Input circuit needed for PC Oscilloscope!

I have bought a PIC pc Oscilloscope but it will only except a 5v input. I have been trying to design an input circuit to reduce voltage that I am looking at from upto 20v amplitude to the 5volts needed by the scope. I am using crocodile clips to simulate the circuit, tried a simple devider and also using op-amp but not able to get any of my circuits working, anyone out there have a decent circuit I can use, please.

hi,
Have you tried a resistor divider as shown in the diagram, if so, what value did the PIC scope show?

Zener diodes begin conduction in the 'reverse' direction when their 'zener voltage' is reached, which should be placed in front of this circuit at the absolute maximum voltage level allowed by the USB scope, to prevent transients from the external circuit from killing it. It's the bare minimum protection I'd put on it, unless you can be absolutly sure the circuit will never get a transient.