• Winning tip: How to build a humane mousetrap

    Date:31 July 2014 Tags:, , ,

    I’m not sure if this falls into the category of DIY hints, but it was an interesting projects and could be of use to your readers. My wife and I live on the edge of a wetland and have some unusual creatures that find their way into our house. We are reluctant to kill them as we can return them to the wetland, and this is normally easy to do, but when the mice arrived, I decided to make a humane mousetrap.

    The box is constructed from melamine offcuts nailed together, with a spring-loaded door and very basic electronics running off a 12-volt DC supply. The mouse smells the peanut butter and bread bait at the back of the trap and walks in. Near the back of the mousetrap, he interrupts an infrared beam that energises a solenoid, releasing the spring-loaded door.

    Over one week, we caught nine mice in the mousetrap.

    The nitty gritty:

    Box Dimensions:
    Originally the box had to be made quickly, so I constructed a fairly rough box out of melamine off-cuts as can be seen in the photo. The dimensions are not really critical, but the idea of the length and width being what it is, was to make sure that the mouse was unable to turn around and exit the trap before the door came down.

    The box dimensions are:
    1) Length: 300 mm
    2) Inside width: 110 mm
    3) Inside height: 130 mm
    4) Door: the door is slightly narrower than the inside width in order for it to slide up and down and a height of 155 mm with 2 panel pins to attach the spring to it.

    The Electronics:
    See the basic schematic diagram for the project. The most important points are:
    1) The infrared LED (d1) and the photo transistor (q1) must be directly opposite each other thereby creating the infrared beam. I positioned this arrangement 80 mm from the back and 20 mm from the trap floor as seen in the diagram.

    2) The solenoid I used was a 12vdc solenoid with a plunger. I had one in my scrap box, so that was the reason for using it. The system is very simple. The spring-loaded door has a hole drilled into it. In the lifted position the solenoid plunger is pushed into the hole and this holds the door in the open position. When the beam is interrupted the solenoid is energised releasing the door.

    The only problem with this arrangement, being the first version, is that the plunger has to be set very sensitively otherwise the magnetic pull of the solenoid will not be strong enough to pull the plunger and release the door. As I used it, I didn’t find this too much of a problem. Originally tried 2 springs, one on either side, but found that 1 was sufficient.

    The component values are:
    1) R1: 560 ohm
    2) R2: 68k ohm
    3) D1: infrared LED
    4) Q1: photo transistor
    5) U1: ULN2803
    6) S1: 12vdc solenoid

    I used a 12vdc power supply for the project as the solenoid is a 12vdc solenoid, but any voltage power supply can be used. The only change that will have to be made is the value of the 2 resistors.

    Rodger D’Alton
    Fish Hoek