The original purpose of the figure circuit, suggested by National Semiconductor, is to keep the brightness of a lamp constant depending on the degree of illumination of a photodiode in a photographic film manufacturing process. Of course, the circuit can be used for other purposes, such as regulating the brightness of environments or controlling the transparency of chemical substances. In this circuit, the light intensity captured by the photodiode is amplified by an operational one, feeding back the LM3524 integrated control circuit. The feedback signal is used to excite a small power transistor that is charged with a small lamp. The lamp provides the excitation signal for the control loop with the photodiode. Note the need to use a symmetrical power source for this circuit.
