# Fully Automatic Night Light

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4

Hi

When we talk about automatic Night lamp first thing comes to our mind is LDR (Light Dependent Resistor).

If we would like to go with LDR ,as it's resistance changes in proportion to light intensity the effective change in resistance is some what slow which results in relay chattering.No matter either you use circuit with transistors or with 555 timer ,we'll face the problem.

Instead , we can use solar panel itself as a sensor ,as the output from the panel is reliable and stable.

Pros:

1. No Relay chattering problem.

2. IC s are not required.

3.Less component count.

4.Either solar panel or power supply value is specific,you can go with any value of your choice.

4.Good isolation between AC and DC.

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## Step 1: Components

we need

1. Solar panel 10v,any wattage

2.Two general purpose Transistors like BC 547

3.2.2 Kohm-- 1

4.1 Kohm--1

5.10 kohm---1

6.18 kohm--1

7.100 uf----1

8.12v,7A relay

9.stable 12V power supply

10 ac LED lamp

## Step 2: Circuit

Connect components according to the circuit provided.

During the day time, solar panel produces voltage around 10 v,as this positive voltage is given to base of T1 it's going to on,T2 going to be off so the relay.

In the night time ,voltage at the base is "0".So T1 is going to be off,T2 goes into on state which in turn switches ON the relay .

The RC network at the base of the T1 provides the buffer time for the transistor for reliable operation of the relay.

## Step 3: Simulation Results

You can test this circuit by applying 10v and 0v ,when base is given with 10v realy should be in OFF state and it should ON in second case i.e with 0v.

This one tested with 10v solar panel & working to my satisfaction.

Note: 1.we need to make common negative terminal by joining both solar panel's and 12v power supply's negative terminal.

2.You can use any solar panel ,but you need to adjust resistor values at base of T1.

Thank you.

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## 4 Discussions

Hi there,

An LDR is not slower than a solar panel, the limiting factor for both is the slow changing ambient light. An LDR can easily detect several kHz.

Your numbered list of "Pro's" does not have much bearing with actual facts either.

"1. No Relay chattering problem."

The relay chattering issue could (should) be solved by adding a bit of hysteresis, by providing a bit of feedback. It is evident, that you have similar problems with the solar panel, since you had to add an RC buffer/filter. It is a design problem, pure and simple.

"2. IC s are not required"

Neither are they for an LDR ;)

"3.Less component count."

Simply not true, see attached schematic.

"4.Either solar panel or power supply value is specific,you can go with any value of your choice."

I am guessing here, but did you mean "Neither solar panel, nor power supply value is specific"?

That would be a stretch - better add a "Within reason" in front of that :)

"4.Good isolation between AC and DC."

The AC-DC isolation is catered for solely by the relay, not what components you use to drive the relay with.

The attached schematic cures your relay chatter problem, whether you use an LDR or a solar panel.

R1 (not needed with a solar panel of course) has to be selected from the LDR at hand and the value can be found by measuring the LDR at the amount of ambient light that you want the shift to occur at. Knowing that value, select R1 for an output from the R1/LDR node of around 0.9V. The actual value depends on the amount of feedback used, so could be made up of a resistor and a trimmer, to enable adjustment.

R_FB is what determines the amount of feedback and its value depends on what hysteris you need and to some extent, the values of R1 and the LDR.

Finally, an 1N4148 is a better choise for the relay, as it is much faster than the 1N400x family (A fast version of the 1N4007 is the UF4007, but 1N4148 is ample for most small relays with less than 100..150mA actuation current). In rare cases, an 1N4148 over the output transistor may cure any issues and an "upgrade" to BC337 will tolerate more current if needs be).

Have a nice day :)

2 replies

Hi Omivent,

Thank you for going through the post,

1. I've tried several circuits which involves ics like 555,transistors even traic like BT 136 which were build around LDR to have an automatic night lamp but none them hardly solved relay chatting/ ac bulb flickering problem even with delay circuit . If we consider night light using 555 timer which tuns on the relay once voltage is more than 2/3 of supply , it also has relay chattering problem.

As solar panel has more area exposed to outside world,it for sure reliable than LDR one.

2. Yes ,your are true right.I mean ics like 555 and opto isolators.

3.yes less component count.

4.we need to adjust resistor values accordingly.

5. yes ,isolation is because of relay only.I'll try your suggestion to go with 4148 instead of 4007.

your circuit needs a resistor at second base.

thank once again ,Have a nice day.

Hi again,

You do not need delay for such circuits, just hysteresis, which, in case you don't know, is a difference in upper and lower threshold - in this case made with feedback.

In plain terms, when the voltage is rising and reaches the upper threshold, the feedback adds a little, to get it into a more stable region faster and v.v for the negative going voltage.

.

"your circuit needs a resistor at second base."

No it doesn't!

Thinking about why you think that and what purpose you think that it would have, will likely teach you more than if I just told you, so I won't

Just consider what its purpose would be.

.

Turning it upside down, I can guarantee you that you have a base resistor on the second transistor that is doing nothing but eventually quenching the circuit a bit ;)

.

Have fun and learn :)

Very nice! Thanks for sharing.