YL-333 Door Alarm Hack – ElectroSchematics.com

The YL-333 Magnetic Antitheft Alarm is an easy to install security alarm system for doors and windows. This battery-operated device works on the principle of magnetic proximity detection. It can potentially be placed on any kind of door/window, and can make a very loud alarm sound (~90dB).

I picked up one of these alarms last month and immediately disassembled it. The first thing that caught my attention was the use of a hall-effect switch in this amended design instead of the commonly employed reed switch!

The method of the YL-333 “set and forget” antitheft alarm is simple. When the encapsulated magnet part of the system is far enough from the AAAx2 battery-powered base, the 90dB thundery scream kicks in and let you know somebody had opened a door (or window) they shouldn’t have.

Since there’s nothing special for the installation, I think I should take a closer look at what’s inside. Here are the pictures of its internal electronics:

So, it is using a minuscule hall-effect sensor (solid-state magnetic proximity sensor).

The annotated figure given below reveals what is under the hood.

As you can see, there are only a few components on the circuit board. The YL3710 IC is the core component responsible for generating the warning signal. I couldn’t find any useful information about this 8-pin IC. It’s not easy to see on the picture but the smaller 3-pin chip package (TSOT23-3) has a marking “6207” which is in fact a Hall-Effect Switch – HM6207.

To satisfy my curiosity, I made a schematic and here’s how it looks:

The HM6207 is a micropower, ultra-sensitive hall-effect switch features a hall sensor, a small-signal amplifier with dynamic offset cancellation and a CMOS output. Either North or South pole of a sufficient strength magnetic field can turn its output active.

Furthermore, the piezo sounder driver section, in practice, is made up of just one extra component. Note that the drum core inductor used there is in fact a 4-pin “alarm booster coil/transformer” (see below). This is also a bit strange as similar Chinese door/window alarm designs are usually based on a 3-pin alarm booster coil.

That’s all about the quick teardown. The moment I learned about its inside electronics I grabbed a couple more. Why? Because this looks like a perfect candidate for many hobby projects as it should be very easy to repurpose it into something more utilitarian!

Last but not least, some hack clues. Pin 5 of YL3710 IC is the alarm enable/disable input pin, and grounding the pin will disable the alarm.

Note at this time that when the magnet part comes closer to the base unit, the hall-effect switch pulls down the enable/disable input pin to disable the alarm. But when the magnet part is far away from the base unit, the hall-effect switch reverses the logic state of the enable/disable input pin (Low/0V to High/3V) so it wakes the alarm sounder up when the door/window opens.

Needless to say, you can open the alarm enclosure (base unit) to do some hacks. For example, isolate the hall-effect switch by cutting the track on the printed circuit board and extend wires from the applicable circuit points as indicated below. This lets you interface your YL-333 door/window alarm with some external electronics later on.

I recommend performing some tests before screwing the cover back on. First, make sure that grounding the “alarm trigger” wire will disable the alarm sounder. Next, bring the magnet part of the alarm system close to the hall-effect switch and then back it off. Verify that you get a legitimate output signal through the “door status” wire. If you don’t, then doublecheck your wires.

I know many people will not be interested in learning how to hack a cheap alarm device but see it provides a nifty enclosure with battery holder, on/off switch, a hall-effect sensor, and of course an electronic alarm sound generator.

Before concluding this post, I would like to show you a simple idea for using an external door sensor switch with the alarm base unit even if it’s too far from it. So, below you can see the scheme for using an “MC-38 Door Window Magnetic Switch” with YL-333. There is much more to come later, so stay tuned!

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