the IR interfacing shown below gives the analog o/p and so for that there can be two ways to IR sensor with interface 8051
either this analog o/p can be given to ADC and then it is interfaced with 8051. this par is expalined on site. go for the link
http://www.engineersgarage.com/microcontroller/8051projects/distance-measure-IR-ADC0804-AT89C51
or to convert its analog o/p to ttl logic use opamp ion compartor mode….
see the comparator ckt below… and for your application it will be more easier, less complex and cost effictive
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