Twilight Switch

042 This inexpensive unit switches the light on at dusk and off again at dawn. The circuit has separate time bases for the on and off delays. The dotted line in the diagram divides the circuit into two halves: A, the light-dependent switch around gate N1; and B, the on-off time base around N2 and LED and relay driver Tl-T2. The voltage at junction R2-R9-C1 is inversely proportional to the light intensity measured by light-dependent resistor (LDR) R9. Schmitt trigger gate N1 toggles whenever this voltage reaches one R1 PI R2 GENERAL INTEREST - 500 R7 D3 Rel RS CIL 1=1 U 12VH47011 • R9 Dl 1.6. 2 BC141 LDR 100n R3 ro N1,N2= IC1=4093 D4 D5 00000 ■• ■••61 ELEKTOR ELECTRONICS JULY 1989 SUPPLEMENT C2 2x min .1N4148 100P 1N4148 T1 BC107 18V .4-A I B-0. •0 894003 - 1 D2 EMI ND. 6 22k 4 E1 115 16V TWILIGHT SWITCH 0 :1 idi of the input threshold levels. Since the difference between these levels is large compared with the voltage span produced by the potential divider, a varia...