Discussion in "Electronics" started by    har    Oct 22, 2007.
Mon Oct 22 2007, 10:52 AM
please give me the schematic for auto answering system.how can i couple the computers audio cable to telephone via audio connector.
Mon Oct 22 2007, 11:18 AM

Circuit Description:
Here is a circuit of a simple telephone answering unit which may be used with any telephone. The circuit consists of three main sections:
1. Ring detector section.
2. A timer controlled electronic switch and telephone line interface.
3. A voice IC having a specific pre-recorded message.
This is not very critical and any other device (such as tape player with message recorded on an endess tape) which does not overload the circuit or the telephone line, may be used. The incoming line is protected by metal-oxide varistor (MOV) RDN 130/14 followed by polarity guard circuit comprising diodes D1 through D4 in bridge configuration. Transistor T1 (MPSA92), having a high breakdown voltage (Vce max.) is used as electronic switch/telephone line interface. It is controlled by transistor T2. Ring detector section comprises capacitor C1, resistor R10 and opto-coupler NEC 2505. (In case of its non-availability, one may substitute it with MC2TE or a similar opto-coupler with additional diode 1N4148 placed with its cathode connected to pin 1 and anode to pin 2 of the opto-coupler.)

Timer NE555 (IC1), configured as monostable flip-flop, is powered by external 230V AC via transformer X1, rectifier diodes D5, D6 and filter capacitor C5. When switch S1 is on, the incoming telephone line as well as transformer X1 get connected to the circuit and D8 LED lights up.
Normally the monoshot IC1 is off (output at pin 3 is low) and so also are the transistors T1, T2 and relay RL1. When ring is received, the opto-coupler operates and takes trigger pin 2 of timer from high-to-low potential, thereby triggering it. This causes the output at pin 3 of IC1 to go high, which causes relay RL1 to energise and create a bypass for the ringing voltage to the opto-coupler as transistor T2 also gets forward biased. Conduction of transistor T2 causes forward biasing of switching transistor T1. This, in turn, causes the positive votage from rectifier diodes D3, D4 cathode junction to be extended to voice IC chip after being limited to steady 6.2 volts by resistor R3, zener D10 and capacitor C2. As a consequence, the voice IC operates and feeds the voice message into the telephone lines via capacitor C6, base-emitter junction of transistor T1 and polarity guard bridge.

The pulse width of timer monoshot (decided by in-circuit value of potentiometer VR1 and capacitor C6) is so adjusted to cover the period of the voice message. At the end of the mono period relay RL1 de-energises and the circuit returns to its original condition. In this circuit about 100 mA is catered for the voice IC. The circuit can be easily modified (such as by use of DPDT relay etc) for operation with different answering devices. For testing the circuit you may use a musical COB IC.

Tags answering machine circuitautomatic answering machineelectronic answering machine
Mon Oct 22 2007, 01:11 PM
its perfect circuit . while using computer sound use audio driver transformer to isolate PC's sound card from circuit since 50v pulse of telephone line may damage the sound card

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Thu Nov 21 2019, 12:30 PM
how to interface 8051 microcontroller with NRF24L01+ module
Sat Oct 12 2019, 10:23 PM
Log in.. then post your code in the 8051 forum
Sat Oct 12 2019, 10:18 AM
Bhadra Sajikumar
i am doing a countdown timer with 8051 for my project, unfortunately our professor had to take a leave of absence before he could teach us most of it.. can someone explain the .asm code of it for me?
Mon Sep 23 2019, 07:37 AM
I am doing a project for my schools wireless security system with cameras,sensors and spot lights using 8051 micro controller, could you help me out with the design and its source code.
Wed Sep 18 2019, 08:48 PM
Caryn Kover
Thank you for your time,
Mon Aug 05 2019, 07:09 PM
@Ismail ..post in forum, give details
Mon Aug 05 2019, 04:33 PM
Ismail balghmi
Hello. Please I won't download
Wed Jul 24 2019, 12:47 AM
@BUSTER123 Please post in the forum
Tue Jul 23 2019, 06:13 PM
hello! newbie here. i am kind of a guy who hates using ready - made existing libraries, so this helped a lot as i was trying to control and write to a character lcd display. however, i can't figure out how to set the cursor position. for example, i want to set it to the 5th column in the first row, so position 5. so according to the table, i am sending 10000101 on the 8 data lines of the lcd (register select pulled low). the cursor does not move there however. and i cant move the cursor left or right too. is it something wrong that i am doing? thank you.
Thu Jul 11 2019, 07:55 PM
@ramos https://www.theengineeringprojects.com/2017/09/l298-motor-driver-library-proteus.html



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