Time based low tire pressure warning sensor

DETAILED DESCRIPTION It is therefore an object of the invention to achieve a LTPW transmitter unit which attains long battery life without an inertia switch.
Another object in such a unit is to avoid repetitious signaling before factory installation.
A LTPW system has a vehicle mounted receiver and a transmitter or sender mounted in each tire for sending pressure data to the receiver which displays a warning when appropriate.
The transmitter includes a pressure switch or absolute pressure sensor coupled to a microprocessor which in turn controls a RF transmitter circuit.
A battery is continuously connected to the microprocessor and the transmitter circuit.
The microprocessor generally resides in a low power consumption sleep mode and wakes up every few seconds to execute software stored therein.
The software is able to define a factory test state and an in-use state.
Initially an IN-USE flag is clear to allow tire assembly, battery installation and factory testing.
The program senses when the tire is first pressurized to a threshold level and then issues only a single signal for the test of the unit.
After the tire is pressurized for a time sufficient for the transmitter to be removed from the factory test state, the IN-USE flag is set; thereafter the unit will be in the in-use state and will issue signals intermittently.
When the microprocessor wakes up and finds that the IN-USE flag is set, it determines from a read pressure timer whether it is time to read pressure; if so the pressure is read and the timer is reset.
The timer value is several wakeup cycles so that the pressure is checked perhaps every 10 to 30 seconds.
If the pressure switch has changed to low pressure or if a pressure sensor detects a significant pressure drop, the transmitter will be energized to send a warning signal to the receiver.
When the pressure is not read, a transmission timer is checked for transmission time.
When it is transmission time, pressure data is read and then transmitted.
Then the transmission time is set at between 3 and 5 minutes by random selection, and the microprocessor enters sleep mode