Many parents face ordeals related to their children wandering around the house at night.  A young mother in Tennessee recently experienced a harrowing night when she woke up to find her 4-year-old son missing only to learn that the boy had stolen a beer, gotten drunk, ransacked a neighbor’s Christmas presents (donning one which was a little brown dress), and was brought home by police.  Luckily, most parents don’t have to worry about more than a living room covered in soap after their child’s wanderings, but it’s still a scary issue nonetheless.

I have tried a few off-the-shelf solutions to combat night wandering and found some low-quality money pits.  One video baby monitor will send a text message to the parent’s cell phone when movement is detected, which would be great if the picture were clear and the camera’s fuzzy static weren’t mistaken for movement every few seconds.  Anyone who has a very sneaky toddler knows that audio monitors are only good for a kid who wants to be heard.

I have a 3-year-old son who likes to sneak downstairs at night, sometimes simply making little rows of toys on the couch, but sometimes making messes and breaking things.  He will even give me a detailed description of his night adventures the next morning, and then describe his plans to me for the next night in the evening.  Even with warning, I still sleep through his antics and am tired of cleaning it up later.

So, when All American started talking to Zilog and learned about their motion sensor modules, I was immediately interested.

I took a siren circuit and hooked it up to an ambient light sensor (so the device will only work in the dark) and the Zilog motion module.  I now have a device that will scream like a European ambulance any time my kid sneaks downstairs at night!  We haven’t had any issues with him since the install because he knows he’ll get caught, and he hates the noise so he won’t get close enough to investigate my device!  : )

The siren module works off a 9V battery, the Zilog module works off 3.3V, and the light sensor takes 5V.  I used a 9V input from a wall wart regulated down to 5V, and the 5V regulated to 3.3V for power.  Not much power is used, so not a lot of consideration had to be taken in this part of the circuit – LDOs worked fine, but regular regulators would have been adequate as well.  

Using a dual comparator and a few resistor dividers with potentiometers, I am able to tune the level of ambient light allowable as well as the sense characteristics and time delay of the Zilog module.  By adding a couple of resistor/LED pairs to the signal lines from the Zilog module and ambient light sensor, tuning the device for different environments is relatively easy. 

While I used parts lying around the lab for the prototype, the official BOM for this project is as follows:

With a little bit of packaging, this small array of circuit boards will help improve my house and give me piece of mind. This ultra-low-cost project has been quick, simple and satisfying and should function for years to come.