Light Dependent LED Intensity Controller Circuit.

The following post explains a simple ambient light dependent LED illumination controller circuit. The light fades or intensifies proportionately i response to the ambient light conditions. With brighter daylights, the LEd illumination gets softer and vice versa. The idea was requested by one of the dedicated members of this blog.

The Request

Dear Sir,

While seeking through the internet for a fully automated day/night LED Time Controller, I found your blog and I was wondering if you could help me with an advice. I want to add some kind of controller to give me smooth transition from sunrise/sunset of an aquarium LED lamp and with what I found on internet so far, seems way too complicated or just expensive for my goal.

I was looking for something simple, without the need of simulating a thunderstorm through an Arduino board with n channels I will never use. I want something that could light up some LEDs at a given time while fading other LEDs, all with a smooth transition. And this has to repeat twice a day, every day.

What do you say, can you help me?

The lamp I have, is the one from …
this configuration:

– for daylight

12 x Cree XP-G2 R5 – 6500 – 7000K

4 x Cree XP-G2 R2 – 2700 – 3200K

2 x OSRAM SSL80 Hyper Red

– for night time


All connected through 5 x KSQ 400mA (with the maximum of 6 LEDs in a row for each KSQ 400mA) to a laptop power adapter.

Now, I don’t know if my LEDs have dimming capability or I have to pass them through some dimmable drivers to obtain the desired effect.

Also, the systems I found so far, are all based on a Arduino and they seem bulky. ex. Neptune (Apex), Profilux, Reef Keeper, DIM4

So, that being said,
thank you in advance for any help.

The Design

The shown light dependent led controller circuit is basically a light dependent PWM optimizer circuit whose duty cycle varies in accordance with the potential difference or level at its cotrol pinout.

As can be seen the circuit includes a couple 555 ICs. IC1 is configured as a standard astable having a frequency of around 80Hz. This frequency is not crucial in terms of the circuit functioning.

IC2 is configured as a PWM generator such that it compares the square wave signal at its pin2 and the triangle wave across its pin6/7.

This results in an output with a particular PWM content at pin#3 of ICs.

However this PWM duty cycle can be varied simply varying the potential difference at pin#5 of IC2.

An LDR can be seen attached across a potential divider preset at pin#5 of this IC. The preset can be used for fine tuning the results as desired.

The LDR resistance level now determines and varies the potential across this sensing pinout which in turn results in a proportionately varying duty cycle at pin#3.

The varying duty cycle causes the connected transistors to conduct accordingly and produce the correspondingly varying intensities over the connected LEDs.

The two transistors are configured as inverters which ensure opposite responses over the LED sets connected across the collector of the respective transistors.