Tuesday, 5 December 2017

LED Escape Lights - Repurposing

My two boys have, for their play 'crib', a large shed. But this time of year they get little use from it, because its dark.

It just so happens though, that Ive acquired a pair of LED Emergency Escape Lights - the sort of thing you see over fire exits. Now, these are supposed to sit there, connected to the mains, keeping an internal battery charged, until such time as the mains fails, and then they illuminate for 30mins, to provide a guide for anyone attempting to escape a dark and possibly smoke filled building.

The pair Ive obtained each have an array of 27 5mm white LEDs, along with the charging and control circuit board and a 3.9v 1500mAh NiCd battery.

 Immediately, the possibility of converting these into 12v 'penthouse' lamps comes to mind. A 12v system is safe enough that I can let the boys have full control over it - so long as the supply is secure. In this case, the supply will be a large Sealed Lead Acid Battery, so all thats needed is a box.

The LEDs are arranged in parallel groups of three, each group with a 1ohm current limit resistor, on a single long PCB strip. The majority of the main PCB in each lamp is the mains switch mode inverter and battery charger circuit.

Most of the mainboards circuitry is therefore of no use for the new application. The only part that is, is the small black device in the center of the photo below

This is a 7135 Low Drop Out constant current regulator. Its job, is to provide a fixed 350mA drive to the LEDs. This works out at around 13mA per LED at 3v.

There is one problem though - the 7135 has a maximum input voltage of 6v! So, a further regulator, or other voltage dropping technique, is still required to bring the 12v supply down to within the 3-6v range of the 7135. Luckily, the industry standard 7805 1A 5v regulator, is incredibly cheap, and I likely have several in stock.

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