A basic LDR circuit?

[JohnnyB]

I'm of a VERY novice standard when it comes to electronics, knowing very little more than a man in the street. Hopefully someone might be able to help me out here. My idea is...a series of LDR's behind a sheet of say, 8inch by 10inch perspex (or similer). The LDR's would be set to respond to light, should one (or more) be exposed to a bright light (even very briefly) this would register on a seperate unit by way of lighting up an LED. For example, during the course of an hour my perspex 'wall' was introduced to a bright light 5 times, I'd know because of 5 lit LED's on my box. The reason for the box is to house most of the components and to keep my 'wall' to a minimum thickness. I believe a Potentiometer would be used to adjust the sensitivity of the LDR's but as I don't want them to react to any ambient lighting (just a strong direct light) I thought maybe 'tinting' the perspex would help, this obviously would be dependant on the LDR's and their sensitivity. If anyone has any ideas on IF/how this would work, I would be most gratefull, I figure the circuit would be quite basic, I just don't know much (anything) about circuits.

 

[ericgibbs]

I'm of a VERY novice standard when it comes to electronics, knowing very little more than a man in the street. Hopefully someone might be able to help me out here. My idea is...a series of LDR's behind a sheet of say, 8inch by 10inch perspex (or similer). The LDR's would be set to respond to light, should one (or more) be exposed to a bright light (even very briefly) this would register on a seperate unit by way of lighting up an LED. For example, during the course of an hour my perspex 'wall' was introduced to a bright light 5 times, I'd know because of 5 lit LED's on my box. The reason for the box is to house most of the components and to keep my 'wall' to a minimum thickness. I believe a Potentiometer would be used to adjust the sensitivity of the LDR's but as I don't want them to react to any ambient lighting (just a strong direct light) I thought maybe 'tinting' the perspex would help, this obviously would be dependant on the LDR's and their sensitivity. If anyone has any ideas on IF/how this would work, I would be most gratefull, I figure the circuit would be quite basic, I just don't know much (anything) about circuits.

hi,
Have look at this link
**broken link removed**

If you Google for 'LDR circuits' you will find lots of ready mades.

 

[JohnnyB]

Cheers, I've seen that page before, but like I said, I'm not very knowledgeable about these things, in that particular diagram, I can't tell where the power is coming from, what the green block next to the LED is, or what transistors are!! Would I at least be right in saying that if I wanted to use about 40 LDR's they'd all have to be connected in series?

 

[TrevorP]

Well as far as I know an LDR is a Light Dependent Resistor. Therefore something like the following circuit is somewhere to start.

What happens is that as the ldr gets more light it's resistance decreases meaning that more voltage is available to the transistor. The more available the more the LED is going to light up. I'm not sure how you're going to make it ONLY react to bright light but you might have to adjust it so that the LED only lights up after a certain brightness.

EDIT: Diode is supposed to be LED.

 

[JohnnyB]

that would be the idea, the LED wouldn't light up at lower voltages but when a much brighter light shone on the LDR, the higher voltage would switch it on! I don't understand the transistor though, what does that do?
Just to add...I know these are silly questions to guys of your knowledge, so I do really appreciate your taking the time to answer them, Cheers.

 

[TrevorP]

Well Transistors can actually be quite complicated but basically the current received at the horizontal node is affecting the resistance between the vertical nodes. Google transistors. There is still stuff I don't understand about them.

 

[JohnnyB]

Cheers, got some good stuff off Wikipedia...I've still a lot to learn though. I am now thinking of a digital counter instead of LED's to register how many times the LDR's have been hit by light, but methinks I'm just making life harder for myself!
I suppose I'll have to consider my power suply in relation to what resistors and the like to use. (this is to be a portable device, probably using AA batteries)

 

[audioguru]

The LDR that turns on a transistor that amplifies the current enough to light an LED doesn't have a memory. So when the light is gone then the LED immediately turns off.

It would take many parts to make a memory that remembers for 1 hour.

 

[TrevorP]

The LDR that turns on a transistor that amplifies the current enough to light an LED doesn't have a memory. So when the light is gone then the LED immediately turns off.

It would take many parts to make a memory that remembers for 1 hour.

Yeah I know I misunderstood what he had first written. What he wants is a hell of a lot more complicated.

 

[Stellarcore]

I've been reading about SCRs a bit recently (oddly enough I've gone through 4 years of computer engineering and we've never covered SCRs), and it would seem to me that it should be possible to make a slight modification to that LED circuit so that the LDR triggers the SCR which remains latched so that the LED stays on until reset.

Then you'd just setup one LED for each LDR and it should do basically what he wants. A counter would be a little more complex, but should be possible with a little more thought.

 

[audioguru]

SCRs would be perfect memories for the LEDs.

 

[JohnnyB]

Thanks for your thoughts on this people, seems I've jumped right in at the deep end here, so I appreciate your suggestions. I'll continue my research, but if anyone feel's to offer more info it's all greatfully recieved.

 

[Gayan Soyza]

Cheers, got some good stuff off Wikipedia...I've still a lot to learn though. I am now thinking of a digital counter instead of LED's to register how many times the LDR's have been hit by light, but methinks I'm just making life harder for myself!
I suppose I'll have to consider my power suply in relation to what resistors and the like to use. (this is to be a portable device, probably using AA batteries)

Take an old working CALCULATOR remove its back cover & solder or fix two wires to the EQUAL BUTTON “=” joints.

Take your LDR circuit (light or dark dependent one) take wires from the output LED or RELAY output & connect it to the EQUAL BUTTON joints wires (Relay output is better).

Power up your circuit also the calculator.

Press “1” key & press the “+” key. Finally press the “=” key now it’s ready to count up.

Give input to your LDR now you can see the number counts up one by one according to your input.

Note: some calculators wont count up when you press the “=” key after entering a number & + key but most do.

 

[Misterbenn]

really people come on here!

this is simple digital electronics - what you what is a variation on a circuit i made just last month.

Firstly get yourself some schmitt NOT (inverter) logic gates ....now stay with me here! if you dont know what a NOT gate is its very simple, whatever (logic) input you give it will output the opposite.
so give it logic 1 (+5v) and it outputs 0 (0v)
give it 0 it gives you 1

The schmitt bit is slightly more complicated but not much, this simply means that there is a deadband between the on / off switching levels. look up hysteresis! put simply if you apply a signal of greater than say 4v that will be seen as a logic 1 input so 0 output.
now your voltage falls to 3.5V nothing changes your in the deadband,
fall to 3V and thats seen as logic 0 giving you output 1
rise to 3.5V and nothing changes because your in the deadband again!

one such gate you could use would be.... a 40106 (look up 40106 pdf in google)

Also you want a decade counter, with this if you apply a pulsed input it counts up to 10. your pulsed input will come from the combined output of all your not gates.

Right now you create a potential divider using a resistor and your LDR, with values selected such that when its dark the output is logic 1 and when its light logic 0. conect to one of the NOT gates input, and conect the output to your counter input. use ten LEDs on the outputs of the counter.
after testing this first one, repeat the same resistor / LDR circuit and conect to the other NOT inputs.

jobs done!

if you want a circuit diagram and values. let me know and i'll c what i can do. you should take a look at it yourself first tho.!

 

[audioguru]

NOT gate?
A NOT gate is not a gate. It is a logic inverter.

 

[JohnnyB]

Thanks MisterBenn, at least I know that I have a do-able project here. I' ve been reading up on as much as I can without getting myself too confused. At the very least, I'm comfortable with the theory behind what your saying, so I have a start point. As soon as the doctors have finished carving me up I'll be able to get started in ernest. May possibly be coming back for that diagram you offered...I'll see how I go, cheers again.