Low tire pressure indicator circuit

摘要

In a system having a pressure sensor on a vehicle tire for generating a magnetic field when tire pressure reaches a predetermined value, an indicating circuit includes a stationary reed switch which is closed when subjected to the magnetic field, a coil in series with the reed switch which develops a magnetic flux when the switch closes to latch the switch closed, and an indicator lamp in series with the reed switch which is continually illuminated when the switch is closed to provide a low tire pressure warning. A test circuit energized when the vehicle ignition key switch is in "start" position includes a pulse forming circuit for generating a current pulse in the latching coil to close the reed switch and provide an indicator light if the indicator circuit is operative. Power to the indicator circuit is interrupted when the key switch is moved from the "start" position to "run" position so that the coil is momentarily de-energized and the switch opened thereby placing the indicator circuit in readiness for sensing a magnetic field from the pressure sensor.