Modulating Sensors

Hi, im trying to make a modulated Sensor For " Line following Robot" sensor, im using LDR and Red Led , im trying to modulate the emitter by a frequency like 685Hz and recieve it , can LDR response that ? i dont have phototransistor here in egypt to repalce it .. any suggesstions if that circuit can work ??
also can i replace the diode 1N4148 by a 1N4001 ??

thanks

any comment guys ?

The 4148 is a signal diode, while the 4001 is a rectifier. The sub should work, unless space is a problem.

LDRs have slow responses. You might want to use a lower frequency. Anything that is not harmonically related to 60 Hz should work.

ok thanks for the reply , what about the circuit for the demodulation ? is this circuit correct in design ? the values for caps , resisotrs ..etc ?
the problem also i want to differentiate between white and blue color (( line following ) , the circuit without modulation it works ... so i want to add the modulation stuff, will this affect the sensor for detecting the two color's thersholds ?