Automatic fading lights

Suitable for 230V incandescent bulbs

Ideal for Christmas decorations

Circuit diagram:

Automatic fading lights


R1_____________470R   1/2W Resistor
R2,R3___________10K   1/4W Resistors
R4,R12__________22K   1/4W Resistors
R5_____________220K   1/4W Resistor
R6_______________2M2  1/2W Trimmer (Carbon or Cermet) (Optional, see text)
R7,R9,R14________4K7  1/4W Resistors
R8_____________100K   1/4W Resistor (See text)
R10______________1K   1/4W Resistor
R11____________Photo resistor (Any type, but see Notes)
R13____________470K   1/2W Trimmer (Carbon or Cermet)

C1_____________330nF  400V Polyester Capacitor
C2,C3,C4_______100µF   25V Electrolytic Capacitors
C5______________10nF  400V Polyester Capacitor
C6_______________4n7   63V Polyester Capacitor

D1,D2________1N4007  1000V 1A Diodes
D3_________BZX79C24    24V 500mW Zener Diode
D4__________Red LED   (Flat, rectangular types preferable, see text)
D5_____________DIAC   Silicon Bi-directional Trigger Device (Any type)
D6__________TIC206M   600V 4A TRIAC

Q1____________BC547    45V 100mA NPN Transistor

IC1___________LM358   Low Power Dual Op-amp

SW1____________SPST   Mains suited Switch


A lamp or, in many cases, a series of lamps such as those commonly used to decorate Christmas trees or shop windows, will make a nice effect if its luminosity will grow gradually and rather slowly from zero to maximum and then will decrease the same way automatically.
This circuit can easily get this lighting effect using a handful of common, easy to find components, and has been designed trying to avoid special purpose chips, bulky, heavy and expensive components as transformers and the like.

After many tests, it was found that, due to the relatively high current required by the Gate of the Triac, could not get satisfactory result with a direct driving of the Triac without using a power transformer.
The solution chosen, therefore, uses a straightforward lamp dimmer circuit, whose control potentiometer has been replaced by a photo resistor with a trimmer in parallel. The photo resistor is in close contact with an LED, whose light intensity is increased or decreased by means of an IC based triangular wave generator.

Circuit operation:

Due to the low current drawing, the circuit can be supplied from 230Vac mains without a transformer. Supply voltage is reduced to 24Vdc by means of C1 reactance, a two diode rectifier cell D1 & D2 and Zener diode D3.
IC1A and IC1B are wired as a triangle wave generator, whose output voltage at pin 7 varies smoothly from about 10V to 15V and vice versa. The duration of a complete cycle can be set between a minimum of about 5 - 6 seconds and a maximum close to 2 minutes through the trimmer R6.
A good visual effect is obtained with R6 set at about 1M so, you can use a trimmer of this value for R6 or you can replace R5 and R6 with a 1M fixed resistor. The values of R8 and R9 were selected in order to drive the LED through Q1 in the more linear way as possible.
The result is a very smooth transition of the lamps brightness, with no flashes, discontinuities or staggering.


The performance of a circuit of this type is influenced by a number of variables, mainly related to the wide tolerances of some parts, namely: LED efficiency, Photo resistor characteristics, dc gain of Q1, Triac sensitivity.
For this reason, R13 should be trimmed in order to have the lamp just off or, perhaps better, its filament barely glowing when the cycle of the triangular wave is reaching the lowest level.
R8 may also require an adjustment: in which case, replace it with a 100K or 220K trimmer.
Particular attention should be paid to the assembly of LED and photo resistor. The use of a rectangular, flat LED, greatly facilitates photocell coupling. In practice, it is enough to join these two components together properly and secure them with black electrical tape, being careful not to form small openings through which the external light could penetrate.