toilet cistern overflow monitor schematic

Retrofit Toilet Cistern Overflow Monitor

Toilet cistern overflows not only waste water but increase the risk for slip-and-fall injuries. The retrofit cistern overflow monitor circuit presented here safeguards your property and helps to maintain a safe and sanitary bathroom environment. The circuit’s water stream sensor detects water discharge through the overflow outlet of the cistern and then automatically turns on a warning indicator. The circuit operates from battery power and can be installed without the need for special test equipment or on-site collaboration.

The Electronics

The circuitry of the system is a simple, true-gating astable multivibrator build around the older, but still useful, low-power monostable/astable multivibrator CD4047BCN (IC1). Here, the astable operation is enabled by a high level on the astable input (pin 5). The output frequency (at 50% duty cycle) at Q and Q outputs (pins 10 and 11) is determined by the RC timing components (R1 and C1).A frequency twice that of Q available at the oscillator output (pin 13) is used to drive the warning indicator (LED1) through a current-limiting resistor (R3). Power supply is provided by a CR2032 lithium-manganese dioxide battery (BAT) with a nominal capacity of 3 V/220 mAh. The water steam sensor (WS1) consists of two short-length stainless steel electrodes, electrically connected on positive supply rail (VCC) and the astable input (pin 5) of IC1. As shown in the circuit diagram, an optional piezo sounder can also be added to enhance its overall performance. This calls for a ready-made 3-V buzzer module for the three-pin connector J1.

toilet cistern overflow monitor schematic

According to the datasheet, timing components used here (R1-C1) should give a typical pulse width of 2.2 R1C1, i.e., a frequency of 4.5 Hz (1/2.2 R1C1) at the oscillator output (pin 13) of IC1. In the prototype, it’s observed as 4.390 Hz (duty cycle 48.6%) — a nearly textbook-perfect value!


The Mechanics

The entire electronics, cooked up on a small perfboard with the battery, should be housed in a small prototype enclosure — made waterproof using epoxy adhesive — including the water stream sensor (two stainless steel electrodes protruded through the bottom of the case). The finished system can be mounted on top of a T-Adapter attached to the overflow pipe in the cistern. The detection level of the water stream sensor enclosed through the top hole of the T-Adapter should be set to occur at a point when an overflow situation is imminent but before water is wastefully discharged. This can be done by lining up the length of two steel electrodes.

finished system picture

In conclusion, the CD4047BCN offered a cost-effective and satisfactory solution for this application. The cost is low and the chip is easy to obtain. As always, avoid the risk of very common counterfeit chips and get yours from a reputable vendor. Living in India, I got mine on eBay. Thanks to National Semiconductor Corporation (NSC) for product information from their CD 4047 datasheet.

Another Flow Sensing Idea

Since designing the above simple circuit for water flow sensing, I have discovered an alternative option in a documentation by Littlefuse ( Littelfuse offers reed switches and reed sensors for use in a variety of flow-sensing applications. One idea to make a flow sensor is to use a reed switch to detect the motion of a paddle fixed with a lightweight permanent magnet, which operates as an integrated unit, as shown below. The reed switch is positioned such that it closes when the magnet on the paddle passes by.


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