Timing circuit to turn off LEDs while minimizing power consumption for a kid's toy

Thread Starter

Jtreebs

Joined Apr 10, 2022
2
Hello,

I am designing a kid's wearable watch that has a button to switch on some LEDs. I want to have some sort of timing mechanism to turn off the LEDs after a set period of time to conserve battery life. I'll be using a couple CR2025s and want to minimize power consumption so the batteries don't need to be replaced for a long time (so I don't think a 555 is an option here?). Safety is my number 1 concern.

Any suggestions to design such a circuit? Any other advice related to safety of components would also be useful. Thanks in advance!
 

dl324

Joined Mar 30, 2015
14,468
Welcome to AAC!

It would be helpful if you had given more thought to the problem so you could include more relevant details in your question. That's probably why your thread has 20 views and no responses.
  1. How many LEDs? What color(s)?
  2. How much area do you have for this circuit? What package will the LEDs be in?
  3. If the circuit is to turn LEDs off, how are they turned on?
  4. How long do you want the delay to be? Is the delay time to be fixed or adjustable? How much current needs to be switched?
  5. Are the batteries going to be in series? Parallel?
  6. What is your definition of "a long time"?
Safety is my number 1 concern.
That won't be much of an issue with coin batteries. How old will the children be? Is this a product that you plan to sell?
 

Thread Starter

Jtreebs

Joined Apr 10, 2022
2
Thanks for the response. Sorry for the lack of information, I didn't realize it was necessary. Apart of taking circuits classes I'm quite a beginner.

I'm using a tricolor (red, blue, green) SMD LED that draws 2-10 mA per color in the 2-3 V range. These will be powered by the two CR2025 batteries hooked up in series for a total 6 V.

I'll set one color at ~5 mA using a resistor connected to the LEDs and will use a pot conected to some custom rotary mechanism to control the voltage through another color. Effectively I expect ~10 mA to be drawn when the LEDs are on. I'm considering adding a toggle switch to swap between two colors but I may just use 2 of the colors depending on how the final device looks.

Currently I'm using a DPST pushbutton switch (OFF - ON) which will connect the battery to each of the resistor-LED paths. The switch will be non-temporary (permanent?) so the LEDs draw current until some timing mechanism opens the circuit or the user pushes the button off.

I was thinking of putting this timing mechanism between the switch and the LEDs. Preferable it would open the LED circuit until the button was pushed two more times (i.e. pushed OFF then ON). The delay doesn't need to be adjustable by the user. I would like to set it so that the circuit opens after being on for 5-10 minutes.

I haven't finished the CAD yet, but I have roughly 1.5 x 1.5 cm^2 PCB area and 0.5 cm height to work with. These numbers are just approximate because I can move things around depending on the final circuit.

Children are 3 and I don't anticipate selling this product currently unless its a huge hit. Ideally I don't want the batteries to run out for months because I plan to make it difficult/annoying to access the interior of the watch because of the age of the children.

Thank you!
 

dl324

Joined Mar 30, 2015
14,468
I doubt that what you want is doable. The batteries will take up most of the volume you have to work with. Go ahead and post circuit questions you encounter, but don't expect anyone to design something for you if there's potential for it to be something you're going to sell.
 

BobTPH

Joined Jun 5, 2013
4,907
Why put the cells in series? You will get no more time out of them than you would out of one cell, unless you plan on using 3 buck converters, which you probably do not have space for.

Get the brightest LEDs you can find and they will work fine at 1mA.

Bob
 

Audioguru again

Joined Oct 21, 2019
4,688
Most of my solar garden lights I built and modified "blink" the LEDs to save power. The on-duration must be 30ms or longer to see the full brightness. Certain blinking frequencies between 5Hz to 30Hz cause a seizure in some people.
 
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