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Autodim add-on for the 6-Digit GPS Clock By JIM ROWE 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. S OON 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. 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 siliconchip.com.au 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 un­ til 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 “Auto­ dim Display Sensor” described here. The hardware was the easy bit, of course – the firmware mods took a bit longer. How it works The new hardware consists of just a few parts on a small PC board. This mounts on 10mm spacers in front of the display unit’s main PC board, in place of the DB-9 connector (CON1) which was originally used to feed in the NMEA-0183 data stream from the September 2009  75 lower than the 2.2kΩ emitter/collector resistor. This makes it suitable for driving one of the analog-to-digital converter (ADC) inputs of the display’s PIC18F877A microcontroller (IC1). In this case, the sensor voltage is fed to the micro’s AN5 ADC input. And that’s really all there is to the hardware side of the add-on, apart from the 100nF capacitor across the 150kΩ resistor. This capacitor is used to filter the LDR’s output voltage, to remove any modulation from the indoor ambient lighting level. +5V ON MAIN BOARD 10k LDR1 (RP-3480)  B B C Q1 BC548 E Q2 BC558 C PIN 8 (AN5) OF IC1 (0V FOR VERY LOW AMBIENT, +4.5V FOR BRIGHT SUN) E 100nF 150k 2.2k GND ON MAIN BOARD BC548, BC558 SC 2009 AUTODIM DISPLAY SENSOR Modified firmware B E The modified firmware for the PIC regularly monitors the voltage applied to the AN5 input (pin 8). It does this by directing the ADC module inside the PIC to measure this voltage. It then tests the measured voltage level and varies the on-off ratio of the display digit switching signals to vary the apparent display brightness, over six levels. As a result, the apparent display brightness varies between virtually full brightness at high ambient light levels down to about 17% of full brightness at very low ambient levels. C 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. TO PIN 8 OF IC1 (PIC) TO +5V LINE TO GND 1 9 0 8 0BC548 240 LDR1 2.2k 150k ERJ Q1 100nF +5V AN5 GND BC558 Q2 10k Building it Fig.2: install the parts on the PC board as shown here. Take care not to get the two transistors mixed up. GPS Frequency Reference. This DB-9 connector is no longer needed if you’re using the GPS receiver module described in the June 2009 issue. Fig.1 shows the circuit details. The ambient light level is sensed by LDR1, a small light-dependent resistor (LDR) which varies its resistance between about 2MΩ in the “dark” and a couple of hundred ohms in bright sunlight. This LDR is connected in series with a 150kΩ resistor across the +5V supply. As a result, resistance changes in the LDR result in corresponding DC voltage changes across the 150kΩ As mentioned earlier, the additional components are all mounted on a small PC board. This is coded 04208091 and measures just 36 x 19mm. Fig.2 shows the parts layout on the PC board. The only polarised parts are transistors Q1 and Q2, so make sure you fit these with the orientation shown. Also be careful not to swap the two: Q1 must be an NPN BC548, while Q2 is a PNP BC558. The LDR is not polarised and can be fitted either way around. Leave its leads about 15mm long so that they can be bent outwards from the PC board after it is fitted to the board. This allows the sensitive “front face” of the LDR to be turned away from the main LED displays when the add-on board is mounted in position. Note: if the LDR is able to pick resistor, the level varying from close to 0V when the ambient light level is very low up to about +4.5V in bright sunlight. Unity-gain buffer The other components in the circuit, based on transistors Q1 & Q2, make up a near-unity gain impedance step-down buffer. This ensures that the light-dependent output voltage is made available at a much lower impedance level than 150kΩ. In greater detail, transistors Q1 and Q2 form a complementary emitter follower, with a source resistance much Table 1: Resistor Colour Codes o o o o No.   1   1   1 76  Silicon Chip Value 150kΩ 10kΩ 2.2kΩ 4-Band Code (1%) brown green yellow brown brown black orange brown red red red brown 5-Band Code (1%) brown green black orange brown brown black black red brown red red black brown brown siliconchip.com.au IC1 PIC 16F877A (8) REG1 78L05 1 + + 4004 CON2 MAIN DISPLAY BOARD +5V AN5 GND AUTODIM ADD-ON BOARD 19080240 ERJ NOTE: FOR CLARITY, WIRES ARE SHOWN ABOVE MAIN BOARD. IN REALITY, THEY LOOP AROUND TO UNDERSIDE OF THIS BOARD Fig.3: only three leads are required to connect the autodim board. Note that the connections are actually run to the underside of the display board. up too much light from the displays themselves, the auto-dimming feature won’t work. Instead, the displays will run at full brightness, regardless of the ambient light level. Only three wires are required to connect the auto-dimming board to the main display board. Fig.3 shows the details. The first step is to solder three 80mm lengths of light-duty hookup wire to the three external wiring points (ie, +5V, AN5 & GND). That done, attach two M3 x 10mm tapped spacers to the underside of the add-on board, using two M3 x 6mm machine screws. The completed add-on assembly can now be mounted in the lower lefthand corner on the main display board. You will have to remove the display board from its case in order to do this. The add-on board is secured in place using two further M3 x 6mm screws which pass up through the upper pair of 3mm holes that were originally provided to secure the DB-9 connector (CON1) – see photos. The three leads from the add-on board can now be fed to the rear of the main board and connected to the Parts List 1 PC board, code 04208091, 36 x 19mm 2 M3 x 10mm tapped spacers 4 M3 x 6mm machine screws 1 light dependant resistor (LDR1) 3 80mm lengths of light-duty hookup wire Semiconductors 1 BC548 NPN transistor (Q1) 1 BC558 PNP transistor (Q2) Capacitors 1 100nF MKT metallised polyester Resistors (0.25W 1%) 1 150kΩ 1 2.2kΩ 1 10kΩ appropriate points underneath. As shown in Fig.3, the +5V lead goes to the +5V supply rail near the output pin of REG1, while the ground wire goes to the earth copper at what was originally pin 5 of CON1. The third centre wire (AN5) goes The autodim board is mounted on two M3 x 10mm tapped spacers. These are attached to the main board using the holes originally provided to secure the DB-9 connector. to pin 8 of the display’s PIC16F877A micro (IC1). This pin was not used in the original “non-dimming” version. Once these three connections have been made, you can slip the finished board assembly back into the enclosure and fasten it in place. All that now remains is to download the new version of the GPS Time Display firmware (0410509B.hex) from the SILICON CHIP website and reprogram the PIC micro so that it knows how to monitor the LDR voltage and vary the display brightness accordingly. Note that the add-on board by itself won’t give you the autodimming function unless you use the revised firmware in your PIC. The updated firmware now replaces the original program whether or not you have the add-on board. However, if you don’t have the add-on board, you must now connect pin 8 of IC1 to the adjacent +5V supply rail via a 2.2kΩ resistor. This is necessary to give the PIC’s AN5 input a de-facto “bright sun” input voltage level in the absence of the LDR sensor board. That’s it! With the add-on LDR sensor board and the modified firmware running inside the PIC, your GPS Time Display will now be able to adjust its brightness according to the ambient SC light level. Issues Getting Dog-Eared? Keep your copies safe with these handy binders. REAL VALUE AT $14.95 PLUS P & P Available Aust, only. Price: $A14.95 plus $10.00 p&p per order (includes GST). Just fill in and mail the handy order form in this issue; or fax (02) 9939 2648; or call (02) 9939 3295 and quote your credit card number. siliconchip.com.au September 2009  77