Motion Activated Dropping Spider, Low Cost, No Programming




Introduction: Motion Activated Dropping Spider, Low Cost, No Programming

About: I am a robotic engineer, and I like to make things and teach others.
In this Intstuctable I’m going to show you how to make a very simple motion activated spider that drops down rapidly and startles anyone that walks underneath, then retracts. I designed this to be an inexpensive and practical electronics project that a beginner can make. The sensor directly powers a transistor that runs a motor to reel down a spider. The reel is attached to a rubber band, when the sensor pulse ends, the motor turns off and the rubber band winds it back up. The speed of the spider drop can be adjusted by simply winding the rubber band more or less, from a slow realistic decent to a startling plummet.

It can be built using only a handful of inexpensive electronic components, some plastic bottle caps, cardboard, a rubber band and a motor. If you have to buy every single part it will be around $20, much less if you have some basics. No microcontroller or programming is required. It works very reliably and best of all, it actually does scare people!

Step 1: Parts and Tools Needed

Note to people who like to scrap stuff:
If you find an old printer you can take apart, you can probably get the motor, the wire, a 3 pin connector, and possible the transistor and the diode for really cheap.

Electronic parts:
I used Radioshack and a site called Radioshack because it had the parts and Tayda because it lets you order in quantities of 1 and the shipping is low. Substitutions are very possible. Any transistor with enough continuous current capacity for the motor should work, in this case 800ma. I encourage scrounging a motor if you’re up for taking stuff apart, they’re in everything. If you're in a hurry to make this before Halloween all of this stuff can be purchased at a radio shack store.

Parallax PIR sensor Module $9.99

1-3/4” Mini perfboard (Comes in pairs) 1.99

Motor 3.49
If you scrounge a motor, try to get one like this with a gear, it will give the spool and the glue that holds it something to grip on to.

4 “AA” Battery Case
Any case that is in series to provide 6v will work, I like this one because it has a switch already installed on it.

If you need to buy this at the store, radioshack stocks the TIP31 and TIP120 either of which would also work.

120 Ohm resistor

1 1N4004 1A 400V Diode

Some wire
If you don’t have some you can strip it out of an old phone cable, use some speaker wire. If you need to buy some there’s a link below, 2ft each of green, red and black 22awg stranded wire should do it.

If you’re uncomfortable soldering you can use this IC socket for the transistor, cut it down the middle with some clippers and you have two transistor sockets for 2 cents:

Craft Supplies:
Plastic spider
A pin
Long Rubber Band (as sold to hold file folders shut)
Cardboard, about 2 square feet of it.
Hot Glue
Paper Glue or Wood Glue
A Paper clip
A milk cap
A 3" to 4" plastic jar lid wide enough to make the drum of the spool
Zip tie
Thread or fishing line
Paper Towel tube or TP tube
Pin or tack
Pumpkin Pail (Only needed if you want to hang it from the ceiling, not needed for hidden over door mount)

Hobby Knife (such as Xacto)
Hot Glue Gun
Soldering Iron
Black permanent marker
wire strippers

Step 2: Build the Circuit

This circuit has only 3 parts.

The transistor works as a switch, letting current flow from the negative terminal of the motor into the collector (labeled C) and out of the emitter (labeled E) When it gets a positive signal from the PIR at the base (labled B). Keep in mind when you follow this diagram that if you use a substitute transistor these pins may be in a different order. Though usually they are as shown.

The diode keeps the motor from shocking the system with back EMF (a pulse of high voltage that can occur when a motor turns off due to the spinning magnets passing the coils). Since the motor is small and the PIR has protection, you could leave this off, but I prefer to keep it safe.

The resistor is to limit the current from the signal wire in the PIR to the transistor. If you leave it out it will still work, it's just another protection feature.

Now, to make the circuit. My preferred method for items that are too simple to fabricate a PCB, like this one, is point to point soldering on a perf board. Each of the components is put through the breadboard, the leads are bent to hold it, then the extra is clipped off. The body of the component should be on the plain side and the leads should stick out of the plated hole side. Solder each component to the breadboard then solder bridge the points that need to be connected by adding some solder and dragging it with the iron until the points merge.

A layout is shown below that allows this circuit to be made with no jumper wires. The white indicates solder bridges. The red and black wires are not stuck down to the board, the ends are just soldered in.

