Having built a battery-powered valve headphone amplifier I discovered that the battery was often fully drained due to forgetting to switch it off. After replacing yet another empty battery, I decided it was time to build a timer to shut down the amplifier automatically. Reviewing my standard usage, a period of 90 minutes seemed to provide the optimal target duration. A circuit based upon an oscillator was quickly discounted due to the potential for audible interference and poor power consumption. Instead, a resistor-capacitor (RC) network together with a Schmitt trigger and MOSFET provided precisely the function required with very little current drain.