This is an automatically latching lock which is powered by a Micro Servo SG90 9G. This makes it easy and inexpensive to interface to most microcontrollers such as Arduino. It is designed to engage with a pin of 3mm to 4mm diameter (not part of this model). When the pin enters the slot, it causes the mechanism to latch. To unlock it, the servo is driven through a range of 45 degrees, which causes the lock to release. The mechanism is designed to have a large mechanical advantage, so it should be able to cope with quite a bit of load without stalling the servo motor.

We are approaching Halloween as I write this, so perhaps you could use it to drop giant spiders on unsuspecting guests :)

You will also need:

• 2x 3mm x 15mm brass shaft (or similar)
• 2x M3 x 15mm screws to secure the housing
• 2x small springs
• Grease

Use a drill to clean up the holes in the moving parts, so that they rotate very freely on the shafts. File or sand the parts to ensure they are smooth, and apply grease before assembly. Test the range of motion of the servo before assembly.

You can actually test the complete mechanism is working well without the servo fitted. Push a screwdriver into the slot to make it latch, then use your fingertip to actuate the trigger. When you are confident that is operating smoothly, connect the servo to your Arduino (or whatever) to find its "rest" or "zero" position, then fit the servo horn in the correct orientation, then mount the servo and test in situ.

I've attached a picture showing a view of the upper part of the housing viewed from below to show the proper orientation of the servo horn when the servo is at its "rest" or "zero" position. When the servo is driven through 45 degrees, it pushes against the "trigger" (the red part) causing the latch to release.

Sample Arduino code is included (example.ino), showing how to drive the servo.