LDR 12V circuit design

[John Tsolomitis]

Hi all, newbie here.

I bought one of those cheap $2 solar powered garden light (to play around with) and as you might guess the light that comes off it is almost non existent. I want to use the casing and wire up a 12V bright LED with an LDR so it can come on at night. I am planning on using around 10 - 15 of these and run them off a PC power supply or simillar.

I was wondering if someone had a little curcuit design on how to do it or quickly draw something up for me with any recommendations that you may want to share.

The LED's I'm planning on using would be the T10 casing type mainly used for automotive applications.

Thanks in advance

John

 

[D.A.(Tony)Stewart]

You could even use the PV sensor to drive an NPN to disable a self biased Type N FET to switch the LED from the low side.

 

[BrunoARG]

There are cheap voltage comparators such as LM339 (check for less parts per package ICs) which can compare a voltage given by the LDR sensor (with its proper resistive voltage divider) and another set by you (with a preset maybe).

If you don't know your LDR value, measure its resistance at the time you want it to make the device to turn on. Make the voltage divider with a resistor of aproximately that value (that would get VCC/2 at the time you want).

 

[John Tsolomitis]

There are cheap voltage comparators such as LM339 (check for less parts per package ICs) which can compare a voltage given by the LDR sensor (with its proper resistive voltage divider) and another set by you (with a preset maybe).

If you don't know your LDR value, measure its resistance at the time you want it to make the device to turn on. Make the voltage divider with a resistor of aproximately that value (that would get VCC/2 at the time you want).

Guys I know bugger all about electronics so what you're telling me goes through one ear and out the other. All I need is a scematic of something that will work with component values. I have no problem reading scematics and building it

Thanks

John

 

[betwixt]

No need to complicate it with ICs and the likes. The lights were solar powered originally so they already have a light sensor in them (the PV panel).

I wouldn't recommend a 12V LED is used, it would probably be too bright and would almost cetainly get too hot. At it's simplest, connect the 5V output of the PSU to all the existing battery connections of all lights in parallel and in each one, change the existing LED for a 'super bright' equivalent or maybe two LEDs in series.

If you really do want to use 12V LEDs, consider the extra metalwork you would need to make to dissipate the heat and the thickness of power cable you need to link each of the lights together. It probably isn't worth the effort. Also consider the possibility of building ONE controller then distributing power to each light, it would use far fewer components than duplicating it in each light body.

Brian.

 

[John Tsolomitis]

I think the battery that it is the units now is a 1.2V rechargeable. Brian you mentioned using the 5V line form the PSU. Does that mean the LED's that are used are 5V LED's? If I get brighter LED's to replace the existing ones, what voltage LED's should I be looking for?

 

[John Tsolomitis]

Brian, I was thinking of using this https://www.ebay.com.au/itm/12V-Pho...Electrical_Test_Equipment&hash=item5d4dd29c49 to control all the lights rather than one for each, as you suggested.

 

[BrunoARG]

Make sure that the solar panel output voltage can turn the led on. You should use the panel as sensor as betwixt said.

 

[betwixt]

John, the first thing to realize is LEDs are not rated by voltage but by current. When a LED is described in adverts as being 12V or 5V it doesn't mean that is the voltage you supply them with, it refers to the voltage they maintain across themselves when allowed to pass a certain current. If you connect a voltage source directly across an LED in most cases it will instantly burn out, you have to start with a voltage a little higher than the LEDs voltage then deliberately starve it of current to keep it within it's operating limits. It is in the starved state that the rated voltage is specified.

In normal solar lights, the LED typically has between 2V and 3V across it and a small circuit is used to boost the battery voltage from about 1.2V up to the LED voltage while also limiting the current it can draw. They have a simple inhibit circuit on them that kills the LED power while there is voltage on the solar panel wires. When it gets dark enough, the panel stops producing voltage and the inhibit is lifted so the light comes on.

