When there exists no water in the tank and zero water is flowing out of the pipe plug, the pump motor could be equipped with an efficient dry run protection system, as shown in the accompanying diagram.
In this section the motor is started by first pressing the push button.
The motor continues to operate for some moments because the 1000uF capacitor and the 56k resistor function as a delay off timer, keeping the transistor turned ON regardless of when the push button is withdrawn.
Water will build up in the little container placed close to the hose pipe's mouth since water is anticipated to come out of the pipe outlet throughout this period. A float magnet and a reed switch relay can be observed within this vessel.
The moment water begins to fill the vessel, the float magnet swiftly climbs to the top and approaches the reed relay, locking it on. In order to ensure that the transistor latches up and maintains the motor's operation, the reed relay immediately supplies a positive voltage to the transistor's base.
The motor shuts off after the delay OFF expires after the predefined length of delay, though, since the reed relay feedback cannot turn ON in without the presence of water.
Safeguarding for Dry Runs with no Reed Switch and Magnet
The above design may be made simpler for those who don't want to utilize a magnetic float sensor setup or a reed switch, as shown in the image below:
The relay is operated by BC547 and BC557 conducting as soon as the push button is pressed. The associated pump motor is turned on by the relay. In an effort to move water via the pipe, the pump motor starts up.
Water that is accessible flows via the pipe outlet and connects the BC547 base to the positive supply. The pump keeps on pushing water constantly as a result of the entire circuit latching through the water contact.
But in the event that water is scarce or dry, the BC547's base cannot be supplied with any water. As a result, BC547 only stays turned on for as long as the 100uF capacitor is charged.
How long the BC547, BC557, relay, and pump motor can stay turned on is determined by the 100uF capacitor along with the 1 M resistor.
The circuit automatically shuts off the pump motor and itself immediately as the allotted time has passed, preventing the motor from heating out because of a dry run scenario.
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