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Subject: Re: Ambient light sensor to dim LED display

Date: 02/04/03 at 12:36 PM
Posted by: Brian Snell
E-mail: bsnell@lara.on.ca
Message Posted:

In Reply to: Re: Ambient light sensor to dim LED display posted by Clem on 02/03/03 at 10:25 PM:

Hi again. Your clock-radio is probably using a 2 phase system for multiplexing the LED digits. Usually, the digit feeds are split up and connected as alternate "pairs" -- with the segmant drives then being able to be run simultaneously to each digit "block". Only the "block" which is pulsed "on" will show a display -- on the next phase the other "digit "block" will show. Both phasing connections are powered from each end of the center-tapped power transformer secondary. All the digits appear to be on due to "persistence of vision" in the human eye, but in reality, each "half" of the display is being turned on and off alternately.

You need to locate the secondary of the transformer (look for wires which connect to diodes). The AC voltage will be around 12 to 16 vac. There will be two wires coming from this secondary which will end up connected somewhere along the row of connections on the dispkay module (probably both close to one end). The connections will probably also be run through a pair of diodes and a pair of resistors (400 to 600 ohms)as well, since each digit "block" will respond to only the positive peak on its half of the AC cycle. If you can locate something similar to this, unsolder one end of one of the resistors and add an extra resistor (say 200 ohms) than see if half of the display has dimmed to a lower level of brightness. Once you have confirmed this, you can then use a DPST switch to switch in or out the added resistors as a "Hi-Lo" selector. If you want to have this done automatically, you can substitute a pair of CdS photocells (or even one "bridge" type which is center-fed" for the resistors. You need to identify the range of resistance required for the cells in order to achieve the degree of dimming to suit your ambient light level, so spend a bit of time trying various fixed resistors until you get in the ballpark. This will tell you what range of CdS cells to obtain (Radio Shck used to sell a bag of assorted types for around $3.00 at one time).

Good luck.

Brian Snell

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