A low-power method and apparatus is provided for adapting to time-varying limitations of a power source, such as a vehicle power source which is in a more-limited state when the engine is off. The supply voltage is monitored for changes using an unclocked, low-power first stage having an analog section, a voltage comparator. Upon detecting voltage changes reflective of a potential power source state change, the first stage generates an interrupt. In response, a second stage transitions from a low-power standby mode to a higher-power active mode. The second stage may include a microprocessor and is configured to confirm or disconfirm the state change. Upon confirmation, further operations are triggered. Upon disconfirmation, the second stage returns to standby mode. The first stage may include an operational amplifier whose two inputs are indicative of the supply voltage, one input having a different response rate to voltage variations than the other.