The tire pressure monitoring system (TPMS) has a positive effect on driving safety, fuel consumption and tire lifetime. Unfortunately, not all vehicles are equipped with this device and it is more or less impossible (or too expensive) to install any existing TPMS afterwards. The solution can be a "plug and play" TPMS and the main requirements are the following:

- can be installed on most vehicles
- continuous monitoring during driving
- warning during driving
- easy to install - do it yourself
- affordable price
- adjustable sensitivity

Most of the vehicles are using the same valve (Schrader valve) for tire inflation, so it is a good basis. The valve cap is exchangeable and it has many features today. There are also caps with tire pressure indications using a spring loaded piston to rise a red flag if the pressure is low. The problem is that the driver cannot see them during driving.

Most drivers have a cellphone (smartphone), so it is also can be a part of the TPMS.
The illustrated valve cap has a spring loaded piston. The tire pressure holds the piston in the upper position against the adjusted spring force. If the pressure decreases the piston sinks in the cap and creates an open cavity on the top of the valve cap. During driving the open cavity works as a resonator and generates sound which is proportional to the pressure. This sound can be detected and analyzed by a smartphone application and give a warning or show the actual pressure. If the pressure decreases further the size of the cavity is increases and the generated sound will be audible for the driver also, so the system works w/o smartphone also.

It looks difficult to find the generated sound in the environmental noise. The airflow over the cap is originated from the peripheral speed and the vehicle speed (driving wind). Luckily the position of the valve cap is always changing on the rolling wheel and airflow components are added and subtracted periodically. The periodicity of generated sound is proportional to the actual vehicle speed, so easy to filter it out from the environmental noise.

The same detection can be implemented in any control unit (ECU) of the vehicle and integrated into the vehicle information system.