DIY Single Pixel Digital Camera

[Artlav] wanted to build a digital camera, but CCDs are expensive and don’t respond well to all wavelengths of light. No problem, then, because with a photodiode, a few stepper motors, the obligatory Arduino, and a cardboard box, it’s pretty easy to make