Digital Meter ATMega8 Circuit

The circuit came out to be very simple and compact. The ATMEGA8 is the lowest pin count AVR controller that I could find that has an onboard 10 bit A-to-D converter. It had no problem directly driving the four digits delivering an average approximately 50 milliamps to the display. The on-chip clock oscillator also saved some parts. I am not sure whether the 8 uH inductor and 0.33 uF decoupling capacitor on the analog VCC was necessary, but I used them as a good practice. Better to put a couple of extra parts on the board than to take the chance of having to take the board back and add them later. The schematic above shows an ISP (In Circuit Programming) connector, which I had originally built onto the board so I could debug the firmware, using the crash-and-burn method - write the code and see if it runs. After I was satisfied with the performance of the meter, I cut off the portion of the board that had the ISP connector on it. Power for the chip is regulated with a 7805 regulator. In this particular case, I used a TO-92 LM78L05 regulator. Measuring the current into the 5 volt input, I find a maximum of about 56 milliamps current drain, when displaying 08.88 volts. When powered from a 9 volt power source, the dissipation of the LM78L05 will be 224 mW. With a 200 degrees C per watt thermal resistance, junction temperature should be about 45 degrees above ambient. A little more calculation showed that the maximum safe input voltage to the LM78L05 is just a little above 12 volts if my maximum ambient temperature is 40 degrees C. In Don’s case, where he would power the meter from 30 volts, it would be best to use a TO-220 version of the LM7805.



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