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.
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.