PHONE/FAX MISSED CALL ALERT

Constructional Project This device detects an incoming call on your phone or fax and lights an LED and latches on a relay, which you can use to switch on a lamp or horn in another room or your garage.

So what’s wrong with listening for the phone ringer or looking at the fax machine’s ‘out’ tray? If you work

at home you need to be able to respond to phone calls or faxes as soon as they come in, but you cannot be hovering over the phone all day; there are times when you have to leave your office or home and if you do not physically lift the phone handset and listen (if you have BT Answer 1571) or check the fax as soon as you arrive back, calls or faxes could be without a response for some time. 

Or maybe you have an office but spend some time in the workshop. Same problem – or even more so if you are waiting on an important call or fax. So this is the remedy. Build this device and it will let you know that a phone call has been made or a fax has been received, without you needing to actually check the phone or fax.

What’s more, you can use it with a remote light or siren to alert you over the noise of machinery!

The Phone/Fax Missed Call Alert hooks up easily to the fax machine’s phone line. It connects in very easily, using a standard low-cost modular cable. The kit of parts should set you back no more than about £15 and you should be able to build it up in a couple of hours at most.

How it works

When someone dials your phone or fax number, the exchange sends out short (200ms) bursts of a 75V 17Hz AC signal – the ‘ring signal’. It’s this signal that causes your phone to ring or triggers the fax machine into answering and receiving the message. Our Missed Call Alert is designed to monitor the phone line and respond to the same ring signal, using it to trigger a flipflop and hence latch a relay.

Download Full Circuit: Click Here

By JIM ROWE
Everyday Practical Electronics, January 2008