Driving 6 White LEDs with 4 AAA Batteries

[DSchafer]

I have a project I was messing with that will use 4 AAA batteries (1.5V each so 6V in series). I will have a PIC10F200 that will turn on 6 white LEDs (20mA, Vf ~ 3.2V). I haven't ever used alkaline batteries so wasn't sure what the best way to do this would be. I am a little space limited. The PIC will flash the LEDs on for about 4 seconds when a trigger occurs and then shut them back off. I would like the device to work for about 2500 triggers without the LEDs losing too much intensity.

I could just drive the LEDs directly from the battery with a 140 series resistor for each to start the current at about 20mA, but not sure how fast the batteries will degrade or how much the voltage can vary from the 1.5V.

I could use a 3.3V LDO and a 5ohm series resistor to drive the LEDs, but not sure how long the batteries would last since the efficiency would be bad starting at 6V input.

I could use a buck regulator, but requires more room and parts/cost. Also not sure if there is any issue with running these from AAA batteries and supplying the initial current when turning the LEDs on.

Bottom line I have never done anything with alkaline batteries and wasn't sure what the best way to go about this is. I might be overthinking it and can just use the first option just fine but wanted to see if anyone here had any experience and suggestions on a cleaner/better solution

 

[JonSea]

Running the LEDs directly from the battery should give a fairly uniform brightness. The voltage over most of the battery range is almost flat.

 

[MikeMl]

Most LED headlamps, mini-flashlights, bike lamps seem to do it with three AAAs. Most White LEDs have a Vf of ~3.2V. Likely there is a resistor in there somewhere?

 

[crutschow]

For a total drain of 120mA from your 6 LEDs, the total drain from the battery for 2500, 4-sec. triggers would be .12*2500*4 = 1200 amp-sec or 1200/3600 = .33 Ah.
An alkaline AAA battery has about 1 AH of capacity at 1V end voltage, so the battery would be drained of about 1/3 its capacity for your 2500 pulses, at which point the battery voltage will likely be about 1.3V.
That will cause some dimming of the LEDs with a resistor to control the current, but may not be that objectionable.
Try it and see.

 

[DSchafer]

Most LED headlamps, mini-flashlights, bike lamps seem to do it with three AAAs. Most White LEDs have a Vf of ~3.2V. Likely there is a resistor in there somewhere?

I was curious and did take a mini flashlight apart that used 3 AAA batteries and they just tied the 8 LEDs it had across the batteries directly with no resistors or anything. I did find an article that said this is pretty common with some of the chinese products and while they are overdriving the LEDs they seem to have few failures. I didn't want to do this though and risk anything burning up since this would be something for the kids to play with.

 

[DSchafer]

Thanks for all the help.

From the curve it seems to drop from 1.5V fairly quickly, so maybe I will size the resistor for 1.4V for each and use 120 ohm instead which will drive the LEDs at about 23mA until the voltage drops and maybe give me a longer brightness. The absolute max current is 25mA.

Looking at LEDs I was going to use either 3mm or 5mm for this and wanted the brightest white I can get. If I look for the highest millicandela rating is that right or is there something else I need to consider?

 

[MikeMl]

/... If I look for the highest millicandela rating is that right or is there something else I need to consider?

Field-of-view dispersion angle.

 

[JonSea]

Are people looking at the LEDs or are they lighting something up? If people will be looking directly at them, diffused LEDs are the way to go.

 

[DSchafer]

Are people looking at the LEDs or are they lighting something up? If people will be looking directly at them, diffused LEDs are the way to go.

They will be inside an enclosure that is standing up like a bottle and the pcb with the LEDs will be on the bottom and the enclosure will be like a translucent red or blue, so basically when outside during the day I want to be able to see the unit lighting up when triggered.

 

[ronsimpson]

For a total drain of 120mA from your 6 LEDs, the total drain from the battery for 2500, 4-sec. triggers would be .12*2500*4 = 1200 amp-sec or 1200/3600 = .33 Ah.
An alkaline AAA battery has about 1 AH of capacity at 1V end voltage, so the battery would be drained of about 1/3 its capacity for your 2500 pulses, at which point the battery voltage will likely be about 1.3V.
That will cause some dimming of the LEDs with a resistor to control the current, but may not be that objectionable.

Thank you Crutschow. We know there are good batteries.
I found AAA batteries at 1.3ah, 1.0ah and 0.53ah.
I have done this circuit several times.
Assume 1ah batteries.
1)I might take the 6V and buck down to about 4V. This would extend the batter life. But I don't see that you need to do that.
2)I might drop back to 3.0V batteries. (two AAA) Then boost up to 6.4V to drive two LEDs in series. For a boost PWM to boost up 2X (about) it will take 2X more current.
(3x2 LEDs)=6.2V at 60mA for the LEDs (did not say that clear...{2 in series + 2 in series + 2 in series}
The PWM will draw approx 150mA at 3V to make 60mA at 6.4V.
Most of the little 4pin and 6pin PICs I used run in the 2 to 3V range just fine.
The PWM will use up 0.5 to 1 square inch but you can use 1/2 the batteries.

 

[ronsimpson]

Here is a very small IC that could take three AAA batteries and boost up to drive all 6 LEDs in series.
The battery voltage will start out at 4.5 V and run down to 3.0V. This range is good for many PICs. The PIC can drive the shutdown pin.

