voice). 5th-graders love talking to each other over the system and they quickly get very excited.
This is what you’ll need to build a sound-powered telephone:
These must have high efficiency. The voice you hear through the system is clear. But it’s not very loud, and unless high-efficiency speakers are used, the voice will be so soft that the kids will probably lose interest. Go to Mouser Electronics (mouser.com) and type in the part number 490-GF0876. Two speakers are needed and the current price is $6.60 each.
You need to mount the speakers in cabinets to protect the speakers from damage in handling. I built my cabinets from 1/2” plywood. They should have outside dimensions 4.5” high X 4.5” wide X 4.0” deep, and they should enclose the speakers completely. You’ll have to cut a round hole, a little smaller than the diameter of the speaker, in each cabinet. The speaker’s paper cone should be mounted just behind the hole. Be sure to put metal window screening or metal “hardware cloth” between the speaker and the wood, to protect the delicate paper cones from kids’ fingers. Ace Hardware sells hardware cloth (it’s also called wire mesh), and I recommend it because it is far stronger than metal window screening. Keep in mind that the gear will be handled by young kids and may even be dropped on the floor, so make the cabinets very strong! I built mine using screws and Elmer’s Glue-All.
These are used to connect the two speakers together. I strongly recommend Monster speaker cable because it is extremely flexible and has low resistance, and both of those qualities are very important in the system. The cable can be expensive but RESET may cover your costs for building the system. I use Monster XPMS Speaker Cable, which is excellent. Amazon sells it and you should buy it in 50-ft. lengths.
You want the two speakers to be as far apart as possible, because you want the kids to hear each other through the system, and not through the air. You should have at least 100 ft. of distance between the speakers, and I always have them 150 ft. apart.
You’ll need to put connectors on the speaker cabinets and cables so you can disconnect the speakers from the long cables. I use 2-pin Cinch-Jones connectors because they are so rugged that they can be stepped on with no damage (and they will be stepped on). Go to Mouser Electronics (mouser.com) and type in these part numbers:
538-P-302H-AB (buy one of this)
538-S-302H-AB (buy one of this)
538-P-302H-CCT (see below for quantity to buy)
538-S-302H-CCT (see below for quantity to buy)
If you plan to buy 100 ft. of cable, buy two 538-P-302H-CCT connectors and two 538-S-302H-CCT connectors. If you plan to buy 150 ft. of cable, buy three 538-P-302H-CCT connectors and three 538-S-302H-CCT connectors.
The connectors with AB suffix go on the speaker cabinets. The connectors with CCT suffix go on the cables. Each cable should get one 538-P connector and one 538-S connector. If you don’t have soldering skills, perhaps you know someone who does.
Inside the speaker cabinets, use two wires to connect the connectors to the solder lugs on the speakers. Use any wire size from #18 to #22.
Normally we take the system outside so we’ll have lots of room. We divide the class into two halves, one half at one speaker and the other half at the other speaker. If it’s raining, we stretch the cables out in a long hall in the school building.
When you see the kids using the sound-powered telephone, it will fully repay your effort to build it. My 5th-graders love it!
If you have any questions you can reach me at firstname.lastname@example.org or 703-830-6379.