Battery Powered Clock

[Dzz't]

Heylo,

I'm looking to build a belt clock on the pager form.

The specs are pretty simple -- HH:MM:SS and DD:MM:YY, pretty simple.

The device will be battery powered with LED display like this: **broken link removed** I'll hafta attach an additional 2 digit display for the seconds, unless there is a 6 digit display about .28" tall going around. (Those 4 digit ones are the only ones I can find)

This can be done, heck if the **broken link removed**, then I can do it. I'm thinking that I might have to use a cellphone battery if power supply is a problem. It'll have to be opened to adjust anything (to make it as water resistant as possible). An aluminum case (probably anodized to a cool dark blue) with a TIG welded belt loop, and we'll figure out how to program the microcontroller so that the date is displayed every 15 seconds for a duration of 2 seconds.

Now, how do I rig up the electronic circuit? What microcontroller with real time clock circuit (crystal)? I see many like this, but they're all AC powered. I don't think battery backup will be required or could fit. I'm thinking that I might also like a 5 LED battery charge circuit which comes on for 3 seconds every minute.

Thanks lads!

 

[Diver300]

You can use single or 2 digit displays.

LED watches that run on batteries only light when the button is pressed, to keep the power consumption low.

 

[Dzz't]

Right, however I intend mine to be on all the time, and probably use PWM to keep power draw down.

Does anyone have a circuit diagram in mind for what I intend to produce?

 

[blueroomelectronics]

Won't work, LEDs draw too much current.

 

[ChrisP]

Right, however I intend mine to be on all the time, and probably use PWM to keep power draw down.

Have you considered using an LCD display with an on-demand backlight?

 

[Dzz't]

No, I'm going for a completely different, unique device.

According to the specs, it requires 1.8v at 20mA (per chip: not sure if that means per digit or per 4 digit display). That equals .36 watts. A cellphone battery may be around 3.6 volts, but I don't know how long it would last -- a week?

 

[Exo]

If that cellphone battery is rated for 1000mAh (an average, some cellphone batteries are less, some are a bit more) @ 3.6V then a circuit drawing 0.36W should (very roughly) run for about 10 hours.

However, that's assuming 100% efficiency , wich is impossible. Using resistors in series with your leds straight from 3.6V will waste a lot more power - so you'll have to use some switching regulator to lower the 3.6V closer to the 1.8V required by the leds. Even then, some power will be wasted.

 

[crutschow]

No, I'm going for a completely different, unique device.

According to the specs, it requires 1.8v at 20mA (per chip: not sure if that means per digit or per 4 digit display). That equals .36 watts. A cellphone battery may be around 3.6 volts, but I don't know how long it would last -- a week?

The 20mA is likely per segment so it will vary with the number being displayed. Thus to display the single number 8 would require 7 * 20mA = 140mA.

But you could probably reduce that current and still have a visible display, depending upon the ambient light.

 

[Exo]

The best way to save current is to multiplex the display so only one display is lit at a time. However, each led will have to be driven with maxium current, otherwise the display will become very dim.

 

[Dzz't]

Now what about PWM? The human eye cannot detect active PWM around 20Hz, but 24 is better.

 

[Exo]

I don't see how it would help. PWM itself won't save power, using a low duty cycle will , but then the leds will become very dim.