555 Timer as an A/D converter
I had a Basic Stamp project that needed to measure a nominal 12 volt battery, and I wanted a simple solution. This is the simplest I could come up with. The 555 timer will put out positive pulses. The
pulse width is inversely proportional to the difference in voltage between the voltage at "ANALOG IN" and the voltage of the 4.7uF capacitor(let's say 2.5 volts). To calibrate this circuit, hook it up to
a Basic Stamp measuring positive pulses, and give the circuit a known voltage. Let's say you get the number 2092 when you give the circuit 15 volts. Your coefficient is 2092 * (15 - 2.5) = 26150.
Now you are ready to measure voltage with your Basic Stamp. Use the formula: voltage = 26150/pulse + 2.5 . You will have to modify this to work within the limits of the Basic Stamp's math.
The accuracy of this circuit rivals many digital voltmeters within the range I tested it (6 volts to 18 volts), about the same as a 10 bit A/D converter. The accuracy will shift with the processor clock
and the +5 supply, so it is pretty good. Conversion time is under 1/10 second. Please note it will not measure voltages below 5 volts. Also, check the accuracy of your +5 volts. If it is 5.2 volts, you will
need to use 2.6 in the formula. A sample program listing follows.
'uncomment the debug lines to get pulse value while calibrating
pulsin 0,1,w2 'I used pin 0
w1=26150 'This is the coefficient you will need to calibrate.
w3=w4*100 'I am going to get around the integer-only Stamp math.
w1=w1-w4*10 'remember the Stamp has left-to-right math
w3=w3+250 '250 is really 2.5 volts
debug w3,"volts * 100" 'we get a reading in hundredths of volts
The software and hardware are released into the public domain.
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