 

[DSchafer]

Here is a very small IC that could take three AAA batteries and boost up to drive all 6 LEDs in series.
The battery voltage will start out at 4.5 V and run down to 3.0V. This range is good for many PICs. The PIC can drive the shutdown pin.
View attachment 105365

Thanks for the info. I will look into trying this. I couldn't find a 3 AAA battery holder that would fit where I needed it so went with the 4, but will see if I can get one to work.

I had assumed I could run the PIC off the batteries 6V, but see that it recommends 5.5V max with absolute at 6.5V so your solution would be better.

 

[ronsimpson]

I have used one or two diodes from a battery to PIC. The diode(s) protect from backwards battery and reduce the voltage by 0.7V each.
If you want to drop back to two batteries I think I can find a IC.

 

[DSchafer]

That would be great, I could try it out and see which works better.

I would like for this to run for about 15 hours before the batteries die. This will have an on/off switch and assuming it would be turned on for like an hour at a time and triggered around 150 times in an hour. So roughly 10 minutes of the hour the LEDs would be on and 50 minutes of the time the LEDs will be off and just the PIC running. So would have to see if 2 AAA batteries would last long enough.

 

[audioguru]

I have a cheap Chinese flashlight with 24 matched white LEDs in parallel that are powered by three AAA battery cells in series. The batteries are "Super-Heavy-Duty" that are old fashioned carbon-zinc and they have a fairly high internal resistance. I put high quality alkaline cells in mine and it made the flashlight much brighter and the LEDs did not burn out but the on-off switch became intermittent instead. Cheap junk.

 

[ronsimpson]

I would like for this to run for about 15 hours before the batteries die.

Some products we made do not have a off switch because the power draw from the micro is so small that the storage life of the battery is equal to the current draw of the micro.
There are a number of ways to put the micro into low power mode. Then when "on" is pushed it just pushes the micro into a higher power state and away we go. The PWM we used used about 10uA worst case, less than 1uA typical, when enable is shut down.

What I am saying is that the LED current is important. The micro will pull next to nothing if you run slow, or use one of the power down modes.
How fast does the micro need to respond to a trigger?
Which PIC?

 

[audioguru]

Do you have a datasheet for the AA cells you will use? Here I buy Energizer and Duracell AAA alkaline cells and their datasheets are online.
You want 150 minutes. The voltage of each cell drops to 1.0V in 150 minutes when the current is about 200mA. Then if each of the 6 LEDs has its own resistor its current will average 200mA/6= 33mA and burn out soon. Use 20mA each and the battery will last for about 200 minutes.

 

[DSchafer]

Some products we made do not have a off switch because the power draw from the micro is so small that the storage life of the battery is equal to the current draw of the micro.
There are a number of ways to put the micro into low power mode. Then when "on" is pushed it just pushes the micro into a higher power state and away we go. The PWM we used used about 10uA worst case, less than 1uA typical, when enable is shut down.

What I am saying is that the LED current is important. The micro will pull next to nothing if you run slow, or use one of the power down modes.
How fast does the micro need to respond to a trigger?
Which PIC?

I was going to use the PIC10F20o. It is will be like a cone that is weighted to keep it upright and when it is hit the LEDs will flash. I was going to try and maybe use a mechanical tilt sensor as an interrupt into the PIC that would wake it up and it would flash the LEDs and then go back to sleep mode. Not sure how well the mechanical tilt sensor will work, but will try it. But yes the PIC will be in sleep mode so drawing very little current. However this is something that could sit in the closet for weeks or months during the winter not being used so thought it would be better to use a switch. Plus if the unit wasn't sitting upright then the tilt switch could be closed and the PIC wouldn't go to sleep mode because I am assuming it would work where say the PIC is in sleep mode waiting and the unit gets hit and the tilt sensor shorts waking up the PIC which would flash the LEDs and then wait to verify the sensor opens back up (the unit is standing upright again) and then the PIC goes back to sleep. Haven't implemented yet, just what I was thinking to keep it from triggering all the time because of the mechanical tilt switch. As I said, I can see the tilt switch ball bearing inside might bounce a lot causing the LEDs to trigger more than once, but it was a cheap solution that I could think of to try and use for this.

 

[ronsimpson]

The tilt switch should be "edge triggered" not level triggered. These things "bounce" too much so you are really looking for a change in level. If the cone is on its side for a month it should trigger once then not re-trigger.
You can "not re-trigger" until the LEDs have finished blinking.
With more code, you could keep track of what percent of the time the LEDs are on. If it is getting triggered 75% of the time, it might be in a car and should just shut down. Probably too much! lol

 

[DSchafer]

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The tilt switch should be "edge triggered" not level triggered. These things "bounce" too much so you are really looking for a change in level. If the cone is on its side for a month it should trigger once then not re-trigger.
You can "not re-trigger" until the LEDs have finished blinking.
With more code, you could keep track of what percent of the time the LEDs are on. If it is getting triggered 75% of the time, it might be in a car and should just shut down. Probably too much! lol

Yes you are correct. I planned on using a rising edge (tilt sensor shorts due to unit being hit) to wake up the PIC and flash the LEDs then wait for some time like say another 3 seconds and verify the tilt sensor has opened back up (unit is upright) or wait until it does open back up and then go back to sleep and repeat. So if the unit was on its side, the LEDs wouldn't flash but the PIC would keep monitoring the pin waiting for the sensor to open back up before going back to sleep. I just thought this would help prevent too many false triggers because as you say I assumed they would bounce a lot it was the cheapest and easiest solution I could think of.