Dear Mr. Per Guldmann,

A good chip to interface 24 VDC would be an 1489. It's cheap, available at any Radio Shack, can handle up to +/- 30VDC input, four gates inside of a 14 pin dip package, and it will output a TTL signal <= 0.7 VDC or 5 VDC.

Thank you,

Charles Bannister