I have created a Lego Mindstorms trap to see which cat was leaving deposits in my robot workshop.
We used the Ultrasonic sensor and a motor to wind up a string to close the door, all nice and simple and I now have the culprit :)
During construction, we had to make it work from an AC adapter rather than batteries as it was to be left on all night, so we made a little adapter. We used inspiration from another design I found on the web at
To build this Instructable you will need
Scrap piece of copper clad pcb board
Soldering iron and solder
8-9v, min 1.5 amp power supply
AA battery connectors form an old toy
Scrap connector pins
Hot melt glue
About an hour of your time
Trouble with the original one was that you needed to remove the battery cover and leave the board exposed which looks a little tatty. Although my version still looks a bit tatty, at least its hidden by the original battery cover.
Step 1: Create the Slide in Board
A better way would be to create a slide-in board that would allow the battery cover to be replaced back over it again. to cover up and protect the board.
**You need to be compentent with a soldering iron and know how to use a meter to check voltage and polarity. I take no responsibility for your problems, you build this at your own risk. **
I had some copper-clad PCB board around and cut a section 38mm x 85 mm, I also scavenged through some old broken battery operated toys to retrieve an AA spring and plate which could be soldered to the copper board to mate with the two main contacts of the controllers battery compartment.
These soldered on very easily. Once I was happy with the position, I cut the copper around them with a knife to ensure they were electrically isolated from the rest of the board.
I then took some old connector pins, anything will do, and soldered them on to the board so they aligned with the original spings along the battery bay and gave the board something to push against to keep it all aligned and centralised. They are not connected to anything, purely to keep it all in place.
Step 2: AC Adapter
You need to choose an AC adapter that outputs around 8-9v DC and at least 1.5 amps , lots of adapters say a voltage on the case, but when you measure them, they can be as high as 18v, so beware and use a meter to measure them!!
You can add a fuse as per the link on the first page, but I had a fused adapter plug so didn't feel the need to bother. Also, you can add a power connector as per the link on the first page, but I decided to keep it simple and wire the adapter straight in.
Make sure you test for the correct polarity and voltage before connecting it to your NXT
A bit of hot melt glue keeps the cable in place and some tape works for strain relief. You need to cut out the little cable access hole in the battery cover of the controller (thats what its for)
It only takes seconds to slide the board in and it works perfectly, just remember to turn off the sleep mode on the controller if you want it to work for extended periods.
All in all, cost me nothing to make and took around an hour to complete.
You can come over and visit us at the UK Robot Group website