Caller-ID Unit Using Micro-controller
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The facility of knowing your caller’s telephone number before answering the call, which has been available to cellular phone users only in India so far, has now been extended to the normal telephone users also in Delhi, Mumbai and some other cities of India through MTNL and DoT since 1st January. MTNL/DoT telephone exchanges transmit the telephone number of calling party just before the first ring while in systems used in USA and China, the data relating to the calling number is sent between first and the second rings. The number of calling party in India is transmitted by MTNL/DoT exchanges in DTMF codes. Two documents containing standards and specifications issued by DoT’s Telecommunications Engineering Centre (TEC) in the form of ‘Generic requirements’ and ‘Interface requirements’ for ‘Subscriber service unit for calling line identification presentation (SSU for CLIP)’ are available from TEC, Khursheed Lal Bhawan, Janpath,
This caller-ID unit has seven main functional blocks: power supply, off-hook detector, DTMF decoder, LCD display unit, micro-controller, memory and keyboard. The key electronic components are shown in Table II. MC68HC705KJ1 is the latest lowcost micro-controller from Motorola. Other parts like ICs MT 8870, 24C08, LM393 and LCD module are standard parts and are available from more than one source.