The circuit works as follows. When light is striking sensor Q1, IC 1-a output is low, and the IC1-b oscillator is off. Thus, IC1-b is high, which results in a low IC1-c output and IC 1-d output. Transistor Q2 is cut, and the lamp is off.
When light drops off, IC1-a output goes high, and IC1-b oscillator turns on, driving the output transistor via IC1-c and IC1-d.
Transistor Q2 can drive lamps up to 500 mA and must be mounted on a heatsink. Sensitivity is adjusted by R1, and flash rate is adjusted by R2.
You can replace the potentiometer with a CdS photoresistor. In this case, also replace R1 with a 100 kg potentiometer.
A schematic diagram of the Dark-Activated Lamp Flasher is shown in Fig. 1.
Transistor Q1 must be mounted on a heatsink. Observe proper positioning of polarized components such as transistors and the electrolytic capacitor.
Q1 is mounted inside a tube and, for a more directional performance, you can include a convergent lens in the front of the tube.
To operate the unit, adjust the sensitivity using R1 to the dark level required to trigger the circuit.
IC1 - 4093 CMOS integrated circuit
Q1 - TIL414 or equivalent phototransistor
Q2 - TIP120 NPN Darlington power transistor
L1 - 200 mA to 1 A, 6 or 12 V lamp (see text)
R1 - 2,200,000 ohm or 4,700,000 Ω trimmer potentiometer
R2 - 100,000 ohm, 1/4 W, 5% resistor
R3 - 2,200,000 ohm - potentiometer
R4 -100,000 ohm, 1/4 W, 5% resistor
R5 - 4,700 ohm, 1/4 W, 5% resistor
C1 - 0.22 or 0.47 µF ceramic or metal film capacitor
C2 - 100 µF, 16 WVDC electrolytic capacitor
