flashing light with twilight switch

The special feature of this flashing light is the optical switch, which auto- matically switches the light on when it gets dark, and switches it off again at dawn. This makes the light ideal as a warning light near obstructions. It may also be used for educational pur- poses to show the operation of tran- sistors in conjunction with optoelec- tronics. Assuming that it is light, the LDR (light dependent resistor) has a low value so that there is sufficient base current through T1for the transistor to conduct. Its collector voltage is then small, so that T2, an n-p-n dar- lington, is off, and lamp L1stays out. When the ambient light reduces, the resistance of the LDR increases until the base current in T1becomes insuf- ficient and the transistor switches off. Its collector voltage then rises, T2 conducts, and L1lights. This process takes place quite quickly, because when the collector voltage of T2 sud- denly becomes nearly 0 V, this poten- 6...10V tial is immediately applied to the...