I'm an artist currently working on an installation piece that requires a little more engineering knowledge than I possess. In short, what I'd like to achieve is a 2 LED blinking circuit with battery efficiency in mind. Preferably fresh batteries should last for a few months to a year or better. I'm guessing the best way would be with two layers of blinking: one blink would be more of a high frequency strobe and just appear "on," the other would cycle the LEDs off and on every second.
I don't know where to start with this, let alone if I'm headed in the right direction. Am I? What frequency would be best for the strobe? Again efficiency is the priority. I don't mind if there's a slight flicker like a GFI switch indicator. Any advice that will get me moving is appreciated.
Best Answer
First off, your high-frequency flashing will give (at best) only a slight improvement in battery lifetime. The idea is that you can increase efficiency by driving the LED at a higher current, and while this is true for some LEDs, it's not true for all.
That said, getting a year out of a continuously flashing LED will take a rather large battery. Let's say that you're flashing the LED with a 25% duty cycle: that is, only on for 25% of the time. And furthermore, let's say that you are running the LED at 10 mA. Then (obviously) in one hour the LED will be on for 15 minutes. The total current drawn will be 10 mA x 0.25 hours, or 2.5 mA-hr. In one day, the total current drawn will be 2.5 x 24, or 60 mA-hr. After one month, it will be 60 x 30, or 1800 mA-hr.
Now for batteries. You can get rechargeable NIMH D cells that will give you 10 - 12 A-hrs of service, so you might think to get a year at lower current drains. However, http://data.energizer.com/PDFs/nickelmetalhydride_appman.pdf suggests that NIMH will self-discharge by 20 - 50% in 6 months, so that pretty much sets that limit.
A bog-standard alkaline D cell will have http://data.energizer.com/PDFs/e95.pdf a capacity of ~ 18 - 20 A-hr and in this application you'd think would give you about 10 months to a year. The problem here is that the lifetime is specified to the point that the battery voltage is 1/2 that of a fresh battery, and LEDs have a minimum voltage that they need.
So, what you want to do is "possible", but only if you can accept the battery size.
What you need to do is get some high-brightness LEDs, a cheap power supply, a DMM and some resistors, and experiment with the LEDs. Find the current level that gives you the brightness you need, keeping in mind that your installation may (or may not, you just have to think about it) be seen in bright light which will require more LED current to appear bright enough. Once you have a target current, only then can you start grinding out the numbers which will tell you how much battery you need.
For example, let's say you have 2 LEDs in series, and they draw 10 mA when on, and their forward voltage is 2 volts each. The LEDs will need 4 volts total. If you use alkaline D cells you can put 6 in series to get 9 volts, and at the end of life they will be putting out about 4.8 volts, giving you about 0.8 volts margin for your current control element. Assuming 20 A-hr for the cells, you'll get about 11 months.
Is that too big a battery? Well, there are other alternatives. For this long-duration application, http://batteryuniversity.com/learn/article/elevating_self_discharge indicates that rechargeable lithium cells (lithium-ion) are possible. http://www.electricwingman.com/power/a123-loose-cells.aspx has a single 20 A-hr cell. Note its size and weight.
Primary lithiums are also a good technical choice. http://www.onlybatteries.com/showitem.asp?ItemID=14509&vfpr=10.30&vfbr=Blue+Sleeve&vfcat=Electronics+Electronics+Accessories+Power+Batteries&vfsku=14509&vfbi=no_bid&sid=gpla&vfsku=14509&vfsku=14509&gpla=pla&gclid=CjwKEAjwuoOpBRCSy6yQm66J1g8SJABrXW48PiokBnHnnAVNcptv7FH19wDGumSYhbMnC9kQH9-IQxoCevPw_wcB is an example, and you could get away with one of them.