light relay system

[greatpeople1]

hey every one
i am a bsc 2nd year electronics student and i neeed information about building light relay system i.e a circuit that automatically turns on lights when its dark

plz giv me detailed information and a probale circuit diagram that how it works
i shalll bei highly indepted to you all


thanx gr8

 

[ljcox]

Use an LDR or photo transistor, a Schmitt Trigger package such as the 40106 or 74C14, etc. and drive the lamp with a transistor - bipolar or MOSFET.

As an electronics student, you should be able to work out the details. If we do it for you, you won't learn as much as you will by doing it yourself.

Post your circuit so we can comment.

 

[mstechca]

An inverter might need to be included in your circuit, depending on the input and output polarities.

Use a CDS (photo resistor) cell and a resistor in series to make a voltage divider.

The center of the divider is the voltage output. Select a voltage that acts as the trigger voltage. Once that voltage is reached, feed it to the S input of an RS flip-flop (4013 IC). and the Q output will indicate that a certain amount of light has passed.

So experiment with a transistor, a resistor, a CDS cell, and a 4013 chip.

I'm sure you can figure out a solution especially if you are taking classes!

 

[ljcox]

You would only use a Flip Flop if you want to latch the output. If so, then you need a reset.

If latching is not required, then a Schmitt Trigger is the best solution.

I assume from his requirement that he does not want a latch, ie. when the Sun goes down, he wants the light to be switched on and when the Sun rises he wants the light to go off.

 

[greatpeople1]

well here m again

well i had found this circuit on this forum so thats where i got some idea how will i do with diagram :? but this circuit diagram is for turning a buzzer when light goes off cant same thing bei used for a bulb as i m only trying to make a model if an actual circuit is not possible

plz assist me as i m no goood in it i.e electronics and i have very basic knowledge of things

thank u all oh replied me back

 

[ljcox]

I can't open your .gif attachment.

 

[greatpeople1]

nd i cant even see it myself

 

[greatpeople1]

well



that is what i m taking as my basic circuit diagram its the first avaiable circuit on tht page nd i want to replace tht buzzer with a bulb what u say thanx in anticipation m really weak in it

:roll: :idea: :idea: giv me an idea will it work

 

[ljcox]

That circuit is a very poor design.

I'll design a better one for you, but I need to know what you want.

My understanding is that you want a light to come on when the LDR is in the dark and to go off when light falls on the LDR. Is this correct?

If so, will the light that is turned on shine on the LDR? If so there will be a feedback effect. And do you need a delay in switching the light on (or off or both)?

 

[Nigel Goodwin]

That circuit is a very poor design.

I'll design a better one for you, but I need to know what you want.

Yes it is poor, but no need to bother designing one (not that there's much 'designing' required), have a look at figure 12 at which does what you want. It can also be improved by adding a high value resistor between pins 6 and 3, this gives it a degree of hysteresis.

There's no need to use an ancient 741, any opamp will do!.

 

[ljcox]

I agree with Nigel. The circuit in the link is adequate. As he said some hystersis is desirable. What supply voltage do you want it to operate from?

 

[greatpeople1]

well thank u all very much

actully sir i dont have much requirment neither do i need any delay in it all i want is i should have a system that will turn on a bulb when its darkness i.e evening light comes in or its to dark
:? help me and is the circuit avaiable in fig 12 good enougf
i want it very simple and costeffective thank u all again

about the power supply i had built a 20v dc power supply so its avaiable to me :?

 

[greatpeople1]

well the powersupply i had made gives me variable output till 20volt dc and about feed back i will employ sipmle principle of putting the bulb upfront and then covering the remaining portion at bak of card board some what will minimize the feedback of light bulb 8)

 

[greatpeople1]

well will any body explain a bit of working of that circuit thts posted by negil :?: :idea:

what will hysteris do i know what hysteris is but who will it affect the circuit plz.... ! :idea:

 

[ljcox]

well will any body explain a bit of working of that circuit thts posted by negil :?: :idea:

what will hysteris do i know what hysteris is but who will it affect the circuit plz.... ! :idea:

I'll explain the circuit as drawn before introducing hystersis.

The threshold voltage is set by P1. Say, for example, it is set to 6 Volt.

With no light, LDR1 has a high resistance so the voltage at pin 2 will be low. Thus the output of the Op Amp will be high and current will flow through R4 into the base - emitter of Q1. Q1 will be on and so the relay is operated.

If the intensity of the light falling on LDR1 is gradually increased, the voltage at pin 3 will rise in sympathy. As it approaches 6 Volt, the output of the Op Amp will start to fall. Thus the current into the base of Q1 will reduce (as will the collector current) and as the light intensity increases further, the current will reduce to the point where the collector current falls below the relay release current and the relay will release.

When the light level is reduced, the relay will operate again once the voltage at pin 2 falls to about 6 Volt.

The point to note is that the base current (and therefore the collector current) changes slowly as the light level changes near the threshold level. So the threshold is not clearly defined.

This can be solved by introducing hystersis. Connect a 100 k resistor between pins 6 and 3. This provides positive feedback. Now when the voltage on pin 2 approaches the threshold, the feedback speeds up the change so the base current into Q1 changes very rapidly.

You can use an Op Amp such as the LM301 or whatever you can buy. Let us know which one is the cheapest and we will advise you if it is suitable.

The supply voltage will depend upon the voltage that the relay needs. The Op Amp will operate over a wide range of voltage. So if you have a 12 Volt relay, then make the supply 12 Volt.

 

[greatpeople1]

sir trhanx for ur selflessness and for helping me

 

[ljcox]

You're welcome.

An alternative solution would be to use a LM311 Voltage Comparitor.

See the attached data sheet. The data sheet shows many examples of how it can be used. You can operate from a single supply voltage of 5 Volt or greater.

Also, you could drive the relay directly from the LM311. See page 2 of the data sheet. Note that you don't need Q1. Just connect pin 6 to gnd. And you could connect a diode across the relay coil rather than use the Zener diode as in the data sheet.

 

[Nigel Goodwin]

An alternative solution would be to use a LM311 Voltage Comparitor.

Except it's hardly an 'alternative', the opamp in the previous link is configured as a comparator - so it's really just the same thing?.

 

[Misterbenn]

wait a min, didn't you say your a second year electronics student??
why do you need help with this tiny simple circuit, shouldnt you be doing this kind of stuff in you sleep.

i mean i'm only a first year electrical and mechanical student but this circuit seems piss easy. All you have to use are basic logic functions .....

 

[ljcox]

An alternative solution would be to use a LM311 Voltage Comparitor.

Except it's hardly an 'alternative', the opamp in the previous link is configured as a comparator - so it's really just the same thing?.

The point is that the LM311 can drive the relay directly whereas you need a transistor if an Op Amp is used.

And, with the Op Amp solution, you may need some level shifting to ensure the transistor is off when the Op Amp output is low (unless it is a "rail to rail" version).