Yes, you can use the board in the advert but all it will do is switch on the power at a predetmined light level. You can use it to turn the 12V on before wiring it to each of the modified lights. Your problem is that in each of the lights you plan to use a high power LED to get more light and for each of those LEDs you need to limit the current they are allowed to draw. The LED will get hot and unless you use some fairly advanced power regulation design, the current limiter will also get hot.

I'm not trying to deter you from trying, just point out that LEDs work quite differently to conventional light bulbs. A conventional bulb gets brighter as you supply it with more voltage, an LED does nothing up to a certain voltage then suddenly turns on at full power, and burns out, unless you restrain it from doing so.

At it's simplest, you wire a resistor in series with each LED to limit the current it can pass. That may be all you need. The calculation of it's value is simple:
1. Pick the current you want the LED to pass (use manufacturers data and drop it to add a safety margin). The figure will probably be in mA. For example if it is rated at 500mA, decide on maybe 450mA to actually use.
2. On the data sheet it will have a figure for "Vf", this is the voltage the LED will try to hold across itself.
3. The resistor value is: (your supply voltage - Vf) / chosen current in Amps. The result will be in Ohms. For example with a 12V supply and Vf of 3V at 450mA the calulation is (12 - 3) / 0.45 = 20 Ohms.
4. The resistor power rating has to be calculated next, the formula is (voltage across the resistor) * (Current through it). So using the same example, the resistor drops 12V down to 3V so it has 9V across it and 0.45A is flowing through it so the power it dissipates is 9*0.45 = 4.05W. You would pick one with a higher power rating than it actually dissipates, in this case I would suggest at least 5W.

So using that example you would buy a 20 Ohm, 5W resistor and wire one in series with each of the LEDS. As I pointed out though, the total power dissipated in each lamp will be that 4.05W plus the heat from the LED itself (~1.5W) so it will get quite hot and be wasting a lot of power.

Brian.

 

[John Tsolomitis]

Thanks Brian,

So if I was to keep it simple (correct me if I'm wrong) I could just buy a brighter LED like this --> https://www.ebay.com.au/itm/50x-5mm...Electrical_Test_Equipment&hash=item27cb8750c1 and hook it up to 5V line and hope it doesn't blow either straight away or within a week.

An idea I had which will probably eliminate all the heat issues and calculations etc, would be to use 12V LED strips (should be bright enough) that I'm already using in my car. Something like this https://www.ebay.com.au/itm/12V-Coo...22011?pt=AU_Lighting_Fans&hash=item4d0e79729b

I would supply 12V to it and the only other thing I'm not sure it will work will be the already built-in sensor for automatically turning the lights on at night. If it doesn't, is it is simple as just connecting an LDR? How?

I have some of that LED strip, I might give it a go.

Let me know what you think guys

John

 

[Audioguru]

The white LEDs in your ebay link have a voltage of from 3.4V to 3.6V and says their current MUST be limited to 20mA By Your Circuit. They will burn out instantly if you feed them 5V.
They are bright probably because the case focusses the light into a very narrow beam.

The LED strips need to have the connectors they sell.
Many solar garden lights do not have an LDR. Instead they detect the loss of voltage from the solar panel to connect the battery to the voltage stepup circuit to light the LED.
If you find an LDR then you must find a circuit to use it with.

 

[John Tsolomitis]

I can cut and solder the LED strips with wires on them so I won't need connectors. The shortest strips you can cut them will give you 3 SMD led's which is plenty bright enough. I just have to stick them in a way that they will distribute the light evenly and will be concealed (mostly) so you can see them through the clear lens of the garden light.

What I need to know from you guys is, if I do use the 12V led strips, can I still use the solar panel to detect light and turn on / off on their own? If so, how will I hook up the 2 wires from it to my LED strips?

 

[Audioguru]

Buy the ebay light controller and a 12VDC power supply to light your LED strips when it gets dark.
If you use an LDR or a solar panel then you need somebody to design and make a circuit to do it.