Phone Call Interceptor
Stop those annoying phone solicitors with this handy device.

 
This device connects between the phone line, an answering machine, and your phones.  When a phone call comes in, the answering machine picks up the call and informs the caller to enter the access code to ring the phone or to wait for the beep and leave a message.  If the correct code is entered, the answering machine is immediately disconnected while the device generates phone ringing tones on an external speaker.  Picking up any phone causes the device to return to its initial state.

The design of this device is greatly simplified by haveing the answering machine 'do the talking' and using a speaker as a ringer rather than actually generating a ring signal (90 volts) to actually ring the phones.   The list of functions left to the device itself are as follows.....
 
  1. Decode the DTMF frequencies from the phone line.
  2. Detect the answering machine picking up the line.
  3. Detect the local phone being picked up or hung up.
  4. Detect the calling party haning up.
  5. Generate a ring signal for the speaker.
  6. Check to see if my bedroom door is closed.
The PIC16F870 chosen for this project has seval analog channels.   One of these is used to capture the DTMF tones and decode the digits.  The actual phone connections are made using a 'hacked' 5 line expandor from Radio Shack.  Inside the casing, seperate connections were made to each of the connectors and run out thru the back of the case.

Since I live in a rather large house (with 5 other people at this time), I used a PA horn for the ringer/speaker.  As a compromise to local sanity, the level of the ringer is reduced when I am sleeping or not at home.  The 'door sense' input detects the 'state' of my bedroom door since that is where the speaker is located.

This is one of the most useful devices I have made lately.  As usual, you can check out both the schematic and source file if you wish to make something like this.