# telephone

Discussion in 'Electronic Basics' started by [email protected], Jul 16, 2006.

2. ### Alan BGuest

Hope you didn't miss this bit:

"The only thing your little intercom cannot do is ring the phone to tell
the person at the other end to pick up. The "ring" signal is a 90-volt AC
wave at 20 hertz (Hz)."
Because you obviously missed this bit:

"...a 9-volt battery (or some other simple power supply) and a 300-ohm
resistor that you can get for a dollar at Radio Shack."

3. ### ChrisGuest

Yup. And if Radio Shack doesn't have a 300 ohm 1/2 watt resistor, then
270 ohms or 330 ohms will do just fine.

Have fun
Chris

4. ### jasenGuest

I've done it.
use a 9v battery. if you get tired of paying for new batteries get a plugpack.

Bye.
Jasen

5. ### feeboGuest

it will.

ummm, try a 9 volt battery

In the pic, the 300 ohm resistor is a non-standard value - if you are
using UK spec phones, go for a 470 ohm - this will extend the battery
life too. Also if you are in the uk, and using modern phones with
electronic ringers, you can make the bell "ring" by rigging a switch
at each end to connect pin 2 & 3 of the standard line jack together.
Press the button to attract the other ends attention then lift the
reciever and wait for them - i.e. it won't work if you have already
lifted the handset.

6. ### Don BoweyGuest

The article told you to use a battery. Do you mean how to get the 9V
*without* a battery? You could use a wallwart.
You do not need the resistor. Modern telephones have a DC resistance of
about 430 Ohms, so the two phones in series will limit the current to about
10.4 mA., which is a good value.

Don

7. ### PuckdropperGuest

Obviously the 9V battery is too hard. The OP needs a simple solution
like using 6 AA batteries, or 7.5 NiMH batteries. (Or 8 and a standard
diode.)

*removes tounge from cheek*

Puckdropper

8. ### Guest

I wired it, and it works.

Out of curiosity, does it matter if the power is on the red wire or
green wire? The wires appear equal.

9. ### Alan BGuest

Terminology lesson: red is tip, green is ring. That means, on a bantam
plug, red is the center conductor (the tip) and green is the outer ring of
the plug. Your simple intercom is a current loop, so under normal
circumstances the polarity of the voltage and the direction of current
aren't important. Here's a crude drawing of how it works, ignoring the
hook switch and ringer circuits:

http://www.privateline.com/archive/howteleworks.gif

Just out of curiosity, how are you going to let the person on the other end
know he or she is being summoned to the intercom?

10. ### PuckdropperGuest

Shouting "Hey! Pick up the phone!" would work. It at least saves you
from having a shouting conversation.

This is an electronics group, though, we can do better. Isn't the ringer
activated by sending a higher voltage on the line?

Puckdropper

11. ### jasenGuest

yeah, it should be AC too... 25 to 20 Hz

Bye.
Jasen

12. ### Guest

Is there an easy way to get this 20 Hz without a frequency generator?

14. ### Alan BGuest

Well, it would be an interesting project to produce 90VAC 20Hz from 120VAC
60 Hz. Food for thought.

15. ### Michael A. TerrellGuest

Ma bell has done it for decades. The old 1A2 type phone system power
supplies did it with no problems. Junk 1A2 systems can still be found
cheap, or for free. They also supply the filtered DC for the talk
circuit, and power for all of the lamps on the phones.

--
Service to my country? Been there, Done that, and I've got my DD214 to
prove it.
Member of DAV #85.

Michael A. Terrell
Central Florida

16. ### jasenGuest

some other sort of oscilator, like a 555, you'll probably need to boost the
voltage a bit though,

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