Electronic Design

Circuit Monitors Pump On/Off Times To Ensure Safe Operation

In a septic-tank-based sewage system, waste water and toilet wastes from, say, a residence drain into the first compartment of a two-compartment septic tank. A grating between the compartments allows just liquid waste to drain into the second compartment. This compartment is pumped out via a float-controlled effluent pump to a septic field, or into a municipal low-pressure sewage line that feeds into a lagoon. If the float isn't properly adjusted, becomes misadjusted, or fails, the pump will come on and stay on until it overheats and seizes up. Or, it can fail to turn on when required.

Additionally, any air leaks on the suction side of the pump can cause it to lose its prime and may result in the pump running continuously, as it struggles to self-prime. System failures can also result from a pump wiring fault, a tripped circuit breaker, a plugged impeller, or a frozen discharge line. In all of these cases, if the pump fails to pump out the second compartment in a timely fashion, the effluent may fill the two compartments. This can cause overflowing, backing up into the house plumbing, and possibly into the basement itself.

People, especially those in rural areas, who depend on septic systems require 24/7 reliable operation, whether they're at home or not. While it's impossible to guarantee flawless system operation for 100% of the time, it's possible to monitor the system performance and generate alarms as needed. However, it's not practical to use electronic level sensing inside the tank due to the extremely corrosive nature of the effluent and resultant gases.

The circuit shown in the figure monitors the pump's off and on times and generates an alarm if the pump is off or on for too long. In this design example, the pump shouldn't be run for more than two minutes in every 24-hour period. This circuit can be used to monitor the operation of other types of systems too.

The ac voltage across the pump motor drives the circuitry via optoisolator U1. While the pump is off:

  • The on-time counter U3 is cleared and held in reset.
  • A 20-bit counter (U4, U5) is incremented at about 10 Hz.
  • LED2 flashes at about 1 Hz to indicate that the off time is being measured.
  • After 220 pulses (approximately 30 hours), U6B is clocked. This turns on LED1, indicating that the pump was off for too long.

When the pump is on:

  • U7A is clocked, turning on LED5. This indicates that the pump has run at least once since the last manual reset via SW1.
  • The optoisolator output pulses (120 Hz) are stretched by D1, R4, and C2 to cause a single logic 1 pulse as long as the pump is on. This pulse clears and holds the off-time counter (U4, U5) in reset and enables a 12-bit on-time counter (U3) that's incremented at approximately 10 Hz.
  • LED4 flashes at about 1 Hz to indicate that the on-time counter is counting.
  • After 212 pulses (approximately 7 min.), U6A is clocked. This turns on LED3, indicating that the pump was on for too long.

The counters and status LEDs can be cleared by manually pressing the Reset pushbutton switch, SW1. The circuit is powered by a 12-V dc wall adapter, and a 9-V backup battery for continuous operation, even during a power failure. The two alarm outputs can be OR'd to drive an alarm buzzer.

It's easier to measure the voltage at the pump terminals than the current actually flowing into the motor. Note that if the voltage is present, but the current is zero due to an open circuit fault in the pump, this will be noticed because the pump will appear to be on for longer than normal.

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