A passive thermal oscillator combines a thermoelectric device and a passive analog electrical circuit to produce a time-oscillating temperature difference. The oscillator makes use of a temperature difference imposed across a thermoelectric device to produce a Seebeck voltage to periodically trigger electrical current to pass through a switch. The periodic electrical current causes periodic Peltier cooling producing a time-oscillating temperature difference across the thermoelectric device. There is no requirement for additional external energy input because the thermal energy generates a voltage that is used as the driving force. The operation is purely passive. So long as there is a temperature difference across the thermoelectric device, then the passive thermal oscillator oscillates. The passive thermal oscillator can integrate multiple energy conversion device technologies to operate cooperatively. The cooperation of multiple energy conversion technologies yields a much higher overall system efficiency than just the conversion of thermal energy into electrical energy.