Lighting Project

[PatrickH]

Hello Everyone! 

I'm currently doing my DT AS level and am at the beginning of my light project. I have had an idea for outdoor guide lighting which incorporates an LDR which will be used as a light sensor. I envision this lighting system being like the lights on a landing strip except on a path outside (2 strips, 1 on either side of the path)- once a certain level of brightness is reached the RGB LEDs will turn on. The light from the LEDs would be subtle and not too bright, just so that the path is clearly outlined with a soft light. Using what skills I have from physics and a little help from the internet I was able to find a simple potential divider circuit which would allow me to use the LDR as a light sensor and a transistor as a switch so that as it gets darker, the resistance of the LDR increases and as a result it's allocated voltage increase (in proportion to the resistance of the fixed resistor that is used) and once it reaches a certain value the transistor will turn on and the LEDs can light up (at least I think this is how it works).

For my research section I need to find out whether or not this idea is feasible and what kind of implications there might be I was wondering if perhaps I could get the input of this website's community as to whether or not you think this lighting system is feasible? I am also rather inexperienced in circuitry like this and have only ever constructed very simple circuits

Thank you, 

Patrick

 

[Sidewinder]

Look up Schmitt trigger & have a think about that first, see what you come up with and come back to us.

If we give you the answers, you'll never learn!

Welcome to the forum.

 

[PatrickH]

I've had a look at Schmitt Triggers and they seem daunting to say the least! As I said I'm not very inexperienced in more complicated circuitry like this (in comparison to the simple single bulb and resistor circuits I've done), however from what I understand, these Schmitt triggers seem very interesting. From what I do understand (which isn't much), once the input voltage reaches above a certain threshold, the comparator in the trigger turns to, lets say, State A, then once it goes below that threshold it returns to it's original state until the threshold is reached again- everywhere I looked there was lots of words not currently in my vocabulary so this is all I could extract from it... I imagine the LDR would influence the input voltage and once that voltage reaches above a certain level (once it is dark enough) the input voltage will be high enough to change the comparator to State A- which causes the voltage to be output at a certain level?

 

[Sidewinder]

They act as a voltage dependent switch, so you are right in your description.

Once the voltage moves between a closely defined threshold they switch on / off.

To use an LDR to control lights, you would need some sort of defined switch.

The Schmitt Trigger could be used to control this switch.

Think about what you could use to control the lights in the form of an electrically controlled switch.

You will NEED something like a Schmitt Trigger device to operate this switch, you can't "just" rely on the changing light to change the voltage, at some point you will need a step / snap change.

Think about it.

If you are doing D&T at As / A level then these are the sorts of challenges you will face, and especially if you move on to Undergraduate study, trust me, been there done that!

 

[PatrickH]

I see! I will definitely include this in my research and consider using a Schmitt trigger in the making part of my project; however doesn't a transistor essentially do the same thing, just without an upper threshold? And the required voltage for a 5m strip of LEDs is 12V, would this be okay as an output for a Schmitt Trigger?  

 

[Sidewinder]

You should be looking for a clean repeatable switching threshold should you not?...

Have a look at what a Schmitt trigger is made up of.

 

[Technician]

Schmitt triggers are used in logic circuits to "square up" a logic pulse.

Where the logic pulse does not have the required rise time, they can

be used to give the next gate the required rapid rise time of the leading

edge.

Consider it yes, but try and think of a way of using a photo-resistor in

such a manner that it controls the base current of the transistor.  In this

way you could eliminate the clumsy potential divider.

I already have a sketch but...............that would not be your work.

 

[PatrickH]

Schmitt triggers are used in logic circuits to "square up" a logic pulse.

Where the logic pulse does not have the required rise time, they can

be used to give the next gate the required rapid rise time of the leading

edge.

Consider it yes, but try and think of a way of using a photo-resistor in

such a manner that it controls the base current of the transistor.  In this

way you could eliminate the clumsy potential divider.

I already have a sketch but...............that would not be your work.

Thanks for the reply Technician,

I think I understand what you mean! I would really like to have a look at this sketch if possible- it would definitely be helpful in trying to get my head around this. At this point in my research, a lot of what I'm doing is gaining influence so if I could see a sketch that is specifically for this type of application then it would really help me in the long run (and it shows that I actually did talk to a specialist). 

