wiring old phones for kids

[Brock]

Ok, my wife bought 2 old rotary phones at a rummage sale with just two wire connections. They are in two different rooms the kids play in a lot. My wife wants the kids to be able to pick up the phones and talk to each other.

I don't need them to ring, or make them dial, just pass voice from one to the other if they were both picked up at the same time. If I remember correctly it is 28vac across the two lines? Does anyone know if this will work?

 

[jrmcferren]

You need to limit the current, the phone will do that to some extent, but not completely. The design I got of Howstuffworks years ago is like this (with mods for permance). Here are the supplies you will need:

-Two telephones (any common battery type dial or non-dial, pulse or tone)
-One 9-volt transistor radio battery
-One 9 volt transistor radio battery snap
-One 300 ohm resistor
-Phone cable (very useful if modular) otherwise just cut off the end and use the wires)
-Solder and soldering gun/iron (you can substitute telecom splices)

These instructions assume non-modular phones, just plug the cable in if modular:
Step 1: Cut the cable in the middle
Step 2: Strip back the jacket
Step 3: Clip off the Yellow and black wires these are not needed (after reviewing optional procedures, you may use these wires)
Step 4: Take either the red or green wire and splice in the resistor with solder and tape or a telecom splice (do not just twist together static will form very quickly)
Step 5: The the wire (red or green) you did not splice and splice in (with solder and tape or telecom splice) the battery snap. Do not install the battery yet.
Step 6: Open the first telephone set (see optional procedures below now so you only open the sets once) if modular skip unless doing optional procedures.
Step 7: disconnect any line cord, keep other wires intact on the network (to preserve set for future use on a regular line, your kids will not likely break them).
Step 8: Strip back the wires and connect them to L1 and L2 clip off yellow and black (unless needed by optional procedures)
Step 9: Follow any optional procedures below
Step 10: Close Telephone Set
Step 11: bang microphone of handset on hard surface to loosen carbon granules (this is not a joke and is the standard procedure)
Step 12: Repeat steps 6-11 on the second set
Step 13: Connect battery and test system.

Optional procedures:
-If the set is modular you can disconnect the line jack and hardwire as above, save the line jack if future line use is expected.
-If the set is a princess set (not a starlite) connect the yellow and black wires to use as light power, one transformer will work with BOTH sets
-To prevent dial pulses from being sent (sends a loud noise) disconnect the dial from the network, you may need to refer to a schematic to reconnect the dial.

If you need any help ask me.

 

[Brock]

I am confused. I am assuming in step 4 you connect to the resistor which is in turn connected to the battery snap? So the resistor is in series with the 9v supply? Doe it need DC or can you use an AC 9v power supply?

I also don't get what your doing inside the phone, L1 and L2 are typically red and black and would already be connected there, I suppose it might not have wires already connected?

 

[PhotonWrangler]

AC is used only to ring the phone. As soon as the phone goes off-hook, AC ringing voltage stops and DC "talk" voltage takes over.

JRMcFerren is correct. All you need to do is wire the two phones in series with each other, the 9v battery and the current limiting resistor. If the phones already have cords on them, you can look for the red and green wires and use those for the series connection. If the phones are missing their cords, L1 and L2 are the contacts inside that normally go to those wires. In telephony-speak, you're connecting to tip and ring.

 

[yuandrew]

I remember the circuit from "How stuff works"

 

[jrmcferren]

Do'h forgot the resistor, I edited the post above, yuandrew has the actual diagram for the circut, although to ease editing, I put the resistor in the green wire instead of directly in series with the battery, the whole circuit is a series circuit anyway.

 

[Brock]

Ahhhhh, I totally get it now. I was thinking the battery was in parallel with one phone then the rest were in parallel with that. It's a series string, makes WAY more since now.

Thanks!