Bracket, you can access the internal spring which sets the sensitivity.For example, by using a very light spring (ie, one that provides just enough force to return the diaphragm to its un-triggered position when the. Pressure is removed), it's possible to get a switch to trigger at just 5cm of water (~0.5kPa or 0.08psi). The external adjustment would then typically give a range of about 5cm to 7.5cm of water, but of course, this will vary depending on the unit and the spring used. Put in a stiffer spring and the adjustment.
The motor cannot be reversed instantly. It needs a brief pause between switching directions, or else it may get damaged. For this purpose, another spin direction control timer (IC2) is employed. It is realised with an IC 555. This timer gives an alternate ‘on’ and ‘off’ time duration of 10 seconds and 3 seconds, respectively. So after every l0 seconds of running (either in clockwise or anticlock-wise direction), the motor stops for a brief duration of 3 seconds. The values of R3 and R4 are calculated accordingly.