Dimming panel LED display

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Dave, I can't do that 15 years ago

Hi,

This is a long shot but I recently bought a digital clock radio with

1-1/2" high characters as my old sight is going fast. The problem is that the red display at night is so bright I can read a book 4 feet from it.

It has a hi-low switch and I figured I'd just pull it apart and up the value of a resistor but to my surprise the switch is double pole. I measured across the contacts with the slider set to the other end and I get 16 Ohms on both, but it arrives there slowly as though cap may be charging. The "bright" side of the switch is open and the pads are not connected to anything so I figured I can add something there.

So, anyone care to hazard a guess what to put across the other end of the switch contacts to make the display less bright?

Thanks

Dave

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Richard Henry 15 years ago

Smoked glass.

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krw 15 years ago

10,000V. It shouldn't take long.

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Adrian Jansen 15 years ago

Welcome to the digital world. Things are no longer controlled in any simple way by resistors, etc. The switch will just be a control read by the internal logic which tells the processor to output either light level 1 or light level 2, whatever these are. Its very unlikely you will be able to adjust those levels externally. Why the switch is double pole ? - probably only because some other variation of this clock uses it, and they have a gazillion of them to use up.

Regards, Adrian Jansen adrianjansen at internode dot on dot net Note reply address is invalid, convert address above to machine form.

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Kevin McMurtrie 15 years ago

The typical hack is that each pin from the clock chip drives a pair of LED anodes at once. The two cathodes in the pair are driven from alternate cycles of AC power taken from opposite sides of a center-tapped secondary winding on the power transformer. This reduces the number of wires in the circuit and gives LED alarm clocks their characteristic flicker.

Changing the brightness should be a simple matter of changing the resistance to the power supply, but there are two alternating power supplies for the LEDs so there are two resistors to change.

Chip -----+----LED----- A power |----LED----- B power

chip power limit Chip --------/\/\/\/\/--------- | A power -----/\/\/\/\/---->|--+-) || (------- AC LED power limit rect ) || ( ) || ( ) || ( GND ____________________________) || ( ) || ( ) || ( ) || ( LED power limit rect ) || ( B power -----/\/\/\/\/---->|----) || (------- AC

I won't see Google Groups replies because I must filter them as spam

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Dave, I can't do that 15 years ago

Thanks Kevin,

I appreciate the ASCII art effort, but I have put this in the too hard bin. Twenty minutes back, I removed the front panel and then shot a few light sprays of matte black rattle-can paint over it. It took 6 passes to get it just right.

Dave

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Dimming panel LED display

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