I had a spare weekend and decided that I would pull something together for Halloween this year using all the bits and bobs I had lying around. This is what I came up with! It is essentially a simple laser projector which projects an image onto a wall whilst transmitting sound to a nearby radio or stereo via an FM transmitter. The intention is that the projector be placed somewhere inconspicuous, disguised as something innocent (in this case, an Altoids tin). When an unsuspecting victim enters the area, the projector springs into life and hopefully scares the living daylights out of them by manifesting a ghost before their very eyes (and ears!). The different parts of this project can of course be taken in isolation and used for other purposes, and I've tried to stick to parts that are cheap or salvageable and as readily available as possible.
Step 1: Parts and Tools
I have split the parts lists into sections in case someone wishes to take a single part of this project in isolation:
- Philips Spot-On Motion activated LED lamp (or similar)
- Multi - turn trimmer pot (optional)
- Laser diode module (preferably high power >10mW, any color)
- ~15mm focal length miniature lens (optional)
- Laser printer acetate film
- Record-your-own-message greetings card, or just the sound module out of one
- FM music transmitter
- 3 x AAA battery box
- Altoids tin or other enclosure
- Soldering iron
- Desoldering vacuum
- Dremmel or other multitool (for customising the enclosure)
- Junior hacksaw
- Access to a laser printer or photocopier
Step 2: Motion Detector (1 of 3)
The motion detector was ripped straight out of a Philips Spot-On motion activated LED lamp I got at a car boot sale for 50p. Bought new you can find them for under £5 on eBay and other such places. The motion detector in these little lights is a PIR type, and is exceptionally easy to hack. All you need is to desolder a few components and you're away!
Begin by unscrewing the back panel and exposing the circuit board. Remove the batteries and wiggle the board to free the battery contacts on either side and the board should slide right out of the casing.
Step 3: Motion Detector (2 of 3)
Next, to start desoldering some stuff:
- Desolder the battery contacts using a desoldering vacuum
- Desolder the three LEDs in the same way
You can keep the 3mm white LEDs for another project, and solder on a 3xAA or 3xAAA battery box at this point if you like.
The LEDs on this lamp are cathode switched, meaning that the transistor responsible for turning them on is connected to the cathode pins of the LEDs. The anodes are connected essentially straight to the +ve battery supply through a series resistor to control the current passing through them. For this reason, we'll be using the cathode pads for the LEDs to switch the different parts of our system on when the sensor triggers.
I decided I wanted to be able to alter the on time of the system as the on time that's set by default is quite long. I picked up a copy of the chip used on the board (it's a BISS0001, datasheet is here if you want to take a look). By a process of deduction and following the traces on the PCB, it turned out that R18 was the resistor I needed to change in order to alter the on time. I replaced it with a multiturn precision pot (1K 10 turn I think), which is easily available and pretty cheap from any electronics hobbyist shop. That way, I could adjust the on time until I was happy with it.
Step 4: Motion Detector (3 of 3)
Finally we'll mount the prepared board in our Altoids tin. Drill out a hole for the PIR detector lens to stick out of in the lid of the Altoids tin and use a hot glue gun to attach it to the lid. I managed to mangle the tin lid because I rushed the drilling of the hole, which needs to be around 12mm in diameter. As a result I had to patch it up with paper. Needless to say, the metal that the tin is made from is very thin, so treat it with care and you'll end up with something much prettier than I did!
Step 5: Laser Projector (1 of 5)
The laser projector is a really simple device that basically consists of an expanded laser beam passing through an acetate that casts a shadow onto a wall. Because the beam is expanded, you'll get much better results if you use a higher power laser diode, such as the one from a 15x DVD writer (red) or a BluRay writer (violet). You can get green laser modules pretty cheaply, but they tend to be longer, and you'd struggle to get everything to fit in the space we have available inside our tin.
The diode I used was ripped from a 15x DVD writer and is capable of putting out over 100mW of light at 650nm, which is a nice ruby red. A word of caution:
BEWARE WHEN USING HIGH POWER LASER DIODES!! The amount of light they put out at their peak is decidedly dangerous to your sight and that of those around you!!
The beam of the laser in this project will be expanded to such an extent that it will be eye-safe once complete, but be super careful about knowing where the beam is pointing when you power it up until it's safely expanded.
For the purposes of this project, I just used a simple resistor to stop my diode from blowing up when connected to the 4.5V power supply, but you can buy modules already housed with a collimating lens and driver if you so desire. DO NOT under any circumstances connect your diode directly to the batteries! That is a one way ticket to laser diode burnout.
Step 6: Laser Projector (2 of 5)
The diode I used had already been mounted in a module with a collimating lens, and the lens locked in place, so I was working with a collimated beam. As a result, I needed to use another lens to re-expand the beam. If you house your diode in a module with an adjustable lens, or buy one with an adjustable lens, you may find that you can use the lens adjustment itself to provide the divergence you need in the beam without resorting to an external lens.