Thank you again,

Patrick

 

[Sidewinder]

I disagree with Technician on the application of a Schmitt trigger, they are used in many and varied applications, and often used as a device to change a varying analogue voltage to a digital signal at a certain value.

Look at what they are and how they are constructed.

Also look at Darlington pairs & the different kinds of transistor and their characteristics.

Try to design with modules rather than components.

 

[Bruspark]

You might want to start by thinking about all these LEDs which will either need a switch for a fairly high voltage if in series or a reasonable total current even if you run them at only 3- 5 mA each for a glow. (Don't forget limiting resistors ). High voltage switching isn't ideal for transistors or DIY so assuming you need to switch 100ma or so at 12v you will find a contradiction between the low current in your resistor + LDR pd to not disturb it and the need to supply several mA into the transistor base so base current x dc gain = switched current x 2 or 3 ( a half on transistor is a hot transistor) A Darlington circuit helps sort

this problem as would using an op amp or power FET but you could also put a small reed relay in the collector circuit and thus switch the light circuit separately and more efficiently immediately.

 

[PatrickH]

I disagree with Technician on the application of a Schmitt trigger, they are used in many and varied applications, and often used as a device to change a varying analogue voltage to a digital signal at a certain value.

Look at what they are and how they are constructed.

Also look at Darlington pairs & the different kinds of transistor and their characteristics.

Try to design with modules rather than components.

I have had a look at the Darlington pairs and I think I understand their purpose; I had no idea that I might need to amplify the signals for my circuit to work. This information is definitely really helpful in my project! 

You might want to start by thinking about all these LEDs which will either need a switch for a fairly high voltage if in series or a reasonable total current even if you run them at only 3- 5 mA each for a glow. (Don't forget limiting resistors ). High voltage switching isn't ideal for transistors or DIY so assuming you need to switch 100ma or so at 12v you will find a contradiction between the low current in your resistor + LDR pd to not disturb it and the need to supply several mA into the transistor base so base current x dc gain = switched current x 2 or 3 ( a half on transistor is a hot transistor) A Darlington circuit helps sort

this problem as would using an op amp or power FET but you could also put a small reed relay in the collector circuit and thus switch the light circuit separately and more efficiently immediately.

I believe the LEDs are wired in parallel but I as I said earlier I am rather inexperienced in circuitry so I only understand a bit of what else you said. The voltage supply needs to be 12V however it is supplied by a transformer which goes into the mains. I am unsure of how I might construct a circuit using a Darlington pair if I have a circuit already built which only has a transformer in it and what kind of implications there might be on it if I am making alterations to how the circuit functions- would I have to strip the wires coming out of the transformer and then start my circuit from there with that being the power supply?

 

[Bruspark]

Use a suitable relay to switch the 12V from the transformer to the LED's. Check the transformer rating e.g. 12V DC 1A  to determine if the relay CONTACTS need to be e.g 1 A DC switching.    The relay COIL should be rated 6 - 8V DC if you are running the transistor part of the circuit off 10 - 12V DC.     Drive the relay using a Darlington pair or Power FET as above.  The Darlington pair and the Power FET each present a much higher input impedance (i.e. lower load) to the LDR and resistor than would a single transistor so they can operate properly over a decent voltage range.   There are some single IC solutions for all this aimed at the solar garden light market but I think you need too much current to use these.  Don't be afraid to experiment with the low voltage circuit . .   theory only takes you so far 

 

[Technician]

 and often used as a device to change a varying analogue voltage to a digital signal at a certain value.

Yes;  that is absolutely correct.   I cannot argue with this.  There are applications where

a function within a control system is completed with contacts on relays.  Sometimes

there is contactor bounce.  The trigger works to ensure that the logic gate receives one

pulse rather than several.  Another vital function is the elimination of noise in digital control

systems.

Darlington pairs used to be called "Super Alpha connections".  That is old speak for increasing

the value of Hfe or current gain of a single device.

 

[Technician]

 I would really like to have a look at this sketch if possible

If you imagine a divider network connected to the base of a transistor,

measure the change in resistance with incident light on a light dependent

resistor and then think of what would happen to the base emitter voltage

of the device as the light intensity changes.