intercom made out of old phones

[the cracken]

This is simply a direct phone line not an intercom but meh! so i can speak and hear at the same time.
so I'm using the switch in the base and the electret mic and speaker in the head set (chucking the pcb). im going to be using a buzzer i buy and the dpdt switch in the base to switch the buzzer on and off but i dont know how to amplify the signal from the electret mic. i know they need power (my electret mic has 2 solder tabs) but the resistor and capacitor sizes seem to vary so much (is there a calculation involved). I have lm741 opamps, can i use them??
thanks for any help!

 

[heyadrian]

Telephone lines usually have a constant 50v supply (low ampage) to supply them (old phones anyway). So you need to general a low current voltage and have it constantly there.

The buzzer (across two wires only) depends on the current.... When the receiver is down and the phone "rings" the curent increases to about 500ma, thus pulling the ringer solenoid. This is why phone companies have an "engaged tone" so when a person is on the phone a 50v 500mA burst doesnt deafen them!

I suggest for an intercom, using a 4 wire system. Cat V cable is cheapest for this I guess.

1 common, 3 others.

Common = Common
Wire 1 = Ringer
Wire 2 = Mic
Wire 3 = Speaker (earpiece)

Save yourself a load of hassle and take the easy route

 

[4pyros]

(chucking the pcb)

Why not use it. all you need to do is provide power to make a intercom work from old phones! Andy

 

[transistor495]

Here you go!

Use old telephones as an intercom

Reply back how it worked for you

 

[KeepItSimpleStupid]

All you should have to do is provide loop power which is about 30 mA from a constant current source. Off hook is about -48 V and on hook is about -5 V. An LM317T configured in a constant current mode should work.

Devices can be purchased to create this sort of intercom where you lift a phone and the other one rings.

See "Use old Telephones as Intercom" on this page: **broken link removed**

 

[heyadrian]

I still keep it stay simple unless youi want a proper project... Depends on your aims?!

 

[4pyros]

I still keep it stay simple unless youi want a proper project... Depends on your aims

Yes you can get it to work with just a current limting resister and a power source. Andy

 

[the cracken]

thanks i'v already looked at that but there isn't a ringer.

 

[the cracken]

because of the voltages involved. i could use a smaller voltage source but i wouldn't have a ringer

 

[4pyros]

Yes no way to get the ringers to work thay are 60-70 volts. some have added piezo buzzers. Andy

 

[the cracken]

here is the really simple ringer circuit im using and i have to buy a buzzer as the phone uses a piezo speaker and i do want to make a oscillator for it.
so all i need to know is how to amplify the signal from the mic to the speaker of the other head set. i know that electret mic needs power but i haven't a clue on how to amplify the signal. could you please give me an explanation of how and why certain components are chosen. thanks (i only have experience with guitar electronics and simple opamp construction and a tiny bit on thermionic valves.)
**broken link removed**

 

[the cracken]

how did they do that is there a circuit diagram?? thanks