This article describes a simple and inexpensive circuit that may be used in conjunction with any microcomputer that has an eight bit port, whose lines can be configured as outputs or inputs, to convert it into an auto-ranging digital frequency meter.The following notes describe its operation when used with the Acorn Master (or Model B with a 2 MHz VIA chip) computer. The USER port is ideal for this application, which requires access to five output lines and three input lines. The circuit is useful in the school or college where an auto-ranging purpose-built meter is prohibitively expensive although desirable. It has been designed to measure frequencies over the range of 2 Hz to 5 MHz with an accuracy of about 0.05%, and it has very adequate over-voltage input protection, with the ability to handle input signals within the range of +70V to −70V.