SparkFun Forums

[njivy]

Which product(s) would be suitable for powering two LEDs from alkaline batteries? The circuit is essentially a flashlight, but my application is more like an LED-lit toy. I'm open to suggestions about off-the-shelf batteries and DC-DC converters.

Constraints:
1. Load: 2 LEDs, 20 mA, 3.3 V, like COM-09850
2. Source: Alkaline batteries, e.g. 4 AAs or a 9-volt
3. Maximize use of the batteries.
4. Pre-assembled DC-DC converter. I'd like to avoid soldering surface-mounted devices.
5. Less than $20 for the converter.

Thank you,

Nic

[jremington]

Pololu has an excellent selection of DC-DC converters.

[njivy]

Thanks @jremington. I took a look at those, and I found a lot of voltage sources. Seems like an LED would need a current source, right?

Option A
Load: 5mm white LED (COM-09850, < 20 mA, 3.2-3.4 V)
Current regulation: FemtoBuck (COM-13716) modified to provide 20 mA. Replace R2 with 4.7-Ohm surface-mounted resistor bought elsewhere. Requires 6 < Vin < 36. Any tips on sources for SMD resistors?
Battery: 9-volt alkaline battery
Evaluation: Efficient. Moderate cost. Requires more skill and available tools.

Option B
Load: 5mm white LED (COM-09850, < 20 mA, 3.2-3.4 V)
Current "regulation": Current-limiting resistor only
Battery: 3 AA batteries, approximately 4.5 V
Evaluation: Inefficient, lots of wasted heat. Low initial cost. Relatively easy.

Option C
Load: 5mm white LED (COM-09850, < 20 mA, 3.2-3.4 V)
Current regulation: DIY circuit (e.g. http://electronics-diy.com/power-led-driver-circuit.php)
Battery: 2 AA batteries with boost converter (PRT-10255) modified to output 3.3 V and operate down to 0.3 V
Evaluation: Efficient? Moderate cost. Requires more skill and available tools.

If rechargeable batteries are used instead, a couple more options arise.

Option D
Load: 5mm white LED (COM-09850, < 20 mA, 3.2-3.4 V)
Current regulation: DIY circuit (e.g. http://electronics-diy.com/power-led-driver-circuit.php). The variable resistance would dissipate the excess voltage as heat, I think.
Battery: 24.5mm rechargeable 3.6V coin battery CR2450 (PRT-10319) with holder (PRT-08863).
Recharging: Adjustable LiPo charger with 24.5mm holder (PRT-14380). Can the load be wired to this board, or does the load need to be disconnected while charging?
Evaluation: Moderate efficiency. Moderate cost. Moderate skill.

Option E
Load: 5mm white LED (COM-09850, < 20 mA, 3.2-3.4 V)
Current regulation: FemtoBuck (COM-13716) modified to provide 20 mA. Replace R2 with 4.7-Ohm surface-mounted resistor bought elsewhere. Requires 6 < Vin < 36.
Battery: 2 x LiPo, 1aH, 3.7 V (PRT-13813)
Recharging: 2 x LiPo Charger/Booster, 5 V, 1A (PRT-14411)
Evaluation: Efficient? High cost. Requires more skill and available tools.

Option F
Load: 5mm white LED (COM-09850, < 20 mA, 3.2-3.4 V)
Current "regulation": Current-limiting resistor only
Battery: 24.5mm rechargeable 3.6V coin battery CR2450 (PRT-10319) with holder (PRT-08863). Resistor to drop voltage from 3.6 to 3.3 V.
Recharging: Adjustable LiPo charger with 24.5mm holder (PRT-14380). Can the load be wired to this board, or does the load need to be disconnected while charging?
Evaluation: Efficient? Moderate cost. Relatively easy.

Conclusion
Looks like I need to invest in a proper soldering iron, stock up on some basic parts, and learn how to solder properly!

[jremington]

Another option is to hack a solar garden light and get out the YX8018 DC-DC converter, which will run off a single AA cell, and requires but a single small inductor. Very low current circuit, so batteries last a long time.

https://ez.analog.com/community/univers ... rden-light

There are other interesting uses for that chip on the web, e.g. https://www.davidpilling.com/wiki/index.php/SolarStuff

[njivy]

The DIY current regulating circuit drops the voltage by 0.5, so options C and D would not work as described.

[Anthony Fremont]

Which product(s) would be suitable for powering two LEDs from alkaline batteries? The circuit is essentially a flashlight, but my application is more like an LED-lit toy. I'm open to suggestions about off-the-shelf batteries and DC-DC converters.

Constraints:
1. Load: 2 LEDs, 20 mA, 3.3 V, like COM-09850
2. Source: Alkaline batteries, e.g. 4 AAs or a 9-volt
3. Maximize use of the batteries.
4. Pre-assembled DC-DC converter. I'd like to avoid soldering surface-mounted devices.
5. Less than $20 for the converter.

Thank you,

Nic

Look up joule thief circuits. They are probably the most efficient way to drive an LED.

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