Hello I am making an analog flex PCB bookmark that lights up when a book closes on it…I am using a comparator and FSR to turn on the LEDS… using a total of 5 0805&1206 LEDS I want to power the circuit with a slim 3v coin battery to keep a slick design…the coin battery I am looking at is 500mAh and the 5 LEDS in series draws a total of 100mA so the bookmark would only stay on for an hr…how can I make this last longer…I know making the LEDS blink or having them turn on for a few seconds then reset the circuit to shut off but just unsure how to do that

 

I fail to see what good is to spend battery energy on something that is actually not in use (in this case book is closed).

100mA is a lot for LED. Few LEDs with that current is blinding. Are you trying to set the book on fire?

If your objective is to draw attention to it, it does not need to look like car headlight. there are high efficiency LEDs that will light up rather intensely while drawing 1-2mA. not long ago i was changing series resistors to limit the LED current from usual 10mA to 0.45mA. suppose you put couple of them and get that current to 5mA. then 500mAh battery should run for 100h or 4 days.

and making the LED blink for 0.1sec every 2sec would reduce current draw 20x so the same coin cell battery should last some 3 months.

does this sound like a usable solution?

 

the other option for "long shelf life" is to not use batteries. create field that can power LEDs wirelessly.

 

I fail to see what good is to spend battery energy on something that is actually not in use (in this case book is closed).

100mA is a lot for LED. Few LEDs with that current is blinding. Are you trying to set the book on fire?

If your objective is to draw attention to it, it does not need to look like car headlight. there are high efficiency LEDs that will light up rather intensely while drawing 1-2mA. not long ago i was changing series resistors to limit the LED current from usual 10mA to 0.45mA. suppose you put couple of them and get that current to 5mA. then 500mAh battery should run for 100h or 4 days.

and making the LED blink for 0.1sec every 2sec would reduce current draw 20x so the same coin cell battery should last some 3 months.

does this sound like a usable solution?

LOL I do not want to set the book on fire. I will def look into using a different resistor value…I just didn’t want the LEDS to be too dim…I was looking into using a chaser circuit to make the LEDS blink

 

I know it seems silly for the LEDs to light up when the book is closed and not in use but that’s just what I wanna do for this project. Also high efficiency 0805 LEDs seem hard to come across…it’s already hard to find blinking ones

 

LEDs in 0805 package drawing 100mA will be smoking hot and if they do not fail open, they will likely desolder themselves. the heat is the enemy both for longevity of the product and for battery life. KT-0805x are in 0805 form and quite efficient. chasers can be done in many ways. 555+4017, mcu, 3 transistors, etc.

 

See :APT2012LSECK-J3-PRV, APT2012LZGCK, APHHS1005LZGCK-V, APHHS1005LSECK-J3-PF

 

Anybody notice the obvious error in the OP?

5 LEDs in series with a 3-volt battery.

Most likely they are in parallel and drawing 20 mA each. (if the current was even actually measured)

So, yea, resistors are the obvious solution keeping it simple.

 

for someone working with SMD parts, it is reasonable to expect that they know the difference between series and parallel connection. so why assume it is an error?

there is plenty of LED drivers that do just that - drive a string of LEDs from low voltage power source. an example of one that can do up to 750mA current though a string of LEDs:

 

And how much current will that draw from the battery to put 100mA into 5 LEDs in series?

The statements by the TS are contradictory.

 

100mA from 500mAh should last 5h. he mentioned that actual run time is closer to an hour. i see no contradiction that battery current is higher than led current. the same is the case with circuit in post #9

 

5 LEDS in series draws a total of 100mA so the bookmark would only stay on for an hr…

5 LEDs in series drawing 100mA with Vf of 3V means output power is 1.5W (5x3V*100mA).
that is typical for illumination LEDs (not for indicator LEDs).
DCDC converter with efficiency of 90% is going to draw slightly more 1.5W/0.9 = 1.67W
if the battery voltage is 3V, current draw is 1.67W/3V=555mA
so 500mAh / 555mA = 0.9h

where is the contradiction?

 

Ok, I stand corrected, it is possible that the OP is using a driver.

But if you compare the output wattage of the 2 configurations it's nuts to be driving them in series.

But yea, you are right.

 

Anybody notice the obvious error in the OP?

5 LEDs in series with a 3-volt battery.

Most likely they are in parallel and drawing 20 mA each. (if the current was even actually measured)

So, yea, resistors are the obvious solution keeping it simple.

Yes they are in parallel with a current of 20mA each…I am very new to this so what current should I aim for…I still want to be able to see a good amount of light

 

I found this chaser circuit online don’t know if it’s something I should consider doing…

 

Personally, I would aim for a single LED drawing 1 mA.
The battery would last a lot longer, and longer still with a pulsing LED.
Look for flashing Xmas tree LEDs.

 

I’ve found LEDS w a current of 2ma which would have a total of 10mA when in parallel thanks to the suggestions above…still using the same 500mAh battery and the correct resistors I’m guessing this will be a much better solution than a blinking circuit

 

But if you compare the output wattage of the 2 configurations it's nuts to be driving them in series.

it depends on application and illumination LEDs are higher wattage. series connection is always preferred since current limiting need to be only done once per string, not once per LED. that also means higher efficiency - energy is wasted once (per string), not once per LED.

also PWM or enable input still allow control.

 

Personally, I would aim for a single LED drawing 1 mA.
The battery would last a lot longer, and longer still with a pulsing LED.
Look for flashing Xmas tree LEDs.

Do u have an idea of what resistor values I should be using for 1ma LEDs

 

it depends on application and illumination LEDs are higher wattage. series connection is always preferred since current limiting need to be only done once per string, not once per LED. that also means higher efficiency - energy is wasted once (per string), not once per LED.

also PWM or enable input still allow control.

Yea, I understand how efficiency works, but this is apples to oranges.

For apples to apples the current would have to be 20mA instead of 100, giving series the efficiency advantage.