By using an $11 DC-DC boost circuit in the Prodmod LED Camera Light I am now able to power it with one, two, or three batteries and can use either rechargeable or alkaline and still maintain a fixed LED brightness. The brightness of the LEDs remains unchanged until the very last moment when the total battery voltage drops below 1V. The flexibility of the DC-DC boost circuit allows you to use either one or two batteries if I wish leaving room for other items in the case. It also allows you to position your mount stud somewhere else if it better suits your camera.
I tested a 5V circuit and a 3.3V and compiled the results. There is an effect on battery life but its not as bad as you might think.
If you have already made the original Prodmod DIY LED Camera Light you may have noticed that the light will start at full brightness and begin to slowly dim as the batteries are depleted. This is because the output voltage is not regulated. As the battery voltage reduces so does the current through your LEDs. Not only does it emit less light, the color temperature can change a bit as well as the current drops.
To regulate a voltage you can typically use something like a LM317 voltage regulator. But you’ll need to supply it with more voltage than you want it to regulate. So if you want 3.6V you might need to give it 5 or 6V. Well, I don’t want to add more batteries to my light just to get a regulated supply. In fact I would rather use fewer batteries.
This is what a DC Boost circuit is for. It converts a low voltage high current supply like a battery and boosts it to a higher voltage yet lower current output. It consumes current from the batteries to get the voltage boost. So even though you output current is low, you input current from the battery remains high. This of it this way: Power=Voltage x Current. Assume the battery has a fixed amount of power, if you increase the voltage in this equation the current must drop – this is the output. But if the current is increased, the voltage must drop – this is the input. There is a calculation given by the Vpack manufacturer that helps explain this relationship. You can download the PDF.
The DC-Boost PCB
So for my first experiment I chose to use the VPack5.0V_AAA_1 from Bodhilabs. These guys are great, they are only $10.95 pre-assembled and free shipping! The circuit is mounted on a small PCB which is glued to a battery holder. It is the smallest one they have and its limited to 100mA output which is fine for this light.
Relocate the mount screw
I removed the PCB from battery holder and wired it up to my original 3LED light. Since I don’t need all three batteries anymore I removed one which made room for the PCB, and it also made it possible to move my mount screw. This is great if the upper right hand corner of the original mount stud was inconvenient.
After I removed the LEDs I went ahead and drilled two new holes in the second battery chamber. One for my Panasonic Lumix off to the side and another for my Nikon near the center. I then relocated the red positive terminal so I now have a circuit for only 2 batteries. you only need one battery, but since we have the space here I settled on using two.
Choose NEW Resistor
Make sure you CHANGE YOUR RESISTOR. You now have a 5V supply where you previously had only 4V or 3.6V. So you’ll need a higher value resistor to limit the LED current. I used 23ohms for my test with only 3LEDs because it was handy and I didn’t mind running the LEDs hot. But you should run your calculations again with 5V supply. This time you are actually sure that number wont change. So 5V-3.3V = 1.7V. And say you want 20mA per LED and have 3 LEDs. That’s a total current of 3×20=0.060A.
So 1.7V/0.060A = 28 ohms.
If you are using 4LEDs the result would be
1.7V/.080 = 22 ohms
Wire it up
You can go ahead and wire up the DC boost as shown – I rearranged direction of the LED leads because I though it would be easier, but you can put them the other way if you want. And you can surely fit another LED to make 4 total. I don’t have a photo of how the wires connect to the PCB. If you need help on that just add a comment below and I’ll add to the post. By the way you don’t have to put the PCB above the screw head like I did, you could put the PCB above the LED chamber, just make sure you protect the PCB from the exposed LED leads and resistor with some electrical tape.
To The Test
Ok, so now we have a steady output voltage, and best of all we get to use only one or two batteries, but how long will it last?I ran tests with one and two batteries for both the 5V and 3.3V version of the Vpack PCB. The batteries were rated for 1000mAh.
Here are the results.
one AAA – 3LED – 22.4 ohm – 80mA = total run time 50 minutes continuous
two AAA – 3LED – 22.4ohm – 80mA = total run time 4 Hours continuous
one AAA – 4LED – 4 ohm – 80mA = total run time 2.8 Hours continuous
two AAA – 4LED – 4 ohm – 80mA = total run time 6.8 Hours continuous!
Clearly the best choice here is the 3.3V Vpack. The reason it lasted longer is because it requires more energy for the Vpack to boost to 5V than it does to boost to 3.3V. Plus with a 5V supply your resistor (28 or 22 ohms) has to waste a bunch of the extra energy as heat. You’ve basically wasted energy twice.
**The hours above are worst case. The actual battery life will be longer than what is shown since the light is ower is turned on and off during normal use.
Conclusion – It’s Not Perfect
With every advantages there has to be a few disadvantages, right? By using a DC boost circuit we gain a lot of flexibility with our input
voltage battery type and voltage. We also gain a constant output voltage to help maintain constant brightness. But its not perfect. As the LEDs heat up their voltage drop changes slightly, and as the resistor heats up it too changes slightly. So even though your voltage is constant your current can still fluctuate. Lucky for us these parts do not get too hot so they wont vary all that much but it should be said that current fluctuations are still possible.
I am sorry I couldn’t show you a graph of output current over time, my low current section of my multimeter isn’t working right.
Add a dimming feature to your LED light. – coming soon!
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