# Simple LED 3.7V circuit?

Discussion in 'The Projects Forum' started by Ludilo, Mar 19, 2015.

1. ### Ludilo Thread Starter New Member

Mar 19, 2015
16
1
Hello everybody.
I'm complete newbie to electronics, yet I would like to build simple and effective bicycle light.

The components are:
1. 1 x 3.7V, 350mAh, 25C LiPo battery
2. 3 x 3.4V, Ultra-bright, 5mm, 18000mcd, 20mA LEDs
3. 1x 22 Ohms resistor
4. 1 x latching switch

My questions are:
1. What is the optimal resistor value and setup (Serial, Parallel.. common R, separate R)?
2. How to keep the voltage steady for 3.4V LEDs (which should be 3.4V, I ques)?

The battery is 3.7V, but when full, it is almost 4.2V, and off course,when connected to LEDs, voltage drops after some time well bellow 3.7V.
Also, with 4.12V, full battery and 22 Ohms resistor, the voltage across the resistor and one LED is 4.06V, more or less.

So, my question is how to keep the voltage steady and keep LEDs working on right voltage while having optimal battery efficiency?

Thanks.

2. ### MrChips Moderator

Oct 2, 2009
14,505
4,271
You operate LEDs at constant current, not constant voltage.

An appropriate series resistance is (4.2V - 3.4V)/20mA = 40Ω

If the battery voltage drops to 3.8V, then (3.8V - 3.4V)/20mA = 20Ω

So use a separate 22Ω resistor in series with each LED. Then connect each resistor and LED combo in parallel cross the battery.

Three LEDs will draw 3 x 20mA = 60mA.
Your 350mAh battery will run for 350mAh/60mA = 6 hours.
The LEDs might stay bright for less than that, about 3 hours.

3. ### Ludilo Thread Starter New Member

Mar 19, 2015
16
1
Hi MrChips.
I have to buy some 22Ω resistors and try them.
Should I be worried about the initial 4.2 voltage?

4. ### MrChips Moderator

Oct 2, 2009
14,505
4,271
If the battery voltage is truly 4.2V then try two 22Ω resistors in series.

5. ### MrChips Moderator

Oct 2, 2009
14,505
4,271
For a bicycle light, consider using a simple CMOS 555 timer circuit using LMC555 or TLC555.
Your battery will last twice as long and you wouldn't need series resistors.

Mind you, just using series resistors for continuous illumination is simple enough.

6. ### Ludilo Thread Starter New Member

Mar 19, 2015
16
1
Well, the battery is 3.7 volts, but once full it "starts", with 4.2 volts.

7. ### wayneh Expert

Sep 9, 2010
13,607
4,408
This will be plenty bright for a "be seen" light, but not adequate for illumination of the path in front of you. I hope that is your plane?

Since the goal is to be seen, I'd consider MrChips suggestion to flash the LEDs. This tends to be attention-getting and saves power by virtue of the LEDs being off when not actually flashing. I believe flashing at 10Hz gives a higher perceived brightness because of the way our eyes and brains work.

8. ### Ludilo Thread Starter New Member

Mar 19, 2015
16
1
Hi wayneh.

Actually, the purpose should be to illuminate the path.
I have tested 3 LEDs with 3.7V LiPO battery and it seemed OK (in a dark room), but I haven't tried it in the "action", on the road...

9. ### wayneh Expert

Sep 9, 2010
13,607
4,408
Hmmm.... I think ~200mW of LED lighting is not considered enough for that application. It would be far better than nothing, but still not what is considered safe. I'd look at the commercial products to see what they use, as a guideline. More. Still more.

Looks to me like 1-2W is more typical for a headlight.

Last edited: Mar 19, 2015
10. ### mcgyvr AAC Fanatic!

Oct 15, 2009
5,081
1,103
I'm with wayne on this..
You need more light to be of any real use..

11. ### MikeML AAC Fanatic!

Oct 2, 2009
5,451
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Powering a 3.7V LED from a 3.7V is problematic.

12. ### Ludilo Thread Starter New Member

Mar 19, 2015
16
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Hello Mike.

The LED is 3.4V, 20mA.

13. ### MikeML AAC Fanatic!

Oct 2, 2009
5,451
1,071
Ok, powering a 3.4V LED from a battery that has this discharge curve is problematic. There is not enough voltage drop (headroom) across the current-limiting resistor to provide proper current regulation.

14. ### Ludilo Thread Starter New Member

Mar 19, 2015
16
1
So true...

With LiPo battery fully charged, almost 4.2V and with 3 x 3.4V LEDs, 2 x 22Ω resistor I still get 4.12V in the circuit.

15. ### Dodgydave AAC Fanatic!

Jun 22, 2012
6,476
1,024
you could use a J112 fet, in constant current mode, works from 3- 30 v. Or buy a ready made one on the web...

16. ### Ludilo Thread Starter New Member

Mar 19, 2015
16
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Hello Dodgydave.

However, I have to ask you: What is J112 FET and what does it do..?
As stated earlier, I'm complete newbie with LEDs and battery.

Jun 22, 2012
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18. ### RichardO Well-Known Member

May 4, 2013
1,663
568
The low power setting on my bike light is about 3 watts. This is enough light to be usable but not at full speed. To ride at full speed (about 15 MPH) or in areas where I am riding in and out of pools of light from street lights I use about 12 watts. 12 watts is a lot of power for extended periods from batteries. That is why I have 3, 6 and 12 watt settings. For instance, your battery would last less than an hour.

Just as important as the power to the LED's is the optics. The beam has to be tight -- just a few degrees wide. And, don't waste light higher or lower than what you need in order to see far enough away to avoid hazards.

Mar 19, 2015
16
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20. ### BobTPH Active Member

Jun 5, 2013
905
160
Have fun. I hope there are not potholes in front of you. I broke my collarbone that way, riding at night with insufficient light. This was when I was eighteen and could still see well.

Bob