Small sensor built into audio jack, held in tweezers

Measuring LED Flicker, With Phototransistor And Audio App

No one likes a flickering light source, but lighting is often dependent on the quality of a building’s main AC power. Light intensity has a close relation to the supply voltage, but bulb type plays a role as well. Incandescent and fluorescent bulbs do not instantly cease emitting the instant power is removed, allowing their output to “coast” somewhat to mask power supply inconsistencies, but LED bulbs can be a different story. LED light output has very little inertia to it, and the quality of both the main AC supply and the bulb’s AC rectifier and filtering will play a big role in the stability of an LED bulb’s output.

Mobile phone spectrum analyzer pointed at light source
The DIY photosensor takes the place of the microphone input.

[Tweepy] wanted to measure and quantify this effect, and found a way to do so with an NPN phototransistor, a resistor, and a 3.5 mm audio plug. The phototransistor and resistor take the place of a microphone plugged into the audio jack of an Android mobile phone, which is running an audio oscilloscope and spectrum analyzer app. The app is meant to work with an audio signal, but it works just as well with [Tweepy]’s DIY photosensor.

Results are simple to interpret; the smoother and fewer the peaks, the better. [Tweepy] did some testing with different lighting solutions and found that the best performer was, perhaps unsurprisingly, a lighting panel intended for photography. The worst performer was an ultra-cheap LED bulb. Not bad for a simple DIY sensor and an existing mobile phone app intended for audio.

Want a closer look at what goes into different LED bulbs and how they tick? We have you covered. Not all LED bulbs are the same, either. Some are stripped to the bone and others are stuffed with unexpected goodness.

Turntable Turns Waveform Generator

In need of a waveform generator for another project, [David Cook] crammed out the old turntable to modify it for a handy hack: By adding a simple reflectance sensor to the pickup he turned it into a waveform generator that optically plays back arbitrary waveforms from printed paper discs.

Continue reading “Turntable Turns Waveform Generator”

Optical Data Transfer Project At Local School’s Family Science Night


[Dave] wanted to show off a project at his 4th-grade son’s school during their family science night. We haven’t heard of an event like this before but it sounds like a fabulous idea! He had a new laser he wanted to include in the project, and noticed that his son was learning about how ASCII maps letters to binary number when the idea struck. He ended up building an optical data transfer system that demonstrates binary code.

This presents a fantastic learning opportunity as the project invited the school kids to select encoded strips like the ones seen above to form a secret message. The laser is pointed at a photosensor which is being read by a Raspberry Pi board. The Python code looks for a baseline and then records increases and decreases in intensity. Since the translucent tokens have either holes or black lines for 0 and 1 the baseline approach does away with the need to clock in the data. [Dave] reports that everyone who tried out the experiment was fully engaged at the prospect of pushing pieces of tape through the sensor and watching their secret message appear on a monitor.

He was motivated to write about this project after reading about data transfer using an LCD screen and photosensor.

Building A Spectrophotometer

What can you make with a toilet paper roll, duct tape, and a graphing calculator? A stand for your homemade spectrometer. This is neither as pretty nor as accurate as a precision scientific instrument, but that doesn’t mean it’s useless. In fact, it works perfectly well for rudimentary observations. Light is shined through a sample solution, passes through a diffraction grating, then shows up as bands of color on the projection surface seen above. The photosensor mounted on the cardboard tube was pulled from a night-light, and is read using the ruler and the multimeter. This results in two data units that are used to graph the results. As long as you’re running test samples as a control this simple setup will yield useful information for the scientist on a shoe-string budget.

[via BoingBoing]