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electronics : phone busy

Phone Busy Indicator

After extensive searching on the Internet, I discovered that no-one made a simple phone busy indicator specifically for the UK. After consulting some experts on the EPE ChatZone message board, I designed and built a simple indicator. There are other more complex designs floating around on the Internet, but I decided that this was the simplest to make and hopefully the most foolproof. The LED will light up when the phone is taken off the hook or in use, so that you know someone is using the phone, before you pick up.


If you want to make your own Phone Busy Indicator you will need to be aware that BT and other telephone operators may not like you fiddling around with your phone line. Also the high voltage (when the line is ringing) could be a safety hazard so don't do anything silly, like touch live wires. As long as you are careful, you should be fine, but I just wanted to warn you so that I don't get the blame if something goes wrong!


Now, on to making the device. To make this simplest of circuits you will need:

Optional extras

If you want to use a single PP3 9V battery, for compactness, you will need: 200 Ohm resistor
Unless you feel confident connecting the components directly, you're going to need a former of some kind:  PCB OR
Strip board OR
Solder tab strip
For aesthetics and for safety, I suggest you use a plastic box to house the circuit in. The one I used was transparent to allow the LED to be seen without the need for a window. Plastic box

The Line-Sensing Relay is the main component of this design. The one I used was a BT55/4 model from RS Components Ltd, order code 200-2452. It needs to be connected to the two main wires of the telephone line, which are generally known as the 'Tip' and 'Ring'. The coils of the relay are energised when the phone is taken off the hook and this makes the connection between the LED and the battery.

The telephone extension cable is used to connect the circuit to the telephone line. The UK (BT) system typically has 4 wires running through the cable, yet there are only 2 we need to be concerned about - the Tip and the Ring. The end connectors have, confusingly, 6 contacts, so only the middle 4 are actually connected to anything. The Tip will be Pin 2 and the Ring will be Pin 5 (note that these pin numbers only apply to the plug of the extension cable, as the numbering is different for a BT socket). See below for a typical wiring arrangement - in this diagram the Tip is the black wire and the Ring is the yellow wire.

phone connector

The diagram below shows how to connect up the components, assuming a 9V battery and BT55/4 relay are used:

schematic

 

Note that one of the wires will need to be connected across the relay in the opposite direction to the other wire (see the yellow wire above). This allows the coils to work with the A.C. signals correctly, otherwise the LED won't light up. Also another point to mention, is that this device needs to be connected in series with the phone you want to monitor, in other words it will only detect a phone that is connected further down the line.

If you are planning to use strip board to arrange your circuit you may find the following diagram useful (again assuming that a 9V battery and BT55/4 relay are used)

stripboard

Here is a photo of the finished article, for anyone who is curious...

completed phone busy

Thanks to John Becker (EPE Magazine), Miguel (especially), Andy and the others at EPE ChatZone

Copyright J.W. July 2003

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