The lens I used had a focal length of around 15mm and was pulled out of the same DVD writer that the diode came from. I glued it straight to the front of the laser module with a few dabs of hot glue, trying my best not to get any glue in the path of the beam itself.
I had to unscrew the rear portion of the module housing to give enough distance within the Altoids tin for the beam to expand.
Step 7: Laser Projector (3 of 5)
Before mounting the laser in the tin, some modifications were needed to hold the acetate in place that would cast the shadow. I cut a strip of metal around 1cm wide from one corner of the tin, starting from just beyond where the curve begins on one edge and continuing to where the curve ends on the other edge. The resulting strip was then straightened out with some large pliers and trimmed so that only a small length was still present with a slot on either side.
I did my modifications with a Dremmel and a diamond cutting disc attachment, but there are many other ways to do it.
Step 8: Laser Projector (4 of 5)
The acetate images were printed at a size of 1cm x 1cm with a laser printer onto laser printer acetate. You could also pront onto normal paper, and then photocopy onto acetate to achieve the same end.
The images were trimmed close around three of the sides, but the third side had an extra piece of acetate left on with a small tab at the top and bottom (sort of a fat 'T' shape overall). The tabs can then be slotted into the slots that were left in the tin to hold the image in place, and allow for easy adjustment or swapping out for a different image later. You will need to flex the acetate image to allow the tabs to slide into the two slots. When you close the lid of the tin, if you tuck in the top tab so that it goes under the lid, this will help to keep the image in place even more securely.
Step 9: Laser Projector (5 of 5)
We can now glue the laser into the tin to complete our projector. The approach I found worked best was to connect power to the laser module (not forgetting to use a driver board or resistor!) whilst fixing it in place to allow for fine tuning of the position. If you use hot glue to fix it in place, you'll have a few seconds to position it by looking at the output with the image acetate in place whilst the glue is cooling. The module needs to be as far back as it can go and pushed against the side of the tin.
Step 10: Audio Transmitter (1 of 2)
The audio transmitter consists of the guts of a record-your-own message greetings card (available from any good card shop) which I received from some generous relative, and the guts of an in-car FM music transmitter (the kind you plug your MP3 player into to get the music on your stereo). The FM transmitter is one I've had lying around for years, and consequently was on the large side and required extensive butchering to get it to fit into the available space. You can easily find transmitters now that are a fraction of the size, and very, very cheap.
The first thing to do is get the boards out of their respective enclosures. The audio recording module is easy, but the transmitter might be harder depending on how well it's sealed inside whatever enclosure it came in. In the case of the audio module I used, I was able to remove the entire section of the PCB that held the batteries and solder wires to carry power to the board, which reduced the size significantly.
In the case of the FM transmitter, the mini USB and audio jack were desoldered and wires soldered on in their place, and a couple of sections were removed with a hacksaw to reduce the size a little. In the case of the transmitter I used, the shield connection of the audio jack was also the aerial connection for the transmitter (the shield of the audio cable acted as the aerial).
Step 11: Audio Transmitter (2 of 2)
Connecting up the transmitter and audio module was pretty straightforward. The outputs from the speaker on the audio module were just connected up directly to the audio input pads on the transmitter. The audio clip was recorded on the module by pressing and holding the record button whilst speaking (or rather, SHOUTING) into the microphone. The play button was then taped shut so that when the power is applied to the module by the motion sensor, it will immediately play back the recorded sound.
As far as the transmitter was concerned, I wasn't keen to have a long aerial wore trailing out of the tin, and it couldn't be hidden inside as the tin completely shields the signal. The solution I settled upon (which is decidedly non-optimal, but it works), was to solder a wire connecting the ground connection of the audio input (which doubled as the aerial connection) to the tin itself, so that the tin became the aerial. This works surprisingly well, although the range is reduced a little.
Step 12: Putting It All Together
The audio module and laser diode module were each wired to the positive battery connection of the PIR board and one of the ground connections originally occupied by the LEDs. The FM transmitter was connected directly to the battery connections, mainly because the FM transmitter needs to be constantly powered, otherwise the radio you have tuned in will be pumping out static (which might just give the game away!)
The audio module was left with all buttons, the microphone and record LED in place so that the box can be repurposed by recording a different message and changing the acetate image. The module and all it's associated bits fit quite snugly in the remaining space in the tin.Take a look at the image of the final layout to see how it all fits together.
I would advise that wires are kept as short as possible, as they have a knack of finding their way into the hinge and getting pinched if you're not careful.
Step 13: Finishing Up
Below is a video of the box from a victim's perspective. It looks somewhat brighter in person, but you get the jist!
I hope you find something of worth in this little project, and please hit the comments if you have any questions!