LED Photoelectric switch

[brettofficer]

Hi All, I would really appreciate some help. I want to run an LED from a battery with a photoelectric sensor to turn the LED on at night. I envisage power would be from a couple of AA batterie. What components do I need to buy to make this happen?
Regards, Brett

 

[danadak]

Do you want it to smoothly dim or switch on / off at a specific ambient light level ?

LED color ?

Google "dimmable led circuit battery 555 timer", lots of hits.


Regards, Dana.

 

[Delta Prime]

Although the power supply says 9 volts the lowest this one could go reliably is 5 volts I would say four double AA batteries
Photoresistor (around 5kΩ in light, 200kΩ or more in dark)
Transistor NPN BC547,PNP BC557
Variable Resistor (potentiometer)100 kΩ
Resistor 390 Ω
Light-Emitting Diode LED

 

[kellys_eye]

Purchase a cheap garden LED ornament (solar powered) that has all the parts, including the battery, for a dollar or so. You can't buy the individual parts for anywhere near this bargain price.

 

[brettofficer]

Do you want it to smoothly dim or switch on / off at a specific ambient light level ?

LED color ?

Google "dimmable led circuit battery 555 timer", lots of hits.


Regards, Dana.

Hi Dana,
Just happy for it to turn on when it gets dark. It is a backlit for a clock to be visible at night so not 100% sure how bright it needs to be yet.
Regards,
Brett

 

[Bluejets]

so not 100% sure how bright it needs to be yet.

One does not need a lot of light at night ........even a single LED running @16mA as shown above would more than likely be too much.
It would certainly drain the 9v battery (assume 216 type) in no time flat.
Better to run off small plug pack.
Start out running the LED around 5mA and see......not difficult to set up a test circuit and see the requirement.

I'd tend to go with #4.

 

[kellys_eye]

Hi,
Thank you so much for that info. How long do you think the batteries might last? Also, what size resistor would you suggest?

Post #7 seems to be from an AI - ignore it.

The brightness could be an issue for you. LEDs don't respond well to having their brightness changed by using a simple resistor. Most variable brightness (for LEDs) is achieved using PWM - pulse width modulation - which adds complexity to your circuit. Use the schematic shown above and try changing R1 (increase its value) until you get something you like. Might be easier to use the schematic 'as-is' and put some paper over the LED if it's too bright.

There are such things as low-current LEDs that will increase battery longevity but changing R1 to 4.7k ohms might do as you require AND save the battery. Using two AA cells and assuming a couple of mA current consumption the batteries should last weeks.

 

[Delta Prime]

I got a horrible feeling you don't know what any of us is talking about. Would you like for us to go shopping for you, so you can just plug in your batteries something off the shelf you can buy at a store?

 

[Bluejets]

LEDs don't respond well to having their brightness changed by using a simple resistor.

New one to me....never had a problem running a 20mA LED on 5mA ever before.

 

[brettofficer]

Purchase a cheap garden LED ornament (solar powered) that has all the parts, including the battery, for a dollar or so. You can't buy the individual parts for anywhere near this bargain price.

Great idea. I think it would need a power source though. Has to run all night, every night for about 6 months. Maybe a phone charger and use cable. I wonder if they come in 1.5 volt?

 

[brettofficer]

I got a horrible feeling you don't know what any of us is talking about. Would you like for us to go shopping for you, so you can just plug in your batteries something off the shelf you can buy at a store?

You are correct. I don't have a very good knowledge of this type of thing, that is why I am on this forum, to get help. I am making an analogue clock for a bedroom that will light up at night

I got a horrible feeling you don't know what any of us is talking about. Would you like for us to go shopping for you, so you can just plug in your batteries something off the shelf you can buy at a store?

Hi Delta, you are correct in a way. I do know some things about what is being proposed, but that's why I am on this forum. I need to learn from the experienced and kind contributors. I don't know much about my question but am learning heaps from others. So don't go shopping for me but, thanks for the offer. Regards, Brett

 

[Delta Prime]

You are correct. I don't have a very good knowledge of this type of thing, that is why I am on this forum, to get help. I am making an analogue clock for a bedroom that will light up at night

Hi Delta, you are correct in a way. I do know some things about what is being proposed, but that's why I am on this forum. I need to learn from the experienced and kind contributors. I don't know much about my question but am learning heaps from others. So don't go shopping for me but, thanks for the offer. Regards, Brett

Shucks! I was going to get a shopping cart full of stuff.
That is the sweetest thing I ever heard ;you are precious!
thank you for your response.