Using a single operational amplifier to make a dual-purpose water level control circuit

The controller made with a single op amp, the circuit is shown in the figure. The switch SA is placed in the water-up position on the right. Connect the DC 12V. When the water level is lower than the low water level B, the voltages of the op amp LM358A's in-phase and inverting input terminals ③ and ② are equal, and its output terminal ① has no output. The 12V voltage lights up LED1 and LED2 at the same time through P3 and P4. There will be a 6V voltage on the ① pin, which is coupled to the VT control electrode through R5 and C5, so that VT is turned on, LED3 is turned on, T2 is turned on, relay K is energized, and the water pump works. When the water level is equal to or higher than the low water level B, the voltage of the op amp LM358A's in-phase input terminal ③ is lower than the voltage of the inverting input terminal ②, and the ① pin outputs zero potential, LED2 goes out, and LED1 keeps on glowing. Due to the double-layer blocking of D1 and D2, the VT control electrode potential will not be pulled down, VT continues to be turned on, and the water pump continues to work. When the water level rises to the high water level A, the LM358A non-inverting input voltage is higher than the inverting input voltage, the ① foot outputs a high level, LED1 turns off, LED2 lights up, coupled by R5 and C5, through the 8V voltage regulator tube VD to the base of T1, T1 turns on, VT turns off, T2 turns off, and the water pump stops working. When the water level drops below A, the output of the op amp is zero level, the water pump still stops working, and when the water level drops below the low water level B, a new round of water filling begins.

Turn SA to the left end of the drainage gear. Connect the DC 12V power supply. When the water level is lower than the low water level B, similarly, the 6V voltage stored in the output pin ① turns on T1 and turns off T2 through D1, L4, and D2, and the water pump does not work. When the water level rises to the low water level A, the pin ① outputs a high level, which turns on VT and T2 through D1 and D3, and the water pump starts to work. At the same time, the output high level is coupled to C6 through R5 and C5 for decoupling. When the voltage filtered by the choke coil L4 does not reach the base of T1, the water level is lower than the A position due to the water pump drainage, and the output pin ① outputs a zero level again. At this time, C6, L4, and C7 discharge the stored charge through R6 and R7. When the water level is lower than the low water level B, the 6V voltage stored in the pin ① turns on T1 and turns off T2, and the water pump stops working. When the water level rises to the high water level A, a new round of drainage begins.