Led project. Led on at nightime.

[Wynge]

Hi, I'm hoping someone can help.
Im looking to get 1 led on at nightime only.
But it can only be 15mm deep. Battery as well. Its for a wooden fridge magnet.
Is it possible to do?
First Post. Be gentle.

 

[betwixt]

Sounds like a pathfinder for midnight fridge raids.

When you say 'nightime' do you mean when it is dark or at particular clock times?

Brian.

 

[Wynge]

No it's simply a fridge magnet for a friend I'm making. The nightime was because I thought a sensor would be easier than a timer.
Thanks for your time.

 

[c_mitra]

First Post. Be gentle.

There is a saying that once a disease is known it is half cured.

So, let me paraphrase the requirement.

You want a visible LED on during night time (say 8pm to 8am), right? or you want the LED to be on whenever it is dark (that is a different problem)?

But you want the whole device to be 15mm thick, right? What about the other dimensions? You may have to use flat batteries like the ones used in cell phones.

But I am confused about "wooden fridge magnet"- all magnets are made of metallic (iron, cobalt, rare earth elements, nickel and some oxides)- or is that a regular magnet embedded in a regular wooden piece?

And you want the battery, magnet and the led all in the same wooden block? Not to talk about the electronics.

But the short answer is, Yes, It's Possible.

 

[Wynge]

Hi, thanks for the reply.
Yes it has magnets inbedded in the back. It's also slightly curved, like the fridge door.
The night time or timed is just for an easy way to illuminate the led. It's not important how it is achieved.
My electronics knowledge is basic. I work with 3 phase.
Thanks again

 

[Externet]

The motion activated types are gentler with battery life instead of the fixed dark-on night lights.
One of maaany on the market ----> **broken link removed**
A single LED garden light would need its battery recharged/replaced to stay on all night.
----> https://ez.analog.com/university-program/b/blogs/posts/hacking-an-led-solar-garden-light

 

[kripacharya]

Hi, I'm hoping someone can help.
Im looking to get 1 led on at nightime only.
But it can only be 15mm deep. Battery as well. Its for a wooden fridge magnet.
Is it possible to do?
First Post. Be gentle.

Should be possible. Here are some bits and pieces you would need --

* a rechargeable power source. Lipo batteries come in all shapes and small sizes down to 10's of mm sizes, and flat. With built-in charge circuits. These have an excellent energy density and a single cell's voltage can drive any LED directly. Here is one example.
* a 'nightime' sensor. This could be any photosensitive item. An LDR is the simplest and even available in 1206 sizes ! Some basic circuitry would be required for this detection. All components are available in very small SMD sizes. Here is **broken link removed**.
* an LED choice - very small SMD leds are available. Green, yellow and blue would use about 20milliwatts for decent brightness, while red and orange (?) even less. But it would be more fun to choose a large LED like these ones. Surely you could have the LED sticking out ??

With a 60maH battery @3.7v you could get around 200mW. Thats enough to power your led for upto 10 hours. A little less since there will be losses elsewhere.
Of course you would need to charge it every day - and that requires a connector. A really small one. And the appropriate charger.

here is a concept circuit drawing..

Hope this is enough food for thought to get you started ?

 

[kripacharya]

A few thoughts on the concept circuit --

The output of comparators is open-drain, so it requires a pullup. Say 10k.
Also the output drive will not be enough for the LED, so an additional NPN BJT (2n3904) needs to be used.

 

[betwixt]

For simplicity, I think I would consider a "Joule Thief" design and single 1.2V rechargeable cell. The on/off control needs be nothing more than an LDR either directly shunting the base to ground or via an additional transistor used as a switch.

Brian.

 

[kripacharya]

For simplicity, I think I would consider a "Joule Thief" design and single 1.2V rechargeable cell. The on/off control needs be nothing more than an LDR either directly shunting the base to ground or via an additional transistor used as a switch.

Brian.

Excellent idea to use an LDR + resistor divider to directly feed & bias a bjt which drives the LED. Some careful choices & experiments required to make it work, but yes the component count would go down a lot. One tricky bit would be to make sure the LED light does not fall on the LDR !!

In addition I did think to use a joule thief config, but decided against for a few reasons --

* it requires a (toroid) transformer + cap + diode + bjt + ...
* it is capable of draining it's battery down to 0.2-0.4v ! And since the power source choice was going to be a LiPo (best energy density/ size) then deep discharging a LiPo is not a good idea since on recharge they tend to overheat/ burn. Non LiPo very small shape-able rechargeable 1.2v cells don't exist - or do they ?
* LiPo is inherently 3.4 - 3.7v so capable of powering any colour of LEDs directly is feasible.

What did also occur to me was to why not put in a micro solar panel for automatic charging ? Now with that we could use a joule thief !! I think i have my next weekend project !!!

- - - Updated - - -

Oh great.

Turns out there is an ASIC which does all of the above in a single package, except for one external inductor. Takes all the fun out of this project.
Check out CL0116 and QX5252
Only thing is they work with cell voltages from 0.9 - 1.5v. So now the search is on for ultra small NiMh butto cells, or similar rechargeables.

 

[betwixt]

Those ICs are actually clever, they use an internal oscillator with inhibit input (for daylight detection) and a low Rds MOSFET output switch. All they need is a fixed resistor to make a potential divider with the LDR at the inhibit input and a small inductor at the output. I have some similar ones here with a different part number.

What I envisaged with the Joule Thief was a small (~8mm) toroid but no output rectifier or capacitor. An LED across the BJT will light on the back kick from the inductor. It is also possible to get small ni-cad button cells that would fit in the available space.

Maxim also make some ICs that will deliver quite high current in a tiny space from voltages as low as 0.7V. For example the MAX1747.

Brian.

 

[Wynge]

Im at work. Will get back later.
Many thanks for your efforts guys.
I don't really want to go down the recharging route.
But I did toy with the idea of saving battery by including motion sensor.
I'll read these through properly later.
Many many thanks for your knowledge.

 

[Wynge]

Hi guys I've had a read through and based on your ideas and thoughts I havemade the decision to double the thickness to 36mm. This should allow a much better battery choice.
I'm hoping this helps?

 

[betwixt]

The battery will still discharge eventually. You have to decide on how bright the LED has to be, noting that in general the brighter it is the faster the battery will drain, and how long it has to operate for. If you are within the Arctic/Antartic circles for example, 'night' can be six months long!

Does it have to be on all night? If not, you have to decide what turns it on other than absence of ambient light. Also, what turns it off again.

Motion sensing is an option but implies you also need a timer so it stays on for a while after first moving. You start to move into the realms of microcontrollers as the need to satisfy more conditions increases. If you are happy to do some simple programming, there are plenty of very low power consumption devices that work on low voltage you can use.

Brian.

 

[kripacharya]

Hi guys I've had a read through and based on your ideas and thoughts I havemade the decision to double the thickness to 36mm. This should allow a much better battery choice.
I'm hoping this helps?

At some point you yourself will have to do some work.