I havent been able to reproduce the full schematic here as I for some reason cannot insert it as HTML,however the part which you need is shown below.
Basically what we are doing is driving a long peice of wire with a digital signal from your detector,allbeit at a very low frequency.
In the schematic shown resistors R1 and R2 act as level restorers which allow the 5 volt signal and the floating signal to drive the logic gate with the correct voltages.Resistor R3 and capcitor C1 form a low pass filter which acts to significantly reduce the amount of radio frequency interference which may have been picked up by the long wire.IC1a Ic1b are CMOS inverters,but they differ from normal in that they have a schmitt trigger input with about 0.5 volts hysteresis at 5 volts supply.The schmitt trigger prevents the inverter from oscillating when driven with a noisy and slow signal such as we have here.The output from the second inverter is now suitable for driving a TTL CMOS or biMOS/NMOS input such as the 8052 interrupt.
I don't know if the diagram is readable by the time you see it,the only thing you must remember apart from the correct power supply for the HCT14 is that the unused inputs must be tied to earth.

Vcc-----|----------------------------------
| IC1a IC1b
INPUT R1 100 Ohm_____ |-----| |-----|
--------|--------| R3 |----|----| |o-----| |o---OUTPUT
| ------ | |-----| |-----|
| 100 Ohm | 74HCT14
| ---C1 1u.
R2 290 Ohm ---
| |
| |
________|__________________|____________
Ground

<Sigh> its really not easy drawing circuits in ascii:(you'll just have to imagine that R1 and R2 are in series