The negative and positive wires on the diagram, and the motor need to be the ones that make the reel lower the spider when connected to the battery pack negative and positive.

To make the wire connector for the PIR use the piece of a socket, or another 3 pin female header and solder it on to the end of the three wires. You could solder the wires directly on to the PIR but I didn't want to fix it to this permanently, and that would make it harder to work with for the install.

Step 3: Build the Reel Mechanism

My first build used a metal jar lid, instead use a plastic lid about 1/2" thick and 3" diameter. Its easier to cut, more rigid and hot glue sticks to it better.

Take the milk cap and poke a hole in the center, it doesn't need to be extremely exact. cut two slots for the rubber band about 1/4" from the center on either side. (first picture) Cut the rubber band in one place and feed an end through each slot in the cap. Tie a knot in each end to hold it on. Double it over and

Use a 1/16 drill bit to put a hole in the center of the plastic jar lid if you're using a motor with no gear, otherwise use whatever diameter makes it fit past the gear. Put two holes close together on the side of the drum wall of the plastic lid between where the two flanges will go. These will be used to attach the string later and putting them in the center of the wall makes it less likely to jump off the spool.

Take a piece of scrap cardboard and cut a tiny slot with your hobby knife, just wide enough to fit over the motor shaft and deep enough that the spacer covers the motor. Place the spacer on the motor shaft. This will allow you to press the parts of the reel down to level them without jamming it against the motor.

Push the plastic jar lid onto the motor shaft, topside against the motor open side away from it, until it is up against the spacer. Keeping it pressed against the motor, apply a small amount of glue. Check that it's fairly steady when spun before gluing it all around, a little wobble is ok, this isn't going to go very fast. Once it's straight enough glue around the motor shaft where it meets the lid.

Now take the milk cap with the rubber band in it, apply hot glue to the remaining part of the motor shaft and press it down on to the shaft. If you're more patient use epoxy or gorilla glue for this part.

Glue around the edges of the milk cap against the jar lid. You can take out a little more of the wobble if you have any here by holding it straight when you glue this.

Now pull out the spacer and check the spin!

Step 4: Finish the Reel Body

To make sides for the reel take some more cardboard from a cereal box and mark out two doughnut shapes with a compass. The inner diameter should be at least 1/2" inside the jar lid which was a 2" diameter in my case, and the outside diameter should be about 3/8" outside of it,  which gave me a 3-1/2" diameter. 

Before cutting out the middle mark a circle the same size as your lid to help you center the part when you attach it.

Cut the cardboard, make sure that the outer edges are very smooth so the string doesn't catch on them.

Center each side using the marks and glue it on.

Step 5: Build the Enclosure

I used cardboard for the enclosure so that this would be easy for anyone to repeat at home, but wood or plastic would work very well if you have the tools to work with them. The inside dimensions are 4”x6”x2.5”.  I left the top mostly open. It should be just wide enough to clear the spool, which as constructed here is 3.5” in diameter. As you can see from the picture I’ve used an existing box that wasn’t quite wide enough and I needed to cut out the sides to make the spool clear.

The top part should be built so it can fold up to access the motor compartment.

Before you close it up, cut a hole the size of a quarter in the bottom of the box, about 1.5" behind the reel. This is to mount the PIR sensor later.

Step 6: Mount the Motor to the Enclosure

Cut 2 pieces of wire 12" in length, strip about 1/2" off each end, loop them through the motor tabs and solder them on.

The motor will need to be positioned so that the rubber band is taken up but not stretched when pulled to the opposite edge. Once you find that point mark it out and put some small holes in the cardboard for zip ties to go through.

Zip tie the motor into place, then put the paper clip over the rubber band and clip it to the other side of the enclosure. Check that the spool spins freely, if it doesn't, adjust it or cut away part of the box. Once the placement is good secure it with some hot glue where the motor meets the cardboard.

cut a small hole in the back top corner of the enclosure and feed the motor wires through.

Step 7: Rig Up the Spider

Make the string only as long as you want the spider to descend with enough extra for the knots at each end. This can work for up to a 15 foot drop, beyond that there won't be enough time for the spider to descend, and the PIR sensor starts to become unreliable.

Poke a hole in the enclosure centered under the spool, the centering between the walls of the spool is crucial, if you put it off the side the string may hop the spool when it retracts.

Feed the thread through the hole you just made, then into the hole in the spool and out the other. Tie it tightly against the drum.

