555 Timer as an A/D converter

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
555 Timer as an A/D converter - schematic

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. `uncomment the debug lines to get pulse value while calibrating loop: `debug cls pulsin 0, 1, w2 `I used pin 0 `debug w2 w1=26150 `This is the coefficient you will need to calibrate. w4=w1/w2 w3=w4*100 `I am going to get around the integer-only Stamp math. w4=w2*w4 w1=w1-w4*10 `remember the Stamp has left-to-right math w4=w1/w2 w3=w4*10+w3 w4=w2*w4 w1=w1-w4*10 w4=w1/w2 w3=w4+w3 w3=w3+250 `250 is really 2. 5 volts debug w3, "volts * 100" `we get a reading in hundredths of volts goto loop

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