8 Line Intercommunication using 89c51

[Alex]

Hi all,
I have some questions about the 8 Line Intercommunication using 89c51 project.

(Link: http://www.electronics-lab.com/projects/telephone/017/)

First of all: where are the source files for the 89c51? ;)

Next, I think I've spotted some errors in the schematics:

• The MT8816 is powered from the wrong supply lines! According to the datasheet the maximum supply voltage (Vdd-Vee) is 13.2 V. So, the Vdd should be lowered at 5 V, or the Vee should be tied to GND (biasing the analog inputs at Vdd/2).
• On the SLIC cards, assuming the phones are connected between TELx and GND, R9 (100 Ohms, on SLIC 1 page, upper left) is connected in parallel to the phone, which is clearly wrong. May be should be 100K? Or it is connected in the wrong place?
  
• In the same area, the relay contacts are miswired: with the current layout, when the phone is ringing, the off-hook detector is disconnected from the phone!
  
• The + inputs of the comparators should have a resistor towards GND to set the on-hook/off-hook threshold (now, when on-hook, both inputs are at the same voltage, so the output is undefined).
  

Thanks in advance,
Alex

[auwal]

Hi
Can you please help us with the corrected version of the 8-line imtercommunication.Thanks Auwal :)