Loop the other end of the thread around the pin or tack and tie it securely, then slide the knot up under the head. Press the pin into the plastic spider at the rear tip of the abdomen. The system will work with the string attached anywhere, but this is the location of the spinnerets on an actual spider.

Now you and take off the paper clip, and wind it up until the spool draws the spider all the way up. Then give it a few more twists. Now to test it, touch the wires to the battery pack. If it tries to pull the spider up you need to switch the wires to the opposite contacts. If it doesn't lower enough, unwind the rubber band a bit, if it doesn't retrieve, wind it up more.

In later steps where  it is necessary to open the compartment again, do it without loosing your rubber band winding by clipping the paper clip to the edge of the reel, this will allow you to swing back the flip top freely.

Step 8: Install the Electronics

Use small screws through the cardboard or tape to stick the circuit board to the side of the enclosure just so it won't be loose.

Use hot glue to stick the PIR into the hole in the enclosure facing down. Feed the wire to the PIR through the same hole that the motor wires come out of and plug it into the PIR.

If you want it to go off whenever anyone is near, leave it as is. If you want it to go off only when someone walks directly underneath it, you will need to give it some tunnel vision by cutting a 1" piece off the cardboard tube and gluing it on around the PIR sensor.

Attach the battery pack to either the side or top of the housing. Make sure the switch is toward the top if you put it on the side.

At this point if you clip the rubber band to the edge, wind it up till it retracts the spider, and turn it on, it should work. It will just go off repeatedly since you're holding it.


Step 9: Install the Reel in the Housing

To mount it hanging from the ceiling in an enclosure first cut a hole in the bottom of the pumpkin pail with your hobby knife, make it big in the middle for the spider to go through with an extension at the side where you want the tube for the sensor to come through. Carefully turn the mechanism sideways and set it into place inside the pumpkin pail.

To mount it above a door simply set the bottom edge of the box on top of a door frame and secure it all in place with a thumb tack.

Step 10: Done

Now it's done, hang it up at a party, or over your porch to scare trick or treaters. It doesn't use much power, so it will last at least all night on one set of batteries.

Variation ideas:
-Install it in a fake hanging plant instead of a pumpkin pail to make the spider even more unexpected
-Replace the motor in this circuit with a relay to power a much bigger motor, wind a couple of bungee cables and drop a prop the size of a basket ball.
-Use the simple motor control circuit for other props, like a rattle snake that shakes it's tail when people walk by.

This is one of my first Instructables, so if anything is unclear let me know and I will update it.

Hope you have fun with this little piece of Halloween tech! If anyone builds this I would love to see pictures, especially if you make a version with a wooden enclosure or another variation.
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    30 Discussions


    9 years ago on Introduction

    Hey there, nice project!
    I'm trying to do something somewhat similar (triggering a dc motor with a pir sensor), and your circuit layout works to trigger the motor, but the motor seems to be triggered even without any motion to the sensor. So basically, the motor turns on for 6 seconds and then off for 6 seconds, constantly.
    I can't figure out what I'm doing wrong.
    Any suggestions?


    Reply 9 years ago on Introduction

    I don't know the specifics but there are a few possibilities. It has a warm up time during which it calibrates to the normal state of what it can see, before that it isn't very reliable. I think its about 30 seconds. If the thing you are activating makes the part that the motion sensor is attached to move, it could be shaking it's field of vision making it detect motion, I ran into this issue when the sensor was in a free hanging pail. It could just be that the field of view is too wide, it sees about 180 degrees all around, so you could try putting cardboard blinders on it to limit the field to the region you want to detect in. Last and least likely, if it is looking at a scene that has stuff that doesn't move, but does rapidly change temperature, or that reflects an area that has moving warm objects or changing heat patterns it would see that as motion.


    Reply 9 years ago on Introduction

    Thanks for your reply.
    Well, I've tried a couple of different things, and unfortunately nothing seems to solve the problem. I even hooked up a spdt relay (with a transistor) to see if that would help. Works perfectly with an LED in place of the motor, but when connected to the motor the relay just keeps triggering every 6 seconds, it's very odd.
    Maybe some sort of kickback of power from the motor? I know next to nothing about electronics so troubleshooting takes a lot of time and research for me to try to understand what's going on with the circuit.
    I'm hoping to use this circuit for an art project, which I'm displaying in a week and a half so I'm freaking out wondering if I can get it to work. Maybe back to the drawing board for me.


