HE­LEN CZ­ER­SKI ON… HOW DO LEDS WORK?

“THOSE BI­CY­CLE LIGHTS HAVE BEEN FLASH­ING AWAY FOR YEARS, AND I’VE NEVER NO­TICED”

BBC Earth (Asia) - - Comment & Analysis - Dr He­len Cz­er­ski is a physi­cist and BBC sci­ence pre­sen­ter Her book, The Storm In A Teacup, is out now

To be hon­est, I should have been bet­ter pre­pared. But I’d bounced out of my flat in the morn­ing with­out check­ing the weather fore­cast and at 6pm, I was curs­ing my op­ti­mism. Tor­ren­tial rain was pour­ing out of the dark sky, and I was wear­ing jeans, on a bike, with­out a proper rain­coat. The slots in my cy­cling hel­met were fun­nelling wa­ter down the back of my neck, and I was cold. I tried telling my­self that Scott had faced far worse on his way to the South Pole, but the nov­elty of that ran out af­ter about 30 sec­onds, leav­ing the re­main­ing 20 min­utes of my cy­cle filled with grumpi­ness. And ev­ery sin­gle traf­fic light seemed to be on red. Half­way home, I was squint­ing out through the small wa­ter­fall tak­ing shape over my face, and be­came dis­tracted by the rain in front of me. The black­ness just be­yond my han­dle­bars was shot through by dashed white straight lines, with spa­ces ex­actly the same length as each dash. At the next red traf­fic light, it was the same: a strictly reg­i­mented pat­tern in the messy fluid chaos around me. My bike lights were clearly flash­ing, but too fast for me to see.

I reck­oned that each dash and each gap was about a cen­time­tre long, and the rain was pretty heavy so the av­er­age rain­drop speed was prob­a­bly about 5m/s. That gives a flash­ing rate of about 250 times each sec­ond, or 250Hz. Why would my bike lights be flash­ing like that?

When I got home, and af­ter one of the most wel­com­ing hot show­ers I’ve had in a long time, I took a closer look at my lights. They’re LED ones, com­pact but as­ton­ish­ingly bright. They’ve got two bright­ness set­tings, and my front light had been on ‘half bright­ness’, which is still shock­ingly in­tense.

When you use a dim­mer switch at home, you’re nor­mally con­trol­ling the volt­age to the light bulb. But this is a rub­bish way of dim­ming an LED light. LEDs (light-emit­ting diodes) work by hav­ing two ma­te­ri­als next to each other, one with some ex­tra elec­trons and one with some ex­tra holes where elec­trons could go. If you push an elec­tric cur­rent through the junc­tion, you ef­fec­tively shove the elec­trons un­til they fall into the holes, and the ex­tra en­ergy left over from that process is emit­ted as light. But in this sys­tem, even a tiny dif­fer­ence in volt­age causes a huge dif­fer­ence in cur­rent (and light), mak­ing it tricky to reg­u­late the bright­ness by al­ter­ing the volt­age. So what man­u­fac­tur­ers do is to pulse the cur­rent – the level of cur­rent stays fixed but it’s switched on and off hun­dreds of times each sec­ond. A hu­man won’t de­tect any flicker in the light as long as the pulse rate is up above about 300 cy­cles per sec­ond, so my es­ti­mate of 250 flashes wasn’t bad. To dim the lights, the num­ber of flashes per sec­ond stays the same, but the pulse it­self gets shorter and the gap gets longer. It’s called ‘pulse width mod­u­la­tion dim­ming’, and it’s used ev­ery­where you find LEDs with a dim­ming op­tion.

What’s fas­ci­nat­ing about all this is that those lights have been flash­ing away for years, and I’ve never no­ticed. It was only when a sin­gle fall­ing rain­drop was il­lu­mi­nated sev­eral times on its way down that the flashes be­came vis­i­ble. Such a low-tech way of spot­ting a very high-tech so­lu­tion… al­most (but not quite) worth get­ting soaked to the skin for!

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