Toilet alarm – ElectroSchematics.com

This alarm allows a distressed person to raise an alert from within a toilet in the event of an emergency. To operate, the user simply pulls the cord of a ceiling pull unit (or presses a panic button) to activate an over door light and panic alarm outside the toilet. Usually this system consists of a master control box which is connected to a pull cord switch, remote audio/visual indicators and remote reset button.

Here are the project details in case you want to make your own disabled toilet alarm. And see how I built one from scratch for less than $10. Let’s start with the schematic!

This circuit uses a BT169 thyristor (T1) together with a 5V electromagnetic relay (K1) to form a dual-channel latch switch mechanism (self-locking switch) which can control external electric/electronic devices as needed. Normally, it does not work but when the pull cord switch/panic button (S2) is touched off, the relay is energized (and latched) instantly. The master power on/off switch (S1) can be flipped then to reset the system by disrupting the dc power supply coming from the 6V battery pack (1.5V AAA cells x 4 in series).

As you can see in the schematic, one BC547 transistor (T2) is used here to drive the alarm status indicator (LED1). The LED indicator lights up slowly after the alarm is armed thru the trigger switch and it effectively acts as an endurance indicant. This indicator lamp driver circuitry built around T2 can be used to ‘communicate’ with an external microcontroller system or the like by replacing LED1 with a suitable optocoupler or a solid state relay.

And, the quick breadboard layout (you must build it on a circuit board)…

I used the “O/E/N 46-05-2CE” relay but almost any 5V relay will work. I picked the O/E/N part because it’s a pretty compact and breadboard-friendly PCB mount electromagnetic relay.

Related Documents: O/E/N Relay https://oenindia.com/php/product_guidelines1.pdf , BT169 Thyristor https://www.mouser.com/datasheet/2/302/BT169D-527397.pdf , Momentary Pull Cord Switch https://www.electricaldirect.co.uk/product/mk-6a-momentary-pull-cord-switch-with-red-cord-white-932149

The relay has two sets of Form C contacts (https://en.wikipedia.org/wiki/Electrical_contacts#Form_C) ie you’ll get a pair of SPDT switches thrown by a single trigger. So, you can exploit a single relay to control two different type devices simultaneously – an AC230V over door light and a DC12V emergency alarm for example. Related Tutorial: https://tutorial.cytron.io/2012/08/01/identify-terminal-pins-of-a-relay-without-reference-to-datasheet-2/

And the “Relay → Load” wiring pointer. CAUTION: Do not attach or remove wiring or connectors when power is applied!

For the panic/emergency alarm part I used a cheap $2 electronic bicycle bell I bought from an online store as it was cheap but will make plenty of noise when triggered. This is the ‘partial’ schematic of that battery powered bicycle bell. Although not very detailed, the circuit diagram contains the key components and their arrangement. I have a reason for those exceptions!

It’s a 108dB piezo-based unit that produces a high pitched warbling siren (able to scrap earwax off the walls – ha ha). The diminutive size bicycle bell runs off of 3VDC catered by two 1.5V AAA cells rests in its own battery chamber. I modified it by simply opening the outer shell, soldering two leads (AB) to the existing bell switch (while keeping the button switch intact) for remote triggering, and re-assembling it after routing the ‘new’ wires patiently through a narrow slot underneath the bell button (I had luck and found the slit there).

The high retail price of traditional over door lights and alarms biased me against a solution that I could have considered earlier. The cheap electronic bicycle bell is certainly a good match for my project. It may also be easy to reverse-engineer its electronics. The bicycle bell made of plastic has four sections – the outer plastic horn, the inner bullet which holds the supporting electronics, the piezo resonating chamber which has a base, lid and the piezo element, and the battery compartment. It will also let us include additional electronics as there’s ample space inside the enclosure.

The final breadboard setup:

While arming the alarm you can see a waveform similar to this one if probed at the collector lead of T2. Yes, when T2 turns to active state we immediately get a high-to-low transition there in a clean way. This can be processed by a microcontroller, if necessary.

I also have a plan to replace the existing electronics of the bicycle bell with my own circuitry (perhaps based on a tiny microcontroller) to drive the piezo disc so that I can render my own alert tone/message through the piezo horn as desired. Electronic bicycle bells/horns are abundant, affordable, and easy to employ and more importantly, there’s no need to get into an acoustic horn (Helmholtz Chamber) design!

I’ve a good feeling about this little project. It’s my hope that it becomes one of those avid hobbyists enjoy building and it’s perfect for those who like modifying designs. If and when I ever enhance mine, I’ll document it completely in a new post. Moreover, note that I looked all over the web to find disabled toilet alarm designs to learn from. I did not borrow anyone’s design/idea but got some theory bits and clever tricks from many sources.

Thanks to everyone who views and shares my post – you make all of this worthwhile 😊

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