Single Cell, White LED Torch Fits in a Matchbox

A little while ago, Makezine blog listed two interesting sites: HOW TO - Make a rusty nail LED night light and ELM - AVR based radio spectrum monitor.

Chan has a great site and he has put up a lot of interesting projects, incidentally many of them based on AVR. One of his projects (not based on AVR) is a pocket LED light. He describes a blocking oscillator based single cell, white LED torch. This and rusty nail project got me thinking and I started experimenting with Chan circuit. The circuit is very robust and it worked with a wide variety of transistors; I tried BC547, 2N2222 and 2n3904.

But the real fun and challenge was in building an enclosure for this torch. I have been a big fan of matchboxes; as a kid I used to collect empty matchboxes everywhere I went. I thought the art on matchboxes reflected the local culture.

Long ago, I built a data logger that could fit in a match box (I never completed it in that form, settling to find a much larger enclosure). But now I was determined to fit the white LED torch circuit AND the AA cell in a matchbox.

Step 1: Build the Matchbox Tray With Pieces of PCB

I took a fairly large matchbox and threw away the tray and decided to build my own tray using a technique I had learnt as a radio Ham: to use pieces of PCB and to solder them together to make a faraday cage of any size. I cut three pieces of PCB. I Drilled a large hole to accommodate the 10mm LED in one of the PCBs.

Step 2: Make a Cutout for the Switch on One of the Side Walls of the Tray

On another PCB, I made a cutout for the On/Off switch as seen here. The body of the switch was bent and soldered to the PCB so that it was secure.

Step 3: Another View of the Switch Soldered on the PCB

Another view of the switch soldered on the PCB.

Step 4: Solder the Three PCB Together

Solder the three peices together to make the matchbox tray. To solder, start by applying solder on the edges of the PCBs and then align two pieces and apply soldering iron. You may need to hold the solder iron for a little while so that the solder melts nicely. Dont move the PCBs till the solder has cooled off.

Step 5: Solder Components and Add a Side Wall to the Tray

Take another piece of PCB for the side wall of the tray. I used hobby drill to create islands of copper on the PCB. Solder the components on the PCB as per the circuit diagram in the next section.

Step 6: Circuit Diagram

This is the circuit from Chan's website (http://elm-chan.org/works/led1/report_e.html) which I modified and hived off two components. The modified circuit is seen here. The circuit is very robust and it worked with a wide variety of transistors; I tried BC547, 2N2222 and 2n3904. The coil I used had 25 turns each, of 26 gauge wire. Resistor R1 changes the intensity of the light. I tried values between 2KOhm to 220Ohm and I settled for 220Ohm. The light is quite bright at this value.

Step 7: Add the Final Side Wall and Fix the 'AA' Cell

Finally, add another PCB for the last side wall. For the cell, I cut a piece of holding spring from an unused cell holder and soldered it on the left wall of PCB in the picture and this forms the negative terminal of the cell. To isolate the positive terminal of the cell, I again cut an island on the right hand side of the PCB and soldered a wire to the switch as seen in the picture.