The Prodmod LED Camera Light as described here was designed to be the simplest way to attach 3 LED lights to your digital camera for video or still pictures. It is the much much cheaper alternative to buying one in the stores. It would only cost $2-$3 in parts or $9.99 for the kit as opposed to $30 in stores.
Since this product design is so simple its battery power is unregulated. This means the current and brightness of the LEDs varies over time. In this article we will discuss how we chose one resistor value as a compromise to deal with this variable supply voltage. Read on to view data plots of current and voltage change vs. time based on tests performed on the ProdMod Camera Light.
If you read the original article you would have noticed that your resistor calculation is based on your power supply voltage so that you can attempt to keep the current to the LED around or below the safe 20mA range. If you assume the voltage is 3.6V because you have three 1.2V NiMh cells you would be sadly underestimating the actual initial voltage. When NiMh cells are fully charged they can produce almost 1.4V which is 4.2V for a set of three cells in series. If you don’t account for the 4.2V you would be driving the LEDs with too much current. This isn’t a horrible thing for such a simple device, but it’s recommended you keep the current under 20mA for each LED otherwise you risk burning them out or reducing their life expectancy.
When your power is unregulated you’ll have to consider how your batteries will drain over time and their voltage will reduce. Since the resistor is a fixed value your LEDs will start to dim as time passes in proportion to your battery’s voltage curve. So our goal is to choose a resistor value that minimizes current to protect the LEDs but also provides enough current to maximize brightness toward the end of the battery life. After a few tests we settled on 10 ohms because it was easier to find and keeps the current around 20mA for most of the battery life. It does however start out around 25mA. So if you want to avoid going over 20mA at any time you will have to try a 12 or 15 ohm resistor.
*Note* the LEDs specified in this DIY kit are actually rated for a max current of 30mA. So you can probably not worry about the 25mA.
A test was performed to show you how the current varies over time.
Three fully charged Energizer 900mAh AAA batteries
One 10 ohm resistor following this circuit design.
The initial closed circuit voltage was measured to be 4.16V.
A multimeter was attached to read the total current draw and send the data to the PC every 10 seconds.
The current was then divided by three to approximate the current driven through each LED.
Two curves were plotted: Total current and single LED current.
Current Vs. Time
As you can see in the plot image the current per LED begins around 25mA. This is above the recommended value but at least the current only reduces to 15mA towards the end of battery life.
The first vertical line indicates where the LEDs were powered off for 13 hrs. This was because the test was performed in two intervals but it also shows how the battery voltage recovers a bit when it’s allowed to rest. You wouldn’t see this if the light was run continuously but that doesn’t represent typical usage anyhow. In fact you would probably turn it on for maybe 2 to 10 minutes at a time and be off for most of the time. If so the batteries might last even longer.
The second vertical line indicates the knee of the data curve and is where the batteries begin to suffer the quickest power loss.
This bend coincides with 15mA of current per LED which we think can be considered an acceptable level of brightness.
After that bend you would see a sharp decline in brightness and is typically when you would plan to refresh your batteries. You can still get sufficient light when the current is 10mA per LED, but in this case the battery begins to lose power significantly faster. The advantage to unregulated power is that you can still get some light out of the LEDs even when the batteries are practically dead. This may come in handy in emergency situations.
Voltage Vs. Time
The voltage was calculated from the measured current. This gives you an estimate of the Voltage curve vs. time. This is what you would typically see when looking at battery specs.
What is most interesting to note here is that the total battery voltage doesn’t even get as low as 3.6V until an hour after it begins its steep drop in power. So using 3.6V in you resistor calculation is way off the mark.
Battery Test Conclusion
It turns out the actual total run time for these 900mAh batteries is somewhere around 11-12hrs depending on how often you let the batteries rest and how dim you’d allow the LEDs to get before wanting to change batteries. This is a bit different from our initial calculation of 16hrs that assumed a constant current draw of 60mA and a battery capacity of 1000mAh. But hey 12 hrs is 3X greater than the $30+ Camerabright product! So we’re happy about that!
The Camerabright website claims their 4 continuous hours is equivalent to 1 year of typical use. I don’t buy that, but in any case our design lasts much longer. If they last one year maybe the Prodmod LED Camera Light can last 3 years?
Now if you do still want to achieve 16 hours of operation, you should firstly use 1000mAh capacity batteries and also consider reducing overall current with a 15 ohm resistor. Together those two changes might get you another hour or two**.
**Disclaimer: There are many more efficient ways to drive LEDs for longer periods of time at constant current and brightness, but this particular tutorial is utilizing simple techniques without the use of ICs or any other circuitry.