A large variety of meters are designed to measure a flow of fluid, such as water or gas, or measure a flow of electricity over time. Typically the flow is used to generate mechanical rotation which operates through a gear train to produce a dial indication of total quantity. Examples are odometers, water meters, gas meters, electrical meters, hour meters, etc. These devices were largely developed before the advent of modern digital circuitry. Today many meters are read remotely, or by a digital computer or microprocessor. Thus an hour meter on a piece of equipment may need to send a digital signal to a microprocessor which controls and monitors that equipment. Water meters for example can be read remotely, for example outside a building or home, to facilitate manual reading. A remote reading may also be accomplished over the phone or by radio or satellite linked to a central computer.
The ubiquitous nature of metering devices means cost, together with high reliability, is a prime consideration. A highly reliable mechanical switch generally means a reed switch, with its hermetically sealed contacts and life of up to one billion cycles without failure. Meter mechanisms have been modified to produce an electrical output by mounting a magnet on the mechanical output dial and detecting rotation of the magnet with a reed switch. Unfortunately, tampering with the meter is always a concern where the meter output is used to calculate a charge to a consumer.
What is needed is a way to convert a rotating dial output to a digital electrical signal which is of greater resolution and which can detect tampering.