I finally got around to building another cheap enuresis (bedwetting) alarm with the help of a couple of inexpensive and easily available electronic parts. This little enuresis alarm is a tricky design because it is missing the usual digital readout panel and intriguing buttons. I am happy with how it turned out, and it was a lot of fun to create something ‘analogue’. Now it is your turn to chime in!
Did you get it?
When a child urinates without control while he/she sleeps, it is called nocturnal enuresis. It is also known as bedwetting. Enuresis alarms are a proven method to cure bedwetting in many children who suffer primary nocturnal enuresis. The wired bedwetting alarm has a moisture sensor pad on a flexible cord that plugs into a body worn alarm box. The alarm box usually attaches to the shoulder of the child’s nightclothes and the sensor cord is run under the pajamas down to the underwear where the sensor is placed to detect the bedwetting. Further reading at https://www.urologyhealth.org/urologic-conditions/nocturnal-enuresis-(bedwetting)
First, look at the basic circuit diagram of my little enuresis alarm. As you can see, the design is elemental and self-explanatory.
This and low-profile design is lightweight tailored to work with a compact single cell (1S) Lithium Polymer (LiPo) battery (3.7V/600mAh). The miniature slide switch (S1) functions as the master power on/off switch to enable/disable the alarm electronics. A red light (LED1) is also included to indicate the alarm switch status – the lamp indicates standby mode of the alarm enuresis.
At the frontend of the circuit is the venerable NE555 timer chip (IC1) wired as a simple moisture sensor controlled electronic switch. DC voltage output supplied by IC1 is used to power up the melody generator chip BT66T-19L (IC2) – the popular equivalent of the UM66 3-pin melody generator IC available in standard TO-92 package (https://datasheetspdf.com/ pdf-file/517048/Formox/BT66T/1). Since the BT66T chip comes in five different part numbers (of course with five unique melodies), you can include a 3-pin socket/female jumper header in your circuit board to shift it at ease when necessary.
The final component is the common NPN transistor S8050 (T1) driving a mylar loudspeaker (SP1). Note that the BT66T can directly drive a piezoelectric element (piezo ceramic disc) but that is not enough to get a reasonable audio output. The loudspeaker I used is a 16Ω/1W mylar speaker. The mylar speaker is loud enough for a nighttime alarm device.
The maximum power applied to a 16Ω loudspeaker will be close to 450mW with a 3.7VDC supply if it’s driven by a standard square wave signal – the RMS voltage of a 0V based/50% duty cycle square wave is: Vrms = Vpk ÷ √2 . Keep this note for your reference hereafter!
Frankly I’m still playing with my breadboard prototype as it’s easier to debug (I wanted to make the end-user design an adept one).
As you might observe, my homebrewed moisture sensor pad is nothing but a flake of the common circuit board (stripboard – https://en.wikipedia.org/wiki/Stripboard). I plugged the moisture sensor wires directly to the breadboard instead of using a dedicated connector, but you can solder a 2-pin JST connector (SEN1 in the schematic) in your circuit board to plug your specific moisture sensor pad with no difficulty. If you do not want to build the moisture pad yourself, try to pick up a “Grove-Water Sensor v1.1” (see below). If so, just link the water sensor’s SIG and GND connections (Leaving its VCC) to the SEN1 connector because the 1MΩ (105 SMD) pullup resistor pre-soldered on the Grove-water sensor module v1.1 is not necessary – you already have it in your circuit (See R1 in my schematic).
Anyway, the moisture sensor pad, which is intended to be attached to a closefitting
underwear or placed over a mattress, needs to be a good size to capture the drops of urine. Also, a dual-sided moisture sensor pad that detects moisture on both sides can be more effective in some situations.
This is the photograph of a pretty good enuresis mat sensor I found online (https://www.vitalcare.com.au/products/).
Back to the project, assembling of the circuit on a common circuit board is easy because all the parts are thru-hole with no surface-mount devices. After construction, put your build inside a waterproof enclosure. Note that you obviously need to employ a dedicated charger for the LiPo battery. An easy way is to buy a $1 LiPo battery charger module (look below) from a webstore – simply Google “Micro USB 3.7V 1A 1S LiPo Battery Charging Module”, and go!
However, it would be better to mount the battery charger module close to the edge of the enclosure to have quick side access to its micro USB connector. I tested the above pictured module since I had a supply, but almost any similar module will do.
And now for something different!
For elderly people who are incontinent, a wireless wet-the-bed alarm is usually more acceptable than the ordinary body-worn wired enuresis alarm. In a typical wireless wet-the-bed alarm, the moisture sensor mat get into the bed is attached by a flexible cord to a wireless transmitter/wireless call point and the system is programmed to alert through the wireless network when moisture is detected.
You can adapt the leftmost part of my basic schematic to build a polished version. I used the 555 IC as an inverting buffer which enables a high impedance signal source to switch a low impedance output transducer.
- Input low (< 1/3 Vcc) makes output high
- Input high (> 2/3 Vcc) makes output low
When the input voltage is between 1/3 and 2/3 Vcc the output remains in its present state. This intermediate input region is a dead blank where there is no response, a property called hysteresis – often a helpful property to give the input a high immunity to noise.
At this point you should note that while running on a 3.7V LiPo battery, the maximum dc voltage available at the ‘loaded’ output (Pin 3) of 555 IC is about 2.4V only. Luckily, that’s more than enough to control certain devices like the VOM1271 photovoltaic mosfet driver from Vishay Semiconductors (https://www.vishay.com/docs/83469/vom1271.pdf). Take time to smell the roses!
Moreover, since it feels that everyone these days loves a wireless device, I also decided for fun to make a little one. The next project will be a quick renovation of my basic design idea covered here, and my intention is to employ a small Wi-Fi module as its radio head. So, try to check back on this page regularly…There might be updates!
So, there you have it. A simple, customizable enuresis alarm. Now it’s your turn to build one for a child who is keen to stop his/her bedwetting and happy to use your enuresis alarm!
Disclaimer: This is a simple hobby electronics project. This communication is not intended nor endorsed as a substitute for medical advice, diagnosis, or treatment. Always seek the advice of your physician regarding any medical questions or conditions.