Abstract: A low voltage single supply, ultra-low power sub hertz timer using a CMOS Schmitt trigger operating in sub-threshold region is presented. Sub-Hertz operation is achieved by controlling the amount of current for charging and discharging a capacitor at the control input of the Schmitt rather than by using large passive components. Pulse width modulation is achieved choosing the width per unit length parameters of transistors used in charging and discharging control blocks for the capacitor. The circuit uses a low supply voltage and can be designed for the higher voltages if required by specific applications. The circuit can produce sub-hertz oscillation with pulse width modulation. The capacitor has a small footprint compatible with integrated circuits. Power consumption is small with short ON times.