A tripwire is one of the most basic ways to set up a simple security system. You run a line across a pathway. Then when someone walks through the line, it activates an alarm. This kind of system is easy to set up and is fairly effective. But there is always room for improvement.
The most inconvenient thing about a classic tripwire alarm is that it requires you to run a physical line from the tripwire to the alarm. This makes it difficult to set up a system where the alarm is far away or inside a building. To get around this problem, I designed a simple remote alarm system that uses a small radio transmitter to activate the alarm wirelessly. So in this project, I am going to show you how to make a simple remote tripwire alarm.
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Step 1: Select a Wireless Transmitter/Reciever
The first thing that you need to do is select a small transmitter to activate the alarm. Just about any radio transmitter can work. I decided to use a wireless doorbell because it was small, battery powered and it is easy to program them so that multiple transmitters can activate one receiver. Some other examples of transmitters that can also work are things like walkie ralkies or garage door openers.
Step 2: Add Wires to the Switch on the Transmitter
The only modification that you need to make to the transmitter is to solder some extension wires to the button terminals so that it can be activated by an external switch. In most cases you will need to remove the circuit board from the housing in order to access the button terminals.
Take two small piece of wire (at least three inches long) and strip the insulation off both ends. Then solder one wire to each side of the button. If your button has four terminals, you need to solder the wires to two terminals that are not connected when the button is not being pressed. You should use a multimeter to check which terminals are normally connected and which ones aren't.
Step 3: Create a Switch That Will Be Activated by the Tripwire
In order to activate the transmitter you need an electrical switch that will react to the tripwire being pulled. I decided to make a spring loaded switch from a clothespin and a couple of machine screws.
Start by disassembling the clothespin. Then drill a hole in the end of both wood pieces that is the same diameter as your machine screws. Insert the machine screws through each hole with the head of the screws on the inside of the clothespin. Tighten a nut on the back side of each wood piece to secure it in place. Then add a second nut onto each screw. This will be used to attach the wires from the transmitter. You may also want to drill a small hole in the opposite end of each piece to attach a line when hanging the tripwire. Once you are done modifying both wood pieces, reassemble the clothespin.
Now you need to attach the wires from the transmitter to the clothespin switch. To do this, take the free end of the the wires, and wrap them around the machine screws between the two nuts. Then tighten the outer nuts onto the wire to hold them in place. Now whenever the clothespin switch is closed, it will activate the transmitter.
Step 4: Setup the Tripwire
First decide where you want to place the trip wire. Look for a location where traffic is restricted to a narrow path such as a hallway if you are indoors or an opening in thick vegetation if you are outside. Also look for areas where you can easily mount the line on both sides. You want the tripwire to be about one foot off the ground.
First cut of a short piece of line that will support the transmitter. Tie one end to the hole on the back of the clothespin. Then tie the other end to some fixture that is near the pathway.
Cut a second piece of line that is long enough to go across the pathway. Add a little extra length for any knots that you need to make to secure it in place. Tie one end to something on the opposite side of the path from the first piece. Then take the free end over to the clothespin.
You can connect the two pieces by putting the free end of the fishing line between the screws on the clothespin. If the fishing line is too thin to keep the screws separated, then you can add a small piece of plastic onto the end of the line.
Step 5: How the Alarm Works
Once your tripwire is in place, your alarm is ready to use. When someone crosses the tripwire, it will pull the line out from between the screws of the clothespin switch. This will connect the terminals of the button on the transmitter and it will send a signal to the receiver. The receiver will then play its tone to alert you that there is an intruder.
Keep in mind that your doorbell has a limited range. You don't want to have the receiver too far away and miss the intruder. You should always test each tripwire with the receiver in its intended location.
Step 6: Optional: Change How the Receiver Alerts You
The receiver will normally play its standard ringtone as the alarm. If you would prefer to have a different notification, there are several ways that you can change it.
One option is to simply change the ringtone. You can check out this project where I use a small recording module to make custom ringtone for a doorbell. There is also the option of simply purchasing a wireless doorbell that lets you upload your own MP3's to use as ringtones.
If you want to be more stealthy, you could also have the receiver alert you by turning on a light instead of using sound. The easiest way to do this is to just replace the speaker with an LED. Start by marking each wire to keep track of which one was connected to the positive/negative terminals of the speaker. Then cut the speaker wires at the speaker terminals and remove the speaker from the housing. Now all you have to do is connect the speaker wires to an LED and a series resistor. To find the value of the resistor that you need, do the following. Subtract the LED's operating voltage from the voltage of the receiver's battery pack. Then take the result and divide it by the operating current of the LED. (example: (4.5V-3.0V)/0.020A=75ohm resistor) When connecting the LED and resistor to the speaker wires, be sure to match the polarity of the LED. The longer lead of the LED needs to be connected to the positive speaker wire.
Step 7: Optional: Setup Multiple Tripwires and Transmitters
Most wireless doorbells have an array of pins and jumpers that lets you set the frequency or security code for the doorbell. This lets you change the settings if you are getting interference from your neighbors. This also lets you have multiple transmitters or multiple receivers for your house. You can use this feature to setup multiple tripwires.
If you want each tripwire to set off a different alarm, then you need to make sure that each transmitter and receiver pair is set to a different frequency. This kind of system will tell you specifically which tripwire has been set off.
If you want to have multiple tripwires all set off a single master alarm, then you need to make sure that all the transmitter/receiver pairs are all set to the same frequency. This kind of system has the advantage of being easier to monitor. You can have a single receiver that you carry around with you. Or you can have multiple receivers in several places throughout your secret base so that you will always hear one when it goes off.