The sensor of this relaxation oscillator with a 741 is a common LDR. The supply must be symmetrical and potentiometer R1 must have approximately the resistance of the LDR at the trigger point. Other operational amplifiers can be used.

The sensor of this relaxation oscillator with a 741 is a common LDR. The supply must be symmetrical and potentiometer R1 must have approximately the resistance of the LDR at the trigger point. Other operational amplifiers can be used.