I was recently requested to repair an optician's illuminated eye-test chart, where a blown lamp had welded the contacts in the on/off switch. Unfortunately, the lamp was a very old 'architectural' one with the electrical contacts cemented to the side of the glass tube, and no longer available in the 18" (450mm) length required. A suitable alternative was required.

Really bright, reasonably-priced LED replacements for T8 fluorescent tubes are now available from many outlets. These are great if you want to replace standard-length fluorescent tubes, but are difficult to obtain in non-standard sizes. I like to use them in cupboards and under shelves, just feeding power to one end, without the bulk of a whole fitting, where standard lengths are not always suitable. That got me to wondering whether they could be 'hacked' to create custom lengths?

Step 1: How?

Out with the trusty old hacksaw, and off came the end-caps, and the whole LED strip slid out of its groove easily. (I cut the non-energised end first, and was careful not to cut too deeply until I understood the construction.) Once dismantled, it was easy to loosen and remove the remnants of the plastic outer tube from the end-caps using long-nosed pliers.

Testing with a DMM set to the 200mA range revealed that all of the LEDs were connected in one series circuit fed by a constant-current ballast supplying 75mA. (Connecting a mA meter across one, two, or more LEDs did not affect the current in the circuit.) Therefore, I could cut-down the LED strip to the desired length, and just solder a 'blob' across the last LED position to complete the circuit and slide it back together and it worked a treat. A custom length LED tube plus a couple of T8 tube connectors salvaged from old fluorescent fittings, and the test chart was better than new.

It is now possible to modify these tubes to virtually any required length to suit your application, with power only supplied to one end. If the tube is in an enclosure, the end-caps do not need to be secured, but if mounted in an accessible position I would recommend gluing the end-caps back on for safety.

Let me know if you think of any novel applications.