In previous articles in this series we have described an A/D converter based on a counter. A ‘real’ A/D converter, however, converts a voltage into a measured value. Usually, this is thought to require an IC; however, as we shall see, a much simpler alternative is available.A real A/D converter with a voltage input can be constructed using a single transistor. The transistor in Figure 1 operates as a comparator, comparing the voltage on the capacitor with a reference value, here 0.7 V. The capacitor is alternately charged and discharged by the DTR output via a resistor, in such a way that the voltage across it is always near to the comparator reference. Depending on the input voltage the output will need to be turned on more or less frequently in order to establish the desired voltage. The count of these events leads to the converted value. Figure 2 shows how the circuit can be constructed.