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check my simple pbx schmetic and suggest your views

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New Member
Dear Sir,
this may take your time but please sir take your time and reply me some suggestion when ever you feel like replying to this.
I know from my behaviour at 8052.com and repeatdly asking questions may have caused you to think that iam a lazy kind of student who just want to finish his home work from forums on internet.... but to tell you the truth really sir there is no one who can guide me about controller in my area and i have to do every thing by my self... i have never went to any university due to some family problems and its only my hobby to make such things.... so please don;t consider my questions as home assignments...as you can see that there is never a time limit i ask you to answer my question.....
Actually sir my biggest dream is to design a small 2 trunk and 8 extensions extendable PBX and for that iam working for last 6 months or so....
and 8052.com is the only web which has provided so much info to me that i got from no where....
Now for accomplishing my goal i have purchased a local 16 Lines Intercom Exchange based on AT89c52 , MT8816 , supports only pulse dailing telephone sets so no DTMF Chip is used....
As i observed the circuit and did some reverse engineering i found that if i under the circuit for one telephone line then all others are just copy paste from hardware point of view,
So i have drawn the circuit for one telephone line.
Now just to do an experiment iam looking forward to make this circuit and connect it with controller in such a way that when ever the telephone set goes off-hook Dial Tone appears in telephone set and remains there untill the user gets on-hook again...
But for this i am looking to use 74HC165 input expansion and 4094BE out put expansion regsiters so that i can use the same circuit for more users....
Please see the circuit and provide me some help about connecting this with controller and how to write codes for above mentioned regsiters ..
Also as i have seen that PCB they have used KA1458 dual operational amplifier to generate call progess tones ... i think the tones are placed on columns of MT8816 and when ever the telephone needs a particular dial tone controller make connection between that column and row. Should i generate Dial tones from controller or use KA1458 or 555 timerchip..



New Member

i can help u on this topic, I have made a project "AT89S51 Based PABx" which support DTMF, 8 telephone as a intercom with trunk call facility. Plz contact me [email protected]
M.Shakeel (Pakistan)


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Well-Known Member
It just shows to go 'ya that there's no circuit like an old circuit. Three and a half years is a twinkling of the eye compared to the upcoming aniversary of the invention of the vacuum triode by Lee DeForrest


New Member
Oh, me too. I didn't see the posted date until Nigel mentioned. This problem should be solved now, or he is still trying it?
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