Telephone & Intercom related schematics

• 1-line telephone status indicator powered from a 3V battery (PDF)
• 4-line telephone status indicator that is polarity independent (PDF)
• 8-line intercom system using 89C51 microcontroller
• 900MHz cordless phone bug
• Assorted telephone circuits (ring generator, in-use indicator, audio interface)
• Audio Visual Indicator for Telephones
• Caller ID decoder (requied Postscript file viewer and/or Ordcad schematic capture)
• Cellular Phone calling Detector
• Cellular Phone calling Detector
• Cellular Phone calling Detector
• Cellular Phone calling detector (detects RF field)
• Cordless phone backup power source using LM317L
• Cut Phone Line Detector
• Cut Phone Line Detector
• Cut Phone Line Detector
• Cut phone line detector
• Cut telephone line detector
• Detecting a telephone ring signal
• Digisound ring modulator
• Digital/Standard Phone Line Tester
• DSL / ADSL phone line filter
• FM Telephone Bug
• FM Telephone Bug
• FM Telephone Bug
• FM telephone line bug transmitter (88MHz to 94MHz)
• Greek telecard reader
• High Quality Intercom using LM380 amplifier IC
• Hold function for Telephone
• Home made PBX / PABX
• Incoming call indicator (ZIP file)
• Infinity bug for remote telephone surveillance
• Isolated Telephone Interface
• Isolated Telephone Interface (phone tap) for tape recorder, PC sound card etc
• Line in use indicator that handles two independent phone lines (PDF)
• Low cost intercom using transistors
• Off line Telephone tester
• Phone "Hold" With Music
• Phone Broadcaster / telehone bugging device circuit diagram
• Phone Busy Indicator
• Phone Busy Indicator
• Phone Busy Indicator
• Phone In Use Indicator circuit diagram
• Phone in-use (manually activated)
• Phone In-Use Light
• Phone line in use indicator
• Phone line indicator
• Phone line to audio interface
• Phone Off-Hook Indicator using CD4049 hex inverter
• 'phone rang' indicator light
• Phone ring circuit schematic using a piezo buzzer for sound output
• Phone to audio interface for SSI202 input
• Phone-In-Use indicator
• Remote Telephone Bell Ringer
• Ringing Phone Light Flasher
• Ringing Phone Light Flasher
• Ringing Phone Light Flasher using optocoupler and relay
• Simple alarm with telephone dialler interface
• Simple circuit to connect telephone equipment to audio mixer
• Simple intercom
• Simple Phone Tap
• Simple Phone Tap Circuit
• Simple Phone Tap circuit diagram
• Simple Skype VoIP analog adapter
• Simple speakerphone
• Simple telephone line tap for intercepting telephone calls
• Smart Phone light
• Soft Musical Telephone Ringer
• Soft musical telephone ringer
• Talking phone dial monitor (PDF)
• Taxi phone automatic dialler
• Telephone amplifier
• Telephone amplifier
• Telephone auto-dialler (useful for an alarm dialler)
• Telephone Hold Button
• Telephone Hold Button
• Telephone Hold Button circuit
• Telephone in use light
• Telephone Line Filter / Protector
• Telephone Line Monitor
• Telephone line monitor
• Telephone line monitor
• Telephone line simulator
• Telephone music on hold interface
• Telephone privacy adapter
• Telephone Record Control using 2N4360, 2N2222 and 2N2102
• Telephone Recorder
• Telephone Recorder
• Telephone recorder to record phone conversations automatically
• Telephone ringing circuits
• The 89C51 PABX
• The Link 4+0 - Internal Intercom (Doesn't offer external line access)
• The Link A2B+1 (the Link Telephone Intercom - DTMF version)
• The Original 2 Phone Intercom Link Design
• Universal telephone hold
• Use old telephones as an intercom
• Wireless Telephone Bug