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Hello Instructables, Tarek here. It is my first instructable please consider this fact.

This is a project that I needed to do for my fish tank. I have a tank which is nicely cycled, aired and lit. But the problem is the duration of light which must simulate the natural lighting condition to ensure ambient environment.

Then my Polyphasic sleep habit. (I hate my sleeping habit)

I was hardly providing proper light and dark situation.

Few days back I suddenly come up with this idea to create a light dependent switch. Now my fishes are happy and I am happy too with my pillow and bed. ;)

The circuit:

The basic of this circuit diagram is not mine. I found it in many places online. I have just added a few components to make it work for my fish tank. So guys take whatever credits you have. I have just enhanced it and built it for me.

For the expert the design says everything. But for me it is not. So I am going to explain a bit and snow you how I make it to work.

Step 1: Components List

IC - 555 Timer IC (non CMOS)

Resistor -

R1 = 22K

R2 = 300K

R4 = 1K

R5= 220 ohm

LDR, LED

Ceramic Capacitor (C1) - 103j / 0.01uF

Capacitor (C2) - 10uF

Variable resistor (VR) - 100K

Transistor (T1) - BC 548

Diod - 1N4007

Relay (RL) - 12V 10A

Power Supply - 12V DC

Old CPU Power Supply (not functional)

Step 2: Mount on a PCB or Veroboard

Sorry for low resolution images. My phone is oldie.I did not have chemicals available in my nearest bazaar so I had to do it on a Veroboard. Careful when you mount the components. I have used a IC socket to enhance the safety of the IC.

At first I mounted and soldered these components and then used some jumper wires to connect them according to the circuit design. after some initial test I applied "100% aquarium silicone" which I had from a previous project. After drying out(silicone) for about 24 hours the jumper connections are now firmly attached to the board. I have also applied silicone to the Relay and attached it on the veroboard.

Careful when you install the transistor. Its pin configuration is explained in the transistor image.

The relay is a 5 pin SPDT (Single pole double through) which has two pin for the circuit and 3 other for the 220V AC appliance like CFL lamp or other low power consumption appliances.

Look for three pin aligned in a row (middle one = A is slightly misaligned). This middle one is the SINGLE POLE (= A). and other adjacent two (= 1,2) is for the circuit.

There are another 2 pin those are for DOUBLE THROUGH (= B, C). Normally when current is not applied to the relay Pin A and one of those double through (B or C) are SWITCHED. But when current is applied through the circuit the internal magnetic field changes the connection to the other one C or B.

That is how it works.

Step 3: How It Works?

Most of the light dependent switch I have seen on internet does not behave well during dusk or dawn. It is the time when those circuits cannot determine whether it is day or night. And they tends to flicker this time. Flickering is surely not good. It will greatly reduce the lifetime of the Relay and not to mention the light itself.

This particular design is free from such problems. It has 2 commemorator and untill a certain threshold limit the working of this circuit stays on a single state.

In the day: Light fall on the LDR causing its to provide a low resistance. so the electricity which can easily pass through the Variable and reach to the circuit. thus the circuit activates and send a signal to the transistor and the transistor activates the relay. choose your connecting pins so that when the relay is active it can on the light of your fish tank.

In the night: As light photon reduces in night the LDR become highly resistant. It do not allow electricity to pass so the IC do not get enough signal and stays off. Which turns off the relay eventually.

You can choose this circuits sensitivity by changing the Variable Resistance value.

Step 4: Final Assembly.

For Assembly I have used an old CPU power supply casing. This is the front side. You can see the LDR and LED for status indication.

I have installed it and it is working just fine. It responds to the room light condition. No need to switch on sacrificing my sleep, It automatically turns off and on. It is just awesome. Yahoo!!!!!!!!!!!!!

Step 5:

<p>Great use of salvaged parts, and good work on controlling the triggering for on/off, many circuits lack that positive shift latch.</p>
<p>Very nicely done!</p><p>Congratulation on sharing your first instructable too. I hope you share many more of your projects with us here! :)</p>
<p>Thanks</p>

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