This circuit was found in an American documentation of the 90s. Optical resources with the LDR as a lens can help in obtaining directivity and sensitivity.
This amplifier with very high input impedance was suggested by Texas Instruments in a manual of...
Using only a ULN5804 integrated circuit, stepper motors up to 500 mA can be controlled from TTL...
Constant current sources are used in applications such as battery chargers, electrolytic tanks, etc....