Helen Czerski
Why LEDs pulse.
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 welcome 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!