Device to show when phone is in use

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

Ken

Another phrase that pays:

. .

I question the veracity of that statement.

****** 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

1. 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

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.

. . . ~~ . +-------+------+||- | [22M] | | 1U | | . | | | +-------|-------+--||--|----------+ . | | 0.01U| | | | + | | . RING---|~ -|--+-||--+ | | | | | . +-------+ | | | | e | | . | [22M] c | |/ | | . | | |/ +-----|2N4403 | | . | +--|MPSA-18 | |\\ | | . | | |\\ | c | | . | | e | | c | . | | | | | |/ | . | [1M] | [2.2M] +----|2N4401 | . | | | | |\\ | . | | | | e | . | | | | | | . +-----+----+-------+--------------+----------+ . .

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.