This instructable produces a simple LED strip that is powered from 2 AA cells and can be controlled by a magnetic reed switch so that it turns on when a door is opened.

This is suitable for cupboards and small spaces like an airing cupboard.

Battery consumption is very low when door is closed so good for occasional use applications.

The electronics consists of a standard boost converter modified to give 12V output and have an enable signal suitable for a magnetic reed switch.

Step 1: Parts and Tools

The following parts are used.

2 AA cell battery box preferably with switch
5V Boost converter based on AL236 (common eBay part). Other boost converters could be used if they have an enable signal but instructions would be a bit different.
12V LED strip. I used 6 LEDs from the strip but other numbers could be used.
Magnetic 2 part reed door switch
Optional 3d printed mounting plate https://www.thingiverse.com/thing:2978871
1 2.2M resistor, 1 15K resistor

Tools need

Scalpel or fine point craft knife
Fine point soldering iron

Step 2: Modifying the Boost Converter

The boost converter I use has a 6 pin AL236 boost IC.

As supplied this has resistor feedback to give +5V out from voltages in excess of 2.2V giving useful life from 2 AA cells.

The voltage output is controlled by a feedback circuit comparing to a 0.6V reference so by changing the feedback any output voltage up to 24V can be obtained. The current handling capacity is more than enough for this application.

The IC has an enable signal but unfortunately it is hard wired to Vin so this has to be changed.

The second picture shows the 2 lower left hand pins shorted together by a short track between the pins. The left hand pin is the enable and the middle pin is Vin. Use the scalpel to 'saw' through the track between these 2 pins. The pins themselves can be used as a guide.

The converter enable mod can now be completed by soldering a 2.2M resistor onto the enable leg and the Vin+. The resistor leg connected to enable can be used to connect to the enable signal. The value of the resistor is not critical but keep it fairly high (> 470K) to minimise standby battery drain.

The converter 12V mod can be achieved by soldering a 15K resistor between the feedback point (bottom right resistor and ground (Vin-). If your module uses different feedback resistors to mine then this value may need to be altered. Mine had a 12K resistor from feedback point to ground for 5V output and the 15K in parallel changes the boost to about 11.2V.

Step 3: Final Wire Up

First connect the battery box to Vin+ and Vi-. With two batteries in check that the boost module gives 11 - 12V output.

Remove batteries and connect the magnetic reed switch to the enable resistor and ground (Vi- or Vout-). Check that output of boost converter is now 11-12V with reed switch open (no magnet near and goes off when magnet is brought close to the switch

Finally connect LED strip to Vout of the boost module making sure Vout goes to 12V and Vout- goes to 0V of the strip.

Mount strip and battery box in a convenient location near door and the two part magnetic switch on door and door frame so that they are close when door is shut.