hello

i'm reviewing a circuit diagram.. which is of a security system with alarm devices connected to its Zones input loops. Each loops contains 4 volts which is always appearing to the input pins of 8051 pins. each sesnor is connected to the zone inputs and each sensor always produces +5V. now what happens is that if i connect the zone to Ground... it brought down the voltage to 0 on the 8051 pins. problem is i have sensors which produces +5V volts in normal situation BUT 0 V when the citcuit is broken which does not SINK current. that means the loop is not connected to GROUND to brough the 8051 pin to zero. when sensor produces 0 V's there is still +5V on the 8051 pin. what should i do. i want to use this circuit because the LOOP always contains the 5V even there is no sensor connected to the LOOP

or please help me find an alternative for this situation

thanks
Shahzad.