Dusk - to - Dawn Light

This circuit turns a LED on when the peripheral light level goes under a specific amount.

Figure 1: Schematic of  Dusk to Dawn Light

Photocell (photoresistor or photoconductor) is

a variable resistor which its resistance changes

with peripheral light. When the ambient light

level is high, the resistance of photocell is low

and vise versa.Therefore, the photocell drop

voltage is small when the ambient light level is

Figure 2: Drop voltage of Photocell

high and vise versa. When peripheral light

level is high, the photocell resistance is

measured (5 k) by a DMM. V(diode) = 0.15 v,

V(base) = 0.08 v, V(photocell) = 041 v. So,

base voltage is lower than its threshold, and

BJT is off (Fig 2 & 3).

Figure 3: Drop Voltages of Diode and base

Hence, LED is in the turn off situation.

When peripheral light level is lower than a

specific level, the photocell resistance (20 k)

and its drop voltage increase. Also base voltage

(V(base) = 0.69 v) goes up and BJT will turn

on. So, LED will turn on.

Figure 3: A Real Circuit

Figure 4: A Video of the Circuit Operating