    Reply 9 years ago on Introduction

    I think you can get it working in that time, Are you sure the motor jiggling the sensor isn't the problem? Because that's exactly the problem I had when it was having the trouble you describe. If you think it may be electrical try putting the motor and transistor on one power supply and the sensor on another with only the grounds tied together. If that fixes it then it is electrical and you need a bit more isolation.


    Reply 3 years ago

    Hi Paul,

    I am making a similar circuit for a shoe polishing machine that starts automatically when the shoe is kept under it. I am not sure whether this circuit will work for that or not. Can you suggest me what type of circuit should I build and if I want to run the motor for a longer time, how can I do that.....
    Please Reply


    Reply 9 years ago on Introduction

    Hmmm... no I haven't tried that, but it sounds promising! I'll let you know how it turns out.
    Thanks again.


    9 years ago on Step 10

    Very clever approach!. What happens if someone stays close enough that the PIR stays triggered? Would the motor wind up the rubber band until it broke?


    Reply 9 years ago on Step 10

    No, the spider actually stops when the string unwinds all the way, the rubber band balances it out so that it is strong enough to turn with the help from the weight of the spider but not after it reaches the bottom where the weight works against it. In the opposite direction it is stopped by the spider hitting the hole the string goes through. In short, if you stay under it, the spider just stays down.


    Reply 4 years ago

    how does the motor not burn out then if someone just stands under the sensor for a long time? As you said, the motor isn't strong enough to pull the spider up, so won't it just draw more and more current which will eventually ruin the motor?


    Reply 4 years ago

    The main reason it is ok is that this PIR gives you a high pulse for
    about 2 seconds, then turns off for about 3 seconds, so if you stand
    under it and keep moving around it will just drop and retract over and
    over. It also helps that in this case I'm using it at very low power so
    stalling doesn't really cause it to heat up much. If it were a big
    powerful motor moving a large prop I suppose I would have set up a
    switch on the reel such that it would turn off when it hit the bottom,
    and turn back on when it finishes retracting so that
    the motor would always stop at the end of travel.


    Reply 4 years ago

    Thanks for the quick response! I have another question: how do you recommend that someone find an appropriate motor for this project? I already built the whole thing, but unfortunately, the motor I used is WAY too strong and the rubber band has no effect on it since the magnets are strong and it takes significant force to turn the armature. It doesn’t stop at the bottom like it is supposed to, the motor just keeps going, reeling the spider back up the wrong way. You said that the motor you used is from a floor sweeper, so you obviously don’t have any specs on it, but is there a test that people can do to see if a motor is too strong or weak without having to construct and glue on a new reel each time they try a new motor? I was thinking about trying this motor:


    Reply 9 years ago on Step 10

    Clever and elegant - Well done!


    8 years ago on Introduction

    Thanks for the great project!

    I picked up my PiR sensor from a different source, and found it only outputs under 2V when active, therefore not enough to get the TIP41C to be fully on to drive my motor. In case anyone else finds the same, an additional transistor (I used a 2N2904) between the PiR output pin and the limiting resistor for the TIP41C sorted this.

    Now we're back on track for Halloween 2012!


    Reply 7 years ago on Introduction

    Hi Paul

    I'm having difficulty matching the motor and rubber band - hoping you found same and have some advice here. The first motor I tried was a small high rpm hobby type and it wasn't powerful enough in the end to wind the spider down against the force of the rubber band.

    I've now re-assembled it with a DC motor scavenged from a printer and I'm having the opposite - the motor is strong enough to wind the spider down, then keep winding until it comes back up again. Even when I've pre-wound the rubber band some, the motor always wins.

    I'm thinking of putting a potentiometer in the circuit to limit the voltage to the motor, but with time against me hoping you have some advice that can help.

    Thanks ! Geoff


    8 years ago on Introduction

    What are the pins for and the paper towel tube for? Also this motor has changed radio shacks motor is has a short shaft and the gear is on the very end. Any idea where I can find a motor with a longer shaft like the one you used? Im still having trouble finding a box for this.


    Reply 7 years ago on Introduction

    The paper towel tube is to narrow down the field of vision of the PIR so it doesn't go off unless someone moves directly beneath it.

    That motor is from a floor sweeper that I took apart. Many printers and other devices you could take apart would have a motor this size, so many would work that I can't really recommend a specific one to buy.