Dark-Activated Flasher (ART035E)

You can use this circuit to turn on a warning lamp automatically at dusk and off at dawn, as described in other projects in this section. This circuit uses a photo transistor as the sensor.

In this project R1 adjusts the turn-on light level, and R2 determines the frequency of the flashes.

The lamp (or many lamps) can be controlled by the relay, but you can also use the circuit to control other loads such as appliances, motors, solenoids, and so forth.

The frequency can be altered by changing

A schematic diagram of the Dark-Activated Flasher is shown in Fig. 1.


Figure 1 - Dark activated flasher
Figure 1 - Dark activated flasher


The layout can be altered to suit the relay used. Any phototransistor can be used in this circuit.

Even a common power transistor such as the 2N3055, without its cover, can act as a sensitive phototransistor.

C1 determines the turn-on and turn-off rate as required by the intended application. See Project 124 for more information about this component.

The relay depends on the controlled load. A mini DPDT 1 A relay can be used to control small appliances and lamps up to 100 W.

IC1 - 4093 CMOS integrated circuit

Q1 - TIL81 or equivalent photo transistor

D1 - 1N4148 general purpose silicon diode

Q2 - 2N2907 PNP general purpose silicon transistor

K1 – 6 or 12 V relay (see text)

R1, R2 - 2,200,000 Ω - potentiometers

R3 - 100,000 Ω, 1/4 W, 5% resistor

R4 - 2,200 Ω, 1/4 W, 5% resistor

C1 - 0.22 µF to 100 µF ceramic, metal film, or electrolytic capacitor (see text)

C2 - 100 µF, 16 WVDC electrolytic capacitor


Circuit Bench