A circuit for detecting Power Supply, what's wrong?

[ubuntu_amateur]

there is a circuit to detect a 5V power supply via parallel port, pls take a look at the attachment.
[/img]
when Power Supply is present, parallel port input will detect Low level.
when Power Supply is absent, parallel port input will detect High level.

usually, if there is no this circuit, read parallel port will get 0x7F at address 0x379.
if this circuit is connected to parallel port, the circumstance is not improved.
the value of reading parallel doesn't change, even if the resister Rb2 is removed.

what's wrong with this circuit? Anyone help figure it out?

 

[zirtapoz]

transistor is ll be on state even if you remove rb2. So when 5 V applied input ll be low. but in absence of 5V still you ll get low cos transistor is in off state and there is no voltage.

 

[XNOX_Rambo]

You don't need that circuit att all. Just connect 5V straight to the port.

 

[ubuntu_amateur]

if there is no voltage to input, read Parallel port will get 1's,
I would like to get 0's when 5V is present.

connect 5V straight to the port, will not detect if the power supply is absent.
Because reading the parallel port will return 1's value when there is no any circuit.

connect GND straight to the port, will get 0's.
if 5V is removed, the port is float, will get 1's.

any suggestions?

 

[zirtapoz]

try to reduce rb2, so that when 5 V absent it has the voltage below 0.7 V.(voltage that comes through paralel port(5V). Cos in absence of 5 V rb2 has a voltage above 0.7V which makes transistor on state. And on state pulls input to low state.

in the same manner use higher Rc1. it works too.

 

[ubuntu_amateur]

zirtapoz, appreciated!
it is ok when rb1=4.7k, rb2=1k, rc1=11k.

 

[zirtapoz]

you wellcome..