As indicated above by a couple of posters, some Hobbs switches come with two contacts, one normally open that starts the hour meter with oil pressure; the other is normally closed and breaks a circuit on oil pressure. The latter is simply wired to the warning light on the panel. If the ignition is on with no oil pressure, the light is on. It goes out on start with oil pressure. (I recall the Hobbs devices are set at around 10psi O.P.)