A few problems with that post #19 circuit :-
1) Standard TTL outputs can source only a couple of mA, so U3 will not be able to drive much base current through Q1.
2) Q1 may have a DC current gain as low as 25, so would need several mA base current to give enough collector current to energise the relay.
3) There is no catch diode across the relay coil, so the back-emf pulse when (if) the relay switches off could kill Q1.

 

thanks eric....your suggestion is valuable but the problem is that i have made hardware of my project so now a large change in circuit is impossible so therefor i m trying to play with this circuit. plz see the attached file. thanks

You can add a comparator with a reference of 1.8V and compare with the input of 1.8V...


In this way if the input reaches 1.8V, the output of the comp will go high (~5V). You need the 10K pullup resistor and have to drop the pulldown on the input of the NOT gate...

Did you notice that you can replace the 2 NOT gates and the OR gate with a single NAND gate?

Allen