Hello group,

The current application I am working on is an alarm clock to replace the mechanical thing sitting on my bed. I plan to use an Atmel AVR uC to control seven-segment displays for the clock display, as well as using the internal timer/counters for the actual timekeeping.

My question here is, I want to make it portable and run off batteries, but I don't want to throw a linear regulator in there that will always be draining the batteries in the form of heat.

The largest drain will be the display, the AVR runs on tens of microamps when sleeping, so I want power regulation that will also be efficient. I am driving the LED segments with 7N2000 MOSFETS.

Any ideas here? Is an LM317 actually the most efficient method here?

 

Hello,

The LM317 is a liniar voltage regulator.
What are the reqirments for the regulator?
What is the voltage of the battery?
What voltage is needed for the AVR?
What current do the leds draw?
Are the leds always on or only on when pushing a button?

Greetings,
Bertus

 

A more efficient method would definitely be a switch mode power supply

 

Thanks for the responses!

>What are the reqirments for the regulator?

Regulation should be 3.3V +/- 0.3V. Current requirements in should never exceed a couple hundred mA.

>What is the voltage of the battery?

I want to use 4xAA batteries. 6V @ approx 10k mA/hrs. I had also considered using C-sized batteries if the current loads turn out to be too high and drain the batteries like crazy.

>What voltage is needed for the AVR?

The AVR can operate down to around 2.8V, and there are special "low-voltage" versions that run on like 1.5V, but they are not available in DIP packages.

>What current do the leds draw?

At max brightness the LED displays can draw 70mA per lighted segment, but I plan to run them closer to their minimums, 4-8mA/segment. Normally there will be perhaps 10 segments lit, more some times of the day, less other times of the day. Say 75mA constant load.

>Are the leds always on or only on when pushing a button?

Always on. Wouldn't be a very useful clock otherwise.

>A more efficient method would definitely be a switch mode power supply

Is there a common IC for this? I am fairly familiar with the linear regulators like the 78xx/79xx series, LM117/317, the LDO regulators etc. I know these are not very efficient since they will be dissipating the extra voltage as heat.

As an alternate solution, if I used 2 C-cell batteries I suppose I could run at 3V without any regulation at all. How low will the voltage from the batteries dip before they are discharged?

Thanks for the help, everyone!

 

I'm afraid that you will go through a LOT of batteries with your current design concept.

Typical NiMH C-batteries might have a 2200-2500mAh capacity. If your average load is 70mA, that gives you between 31.4 and 35.7 hours of operation before the batteries are fully depleted. Typical NiMH AA batteries might have 1800-2000mAh capacity, giving you 25.7 to 28.5 hours of operation.

If you wish to make it portable, you'll need to have the display wired for "on demand" time (ie: pushing a button to see the time). However, if it's running from wall power (ie: transformer supply) you can use the batteries as a backup to maintain the time in case of power failure.

 

install rechargeable batteries with an input jack for the appropriate wall wart to keep the batteries charged and have the best of both worlds.

 

Yeah, when I did more calculations I realized that the batteries would only last a day or two.

Time to figure out how to reduce the power consumption of the display.

 

Yep, what VoodooMojo said.

You'll need a simple battery charging circuit. You'll probably be OK to constantly charge the batteries at less than 5mA.

If the clock is running from mains power, the LED display can be on all the time, but in battery backup mode must be "on demand" only.

 

You may also try using the SMPS circuits used for charging the Cell Phone batteries, you may do slight variations in the circuit to use it as a Power supply for your own purpose and also use it to charge the rechargeable battery. I would suggest you go for the cheap SMPS chargers ( china made). I have used them and they work fine

 

You'd do better to use an LCD display rather than LEDs. As others have said, you just can't run an LED display for long on batteries. Actually LCDs have the problem that they don't show in the dark, so you'd need a backlight, either on all the time (battery killing!) or coming on when you press a button.

How about a clock where the display lights up when you just wave your hand in its vicinity? That would be fun. It should be possible with a capacitive sensor.