A signal processing circuit comprises a difference amplifier receiving an input signal and a feedback signal. A bistable circuit switches repeatedly between states only when the difference amplifier''s output is non-zero and positive. The feedback signal is developed by a switch that is closed to apply to an integrator a reference signal in response to pulses from the output of the bistable circuit. A source providing a pulse train clocks the bistable circuit. A heat meter in which the input signal comes from a temperature sensitive bridge and the pulse train comes from a flow meter is described.