Need help - LDR circuit with rgb led's

Thread Starter

Carr1811

Joined Apr 4, 2017
11
Hi,

I am currently doing a project for my GCSE's which requires a LDR circuit which responds to a decrease in light. This part works. However the rgb LEDs do not function properly even with sufficient voltage, they are 5v led strip. Could some please tell me a solution to this or a circuit diagram that would work.

Thanks
 

Thread Starter

Carr1811

Joined Apr 4, 2017
11

That should be a link to the picture and these are the spec for the strip led
Specification:
Type: LED strip light
Material: Plastic
Waterproof: Yes, IP65
Chip: SMD 5050
LED Quantity:
2m:120
1m: 60
0.5m: 30
Light Color: RGB / Warm white (2800-3200K) / Cool white (6000-6500K)
Luminous Flux.: 80-100LM/W
Power: 2.5W/m
Beam Angle: 120 degrees
Voltage: DC 4.5V
Control Box Battery: 3*AA battery (not included)
Length: 2m/1m/0.5m (6.5"/3.3"/1.6")
Net Weight: 40-80g / 1.4-2.8oz
 

Thread Starter

Carr1811

Joined Apr 4, 2017
11
I just want to clarify that the LDR is functioning as it should, the circuit activates at low light and deactivates in high levels of light. However the issue is that the output, the rgb led circuit is not functioning as it should. It does work as i have tested it with 3 AA batteries and it functions normally then but as soon as i put it in the circuit it seems to not get enough power to change colours as stays red ( i assume it is power as the red led would require the least energy to power). I hope this sheds a bit more light on the situation (no pun intended)
 

AnalogKid

Joined Aug 1, 2013
8,231
How much current does the LED strip need? 3 AA batteries can make a lot more current than a 9 V battery. You might be overloading the battery. Or the transistor is not turning on correctly. When connected to your circuit, what is the voltage across the LED leads?

ak
 

Thread Starter

Carr1811

Joined Apr 4, 2017
11
Please could you tell me how to work out the current required for the led's. The LED's also only get 3-4 volts when the circuit is activated. It says on the website that they are 5 V LEDs.
 

djsfantasi

Joined Apr 11, 2010
5,849
What website?

I can only sigh when a TS assumes that we have control of his computer and can manipulate his browsing history (by the way, some seem to like "XXXVideo.ru").
 

Thread Starter

Carr1811

Joined Apr 4, 2017
11
Yes but the issue is that even with the appropriate 4.5 V of battery (3 AA batteries which the unit ran off to begin with) the circuit doesn't function properly within the LDR circuit.
 

djsfantasi

Joined Apr 11, 2010
5,849
Now I see the specs. The web url might have helped more, but I'll work with this for now.

First, I don't see where the LEDs are 5V LEDs.

We are still missing an important piece of info; at least I couldn't find it. How long is the strip? The numbers depend on whether it is a 2m, 1m, or 0.5m long.

But I'll base this example on a 1m strip and leave the calculations for other lengths for you.

A 1m strip is rated at 2.5W. And the recommended supply voltage is 4.5VDC. using the equation for power...
P = V * I
Substituting values...
2.5W = 4.5 V * I
I = 4.5V / 2.5W
I = 1.8A (or 1,800mA)
A 9V battery can't supply this. In short, I just cain't work

I looked up Aa batteries as well. Brand new, they might work. But they'll die quickly with 1.8A. Maybe twenty minutes?

The last possibility is the circuit itself. Test on a breadboard just the switching part of the circuit (base resistor and two transistors. Manually switch it and confirm that it works at operating voltage. Then add in the voltage divider.

Once you know that parts work, try with a 5A bench supply at 4.5V. Then finally, with the batteries.

Good luck
 

LesJones

Joined Jan 8, 2017
2,420
Hi djsfantasi,
You have made a mistake in rearranging the equation.
2.5W = 4.5 V * I
I = 4.5V / 2.5W
It should be I = 2.5/4.5
I = 0.56 amps (Which is still too high for the PP3 battery to supply)

Carr1811,
Can you measure the battery voltage when the circuit is lighting the LEDs. Also what is the value of the resistor between the LDR and the base of the transistor. It is not marked on your schematic.

Les.
 
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