Intercom Circuits

Intercom Circuit with a LM1458 opamp

The example below illustrates using an op-amp as an audio amplifier for a simple intercom. A small 8 ohm speaker is used as a microphone which is coupled to the op-amp input through a 0.1uF capacitor. The speaker is sensitive to low frequencies and the small value capacitor serves to attenuate the lower tones and produce a better overall response. You can experiment with different value capacitors to improve the response for various speakers. The op-amp voltage gain is determined by the ratio of the feedback resistor to the series input resistor which is around one thousand in this case (1 Meg / 1K). ..

Telephone Line Simulator Schematic

The following text describes construction, testing and use of an electronic telephone line simulator of my own design. This self-build project can simulate a telephone call / connection between any two local telephone devices. This for a fraction of the outlay of some commercial units, many with cost-increasing features that usually are not required. LineSim makes all the following actions possible without occupying a telephone line or incurring any call charges. At some time, you must have wanted to do at least one of these things. ..

Simple Intercom circuit

The circuit comprises a 3-stage resistor-capacitor coupled amplifier. When ring button S2 is pressed, the amplifier circuit formed around transistors T1 and T2 gets converted into an asymmetrical astable multivib-rator generating ring signals. These ring signals are amplified by transistor T3 to drive the speaker of earpiece. Current consumption of this intercom is 10 to 15 mA only. Thus a 9-volt PP3 battery would have a long life, when used in this circuit. For making a two-way intercom, two identical units, as shown in figure, are required to be used. Output of one amplifier unit goes to speaker of the other unit, and vice versa. ..

Easy intercom 2 wire circuit

This Intercom is powered by two 9volt batteries and uses only current when the Intercom is used. Both units are connected via a two-wire little cable or simply two wires (dotted lines). The loud speakers act both as loudspeaker and as a microphone. When you press S1 and speak into the loudspeaker then this signal is amplified by the transistor stage and made audible in the right loudspeaker and vice-versa. An added benefit of this system is that when the switch is pressed it is quiet, not even annoying noise. This circuit has worked for me to my full satisfaction for many years now. ..

Baby watch Intercom Circuit

It's a very simple circuit, for the follow-up of baby. It can however be also used for other use, as intercom etc. In the place of M1 we can we use a simple electret mic capsule. The regulation of gain becomes from the R1, whoever can be trimmer or potesometer. Has been added also a small automatism, round the RL1 and battery 9V for the case where we have interruption of AC Line. The life of battery is enough big, one and the consumption of current is small, roughly 30 mA, for loudspeaker 8R /0,4W. The transistors do not need heatsink, one and work in Class B. ..

Radio Glen TBU

The heart(s) of the unit are the ETAL P3400 line modules available from Farnell. These modules mop-up all of the line interface functions like ring detection, physical hybrid, on-off hook handling etc. This means that the TBU as a whole would probably pass PSTN type approval testing if it was subjected to it. This sheet shows the differential amplifiers which take the signal from the RX side of the hybrid, and the differential amps which accept the mixer TX output for sending to the TX side of the hybrid. The TX signal is bandpass filtered with -3dB points of 200Hz and 3500Hz at -12dB/octave. This avoids sending unecessarily wide audio to the telephone line and makes cancellation of the TX crosstalk easier. ..

Bi-directional intercom system

This is a very small telephone exchange attached only two ordinary Tele phones. Fully bi-directional facility with an intercom system. When a phone is in off hook condition , Exchange will detect it & send ringing pulse to other phone. When it will be picked up , the connection will be established between both phones. This will be taken place vice-versa...

Advanced Telephone Intercom

In order to achieve a status between two levels of switching for two calls at the same time, you must first establish privacy on each call. Otherwise, you will end up with all four phones talking to each other, and that just won't do. So the Link 'P' is an intermediate stage between the basic Link design, and the fully blown one (yet to be realized, but not that far away,) with all the 'bells and whistles' features. In the diagram above, you can see in block format, how this is achieved. The RTC (Ring Trip Circuit,) Reg. U (Register, which acts like an old style Uniselector,) and the STG (Service Tone Generator,) are all part of the basic Link design...

2 Phone Intercom System

This is the circuit that sparked off all the other Link Telephone Intercom designs. Originally designed back in 1996 with heavy duty relays and their contact banks, it was updated late last year with the addition of IC1 in place of a simpler transistor multivibrator, the replacement of all relays and contact banks with optocouplers, and the addition of the two transformers as part of the transmission bridge (see explanation below) which was comprised partly of the relay coils. If you just want simple ?no frills? communication from one phone to another, then this is the right circuit for you! This version is much more compact, more economic on power drain (yes, you can run it off of batteries ? eg: a 12 volt Gel Cell!) and has no moving parts, except for the switch hooks inside each of the phone handsets...

4 Way Intercom system

This version of the Link is for those who really need a good cheap intercom that will work reliably, but without access to an outside Telco line. This could be in a pre school, a hobby farm or a small workshop or factory, where external phone traffic needs to be kept separate and protected from phone abusers and kids playing around? It follows mainly the principals of the Link pulse dial circuit, but with the modifications and additions of the Link A2B+1 for DTMF dialing...

Telephone Intercom

The Link circuitry is simple and efficient, employing just two ICs, half a dozen transistors, and a handful of garden variety components. It all runs on 12 volts and is easily assembled. You can have your own home intercom between the kitchen, the garage, the rumpus room and at your poolside ?barby? and all for less than $100! The ?Link? intercom has been designed in such a way that you can buy parts for it ?off the shelf? at just about any decent electronics retail chain. It uses old pulse dial handsets and replaces the AC bell set with a 9 volt DC buzzer. The whole circuit runs from a 12 volt regulated DC supply and is suitable for short term battery operation (eg: ?Gel Cell?)...

The transmitter for the wireless headphones is built around a CD4046 CMOS phase-locked loop, coupled with a driver transistor, and a pair of infrared LEDs. ..

Very simple and useful circuit for communication between two person. The Q1 is used to amplify the weak output signal of speaker when one push his (her) side push-button to speak. The base-common configuration of the Q1 pre-amplifier is for matching low impedance speaker with input impedance of the pre-amplifier, then it can amplify the output speaker signal well. To drive the speaker, the circuit use an audio power amplifier IC, LM380...

Using this low-cost project one can reproduce audio from TV without disturbing others. It does not use any wire connection between TV and headphones. In place of a pair of wires, it uses invisible infrared light to transmit audio signals from TV to headphones. Without using any lens, a range of up to 6 metres is possible. Range can be extended by using lenses and reflectors with IR sensors comprising transmitters and receivers. IR transmitter uses two-stage transistor amplifier to drive two series-connected IR LEDs. An audio output transformer is used (in reverse) to couple audio output from TV to the IR transmitter...

This circuit consists of two identical intercom units. Each unit contains a power supply, microphone preamplifier, audio amplifier and a Push To Talk (PTT) relay circuit. Only 2 wires are required to connect the units together. Due to the low output impedance of the mic preamp, screened cable is not necessary and ordinary 2 core speaker cable, or bell wire may be used...