TIMING UNIT

A fail-safe long interval timer has a capacitor which is charged progressively and the voltage across which is sampled momentarily at intervals, without fully discharging the capacitor, by connecting a pulse transformer across the capacitor for the duration of very short pulses which act on a solid state switching element. When the sampled pulses reach a threshold amplitude level a threshold device is triggered, after a time interval which will be determined by the charging circuit and the capacitor as well as the characteristics of the sampling circuit.