Automatic fading lights

Posted in LED & Light, on 2015-09-05

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 The solution chosen, therefore, uses a straightforward lamp dimmer circuit, whose control potentiometer has been replaced by a photo resistor with a trimmer in parallel.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.


  • The Photo resistor is not critical and almost any type can be used. However, please note that the small sized samples are frequently unable to withstand voltages higher than about 70 - 100V. Larger types are usually rated at 320V and should be fine.
  • If your mains supply voltage is 115 - 120V, you should make the circuit the following changes: the value of C1 should be doubled. Use two 330nF 250V capacitors wired in parallel, or a single 680nF 250V capacitor. Moreover, though not mandatory, the working voltage of C5 can be reduced to 250V and a 400V Triac can be used for D6 as, for example, the TIC206D.

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