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.

AT Mega 8 digital Meter
source :

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