Double power in hard mode:
Enter 55558888 as a password.

Disable hard mode:
Enter 33335555 as a password.

Protection from police:
Enter 33333335 as a password.

Double credits in Defender mini-game:
Enter 33335588 as a password.

Level	Password
1-2	35--3383
2-1	3---5--8
2-2	-358-358
2-3	----3333
2-4	-3-35555
3-1	----3355
3-2	-3-5---8
3-3	33335588
4-1	33--55-8
4-2	33335555
4-3	44445555
4-4	55555555