Rongge

The circuit is based on a 555 IC (Bipolar or CMOS) for sensing the minimum and maximum water levels and turns a MOSFET on/off which directly controls a 12V DC pump motor. Or, it can power a relay coil to switch high currents/voltages, DC or AC. ‘Trigger’ and ‘Threshold’ pins (2 & 6) are used to detect the maximum and minimum levels, respectively. The two voltage comparator op-amps inside the 555 control the output, turning it on/off. Looking at the circuit diagram you will notice that the ‘Trigger’ pin (2) is marked ‘HIGH probe’, despite being triggered (output goes HIGH) when the voltage drops below 1/3 of the supply voltage and, the ‘Threshold’ pin (6) is marked ‘LOW probe’ while it is ‘reset’ (output goes LOW) when the voltage rises above 2/3 of the supply voltage. The circuit works as follows: Three (3) probes are immersed in the vessel. (usually from the top) One is the ‘GROUND’ probe, going to the level a little lower than the minimum desired level. This is the ‘common’ (or ‘...

This circuit operates an automotive windscreen washer pump to fill a 20-litre drum from a 205-litre water reservoir. The drum is suspended above a drip line, which irrigates a vegetable garden. Two stainless steel probes mounted in the drum act as sensors for the system. One probe is positioned at the high water mark, the other at about half-full. The pump power is switched by a 12V automotive relay (RLY1). Two op amps (IC1a & IC1b) connected as voltage comparators form the basis of the circuit.  Initially, assume a falling water level with the pump switched off. When the water level exposes the lower probe, the non-inverting input (pin 5) of IC1b rises to about 7.4V. With trimpot VR2 correctly adjusted, this will be higher than the voltage on pin 6. The output (pin 7) therefore swings high, biasing Q1 into conduction. This in turn causes Q4 to conduct, switching on the relay and starting the pump.  In addition, when Q4 switches on it supplies base current to Q3 via a 6.8k...