A non-intrusive flow detector consists of an easily adjustable visible light optical sensor assembly, producing a spot of light, which attaches non-obstructively and non-invasively to a fluid flow meter. The spot of visible focused light requires less than one milli-amp. The sensor monitors dial movement of a least flow dial or wheel and generates an electrical signal representative of a flow through the meter. The signal is then transformed by a 10-bit A/D converter and further processed. The flow rate and consumption is displayed in suitable units on a LCD. The microprocessor can be programmed to detect extremely small flow rates. With an appropriate sample time programmed, a flow of a few drops per minute would be detected. This embodiment aims to provide an improved method and means for enabling a consumer to monitor consumption of a metered source. This embodiment can be utilized as a sprinkler alarm.