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Autodim Add-On For The 6-Digit GPS Clock

Here's a low-cost add-on for the Digital GPS Time Display published in the May & June 2009 issues of SILICON CHIP. It senses the ambient light level, so that a modified program running in the display unit?s PIC controller can adjust the LED brightness to a comfortable level - ranging from full brightness when the ambient light level is high down to dim when the ambient light is very low.

By Jim Rowe

Soon AFTER THE basic 6-Digit GPS Time Display was published in the May 2009 issue, we received emails from readers who were disappointed that we hadn’t provided the design with an “autodim” facility. And they wanted to know if such a feature could be added in.

Click for larger image
Fig.1: the circuit uses an LDR to sense the ambient light level. The resulting DC voltage across the 150kΩ resistor is then buffered by complementary emitter follower stage Q1 & Q2 and fed to pin 8 of the microcontroller on the display board.

Unfortunately, trying to incorporate dimming on the existing clock PC board is quite difficult. The conventional way of doing it would be to use a transistor and LDR circuit to control the emitters of all six common-cathode driver transistors, Q15-Q20. We have used this scheme on quite a few past projects which had a PIC micro and 7-segment LED displays but a quick look at the PC board pattern shows that it would be quite impractical.

This presented a real dilemma un­til we came up with an alternative scheme: use an LDR and transistor circuit to allow the PIC micro to directly monitor the ambient light level and then change the duty cycle of the multiplexed drive to the 7-segment displays.

So we set out to develop the “Auto­dim Display Sensor” described here. The hardware was the easy bit, of course – the firmware mods took a bit longer.

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