Hi There

I was wondering if some one could help me, I am building a very simple LDR circuit that turns on two led's when the light is shined on the LDR. I have based the circuit around a voltage divider,

http://www.doctronics.co.uk/voltage.htm

the LDR at the top has a resistance of about 2.5k when in light and the bottom resistor is 4.7k and it seem to work, on my multimeter i am reading just under 4v which is right for the LED's but there are not very bright especially compare to when i feed 3v direct from the batteries to LED's, i pruseme the difference is something to do with the amps but i've now a bit stuck about how to get the LEDs as bright as i want

Any help on this would be greatly appricated

Cheers

James

 

Have you tested the circuit's voltage with everything connected properly??

Another thing that you could try testing is increasing the voltage by another 1.5V cell.

Good luck,
Tanner

 

You need to use a transistor as a switch.

Here is one way you could do that:

The 2N3904 could also be a 2N4401, 2N2222, or a wide variety of general purpose NPN transistors.

 

Hi Guys

Thanks for replying, I have increased the voltage before and managed to fry one of the LEDs so i am a bit nervous about doing it again.

SgtWookie: Thanks for posting the circuit diagram, i have now followed it using a C548 transistor as i don't have a 2N3904 but will be buying one tomorrow, however it still hasn't fixed my problem as the led brightness isn't as bright in the circuit as attached directly to the battery. Do you think this is something i could fix or is it just one of those things ?

Cheers

James

 

I have increased the voltage before and managed to fry one of the LEDs so i am a bit nervous about doing it again.

You need to give us more information about the LEDs that you are using.
What is their typical Vf@current rating?
Normally, it's specified as something like 1.7v @ 20mA.
With the values in the circuit I posted, you should get around 16mA current flow through the LEDs if their Vf is 1.7v.

SgtWookie: Thanks for posting the circuit diagram, i have now followed it using a C548 transistor as i don't have a 2N3904 but will be buying one tomorrow, however it still hasn't fixed my problem as the led brightness isn't as bright in the circuit as attached directly to the battery. Do you think this is something i could fix or is it just one of those things ?

Like I mentioned above, we need to know more about your LEDs. They all have different specifications.

We have no clue as to where in the world you live. If you're in Europe or Asia, you may have a considerably different selection of transistors than we have here in the USA.

 

Hi SgtWookie

Again thanks for replying

The LED i am using is not you normal LED here's the spec

N24FN
Colour: Green
Forward Current max: 100mA
Forward Voltage max: 3.8V
Reverse Voltage max: 5V
Power dissipation: 114mW
Light output min. @ 20mA: 10000mcd
Light output typ. @ 20mA: 22000md

bought from here
http://www.maplin.co.uk/Module.aspx?ModuleNo=220752

I live in the UK, I am not sure exaclty what LDR i am using as i bought it quite a while ago but i think it a bit like this one but it measures about 2k when bright

http://uk.rs-online.com/web/search/searchBrowseAction.html?method=getProduct&R=0651507

Cheers

James

 

You increased the voltage by 1.5V and it fried an LED??

Might have been too much current?

The LEDs I play with can take a serious beating.

 

Rather embarassing i had the Transistor the wrong way - doh

It works perfectly now many thanks

Thanks

James

 

I do this often, so often in fact, I've drawn 3d pinouts for all my transistors, because I confuse the datasheet pinouts :S

 

Rather embarassing i had the Transistor the wrong way - doh

Don't be embarrassed. Most of us have done the same at one time or another. The few that haven't are liars and theoreticians.

 

Thanks Guys

I feel a bit better now

Cheers

James