# Device to show when phone is in use

Discussion in 'Electronic Design' started by Andy Petro, Mar 27, 2006.

1. ### Andy PetroGuest

Bell used to sell a device that told you when a phone was connected.
You placed it in series with the line and a light would indicate when
the line was busy. Any idea as to how to build one as they are no
longer sold.

2. ### Ken TaylorGuest

Search on "telephone in use indicator". Plenty of circuits and ready-mades.

Ken

4. ### Joe G \(Home\)Guest

****** Warning Some Countries forbid connection of non-government approved
telephone equipment OR tampering/tinkering with telephone line ****

*** If info is for educational purposes only ******

1st Understand the concept of phone not in use and hung-up.

How do you detect a phone is off hook.... what happens to the electrical
signals? Ask yourself those questions.

When the phone is on-hook (hung up) the phone has a higher DC resistance...
as if it were not connected....

When the phone is picked up a lower DC resistance is applied by the hook
switch to the phone line.

The lower DC resistance has 2 effects

1. Draws current from the phone exchange typically 20mA to 90mA depending
on how far from the exchange

2. The voltage drops from typically 48 volts to typically 6v ~ 18v
depending on how far you are away from the exchange.

OK.... you how have two effects you can measure....

current ... which requires a series in-line device

voltage.... measure the voltage across the phone point.... parallel circuit.

NOW.... when you measure electrical signals you should not effect the item
you are measuring.

So a series current or parallel voltage measurement is required.

Note... there are other complex factors involved..... but this info may
partly explain the DC characteristic of phone and exchanges.

JG

5. ### Fred BloggsGuest

The indicator circuit is required to present an ultra-high impedance to
the line when on hook. This little flasher from Tech-Lib does the job:
View in a fixed-width font such as Courier.

..
..
.. ~~
.. +-------+------+|<|-[180]-+
.. FWB | | | + |
.. +-------+ | | | -----
.. TIP ---|~ +|--------+ [1M] [1M] | 9V ---
.. | | | | | | Batt |
.. | -|>|- | [22M] | | 1U | |
.. | | | +-------|-------+--||--|----------+
.. | | 0.01U| | | | + | |
.. RING---|~ -|--+-||--+ | | | | |
.. +-------+ | | | | e | |
.. | [22M] c | |/ | |
.. | | |/ +-----|2N4403 | |
.. | +--|MPSA-18 | |\ | |
.. | | |\ | c | |
.. | | e | | c |
.. | | | | | |/ |
.. | [1M] | [2.2M] +----|2N4401 |
.. | | | | |\ |
.. | | | | e |
.. | | | | | |
.. +-----+----+-------+--------------+----------+
..
..

6. ### Ian StirlingGuest

http://cgi.ebay.com/PHONE-TAP-DETEC...ryZ73336QQssPageNameZWDVWQQrdZ1QQcmdZViewItem
Was found in several seconds of searching.
Minus the bullshit about phone tapping, looks like it's the right
device.
Presumably there are cheaper ones.