In this design, i will be using a 555 timer IC in monostable configuration to achieve no-contact automatic sanitiser dispenser.
When trigger is connected to a sensor, the monostable circuit itself can start pump.
but we encounter a problem with the standard monostable circuit, where the output stays high as long as the trigger pin is held LOW / Triggered.
But that is a problem in our application, as the liquid overflows the pams and starts to drip to ground, hence wasting the liquid.
so, by adding an RC (resistor, capacitor) in between trigger pin and trigger source, with +ve of capacitor to trigger source and -ve of capacitor to 555 trigger pin, we can make this circuit auto cut-off after the monostable pulse time set by (Rv and Cv) at 555 IC.
The RC couple in between the trigger circuit acts as trigger pulse, where, even if sensor is countinuously triggering, only a single pulse is sent to the 555 IC trigger pin.
This happens because, when the trigger is detected, the pulse is sent to 555 trigger pin only till capacitor is fully charged, after that the capacitor blocks the trigger signal.
on removal of trigger signal from sensor, the capacitor gets discharged by the resistor in parallel to it, resetting the RC pair, ready to accept new sensor trigger / signal.
Hence, for each time hand detected by sensor, liquid is only dispensed for only the set amount of time even if hand is placed below the sensor indefinitely, and it only re-dispenses liquid on removal of hand and placing it again.
by using this circuit, we can limit liquid usage and also avoid wastage.