Long-life circuit for LEDs and a clock

[dabuttery]

Hello- I’m new to this forum and want to pick your brains on a project. I do have a little background in electronics, but it has been a while, so bear with me. I’m trying to decorate a battery-powered wall clock with a few LEDs, and could use some assistance in creating this circuit.

For starters, the clock is a typical little 1.5V quartz step movement, not a continuous sweep, which essentially means it draws appx. 0.3mA once each second to advance the hands.

I’m using four (4) ultra bright 5mm LEDs in this project, which will be on all the time- i.e. 24/7. The specs for these are as follows:
Forward voltage 3.0V min, 3.4V typical, 3.8V max
Reverse voltage 5.0V
Forward current 40.0mA max
Peak fwd current 100.0mA max
Reverse current 10.0microA max

For convenience and long life, I want to try to run the clock and LEDs together off a single battery pack that holds six (6) 1.5V D-cell batteries. The desired circuit should drive the LEDs as bright as possible, but also be frugal with power consumption. I know these two requirements are at odds with each other, so if need be, the brightness of the LEDs is secondary (but only slightly) to battery life.

To this end, I’m wondering about using some sort of IC driver to cycle the LEDs in a high-frequency on/off mode to conserve power, with a freq so high the pulse is not visible to the naked eye. I’m also interested in integrating some sort of photo resistor in the circuit to dim the LEDs 20-30% or so at night, also to conserve power.

So, what do you think? Any other power conservation tips? Any tricks to increase the light output without burning up the LEDs? Also, can you advise me on specific components for the circuit. Thanks for your time and consideration!

 

[audioguru]

Your question is on this site too?
24/7 is a long time to light LEDs. You will be replacing the D cells each month.

 

[Russlk]

I suppoose that if you pulse the LEDs at 20 mA and 25% duty cycle, they will appear brighter than if you fed them DC at 5 mA. You wil loose power in the efficiency of the switching circuit, but the efficiency should be better than 75%. Look at the LTC1872 data sheet, it has a LED driver circuit. That circuit supplies DC to the LEDs but it can be modified to provide pulses.

 

[audioguru]

I suppose that if you pulse the LEDs at 20 mA and 25% duty cycle, they will appear brighter than if you fed them DC at 5 mA.

No, they would look the same. For continuous PWM'd current, vision averages the current.
Some people think that vision works like a peak detector. It does only for spaced individual blinks of an LED.

Some LEDs are slightly more efficient at high current. Most are a little less efficient at high current.

 

[Roff]

Perhaps you need to **broken link removed** to the clock. That should save some power. :roll:

Here is a quote from **broken link removed**:

An advantage of pulse drive is that the human eye behaves as a partially integrating and partially peak-reading photometer. As a result, the eye perceives rapidly pulsed light somewhere between the peak and the average brightness. This means that a low-duty-cycle, high-intensity pulse of light looks brighter than a DC signal equal to the average of the pulsed signal. An advantage of multiplexed operation, therefore, is an improvement in display intensity for a given average power consumption.

I don't know if it's true - it's just something I found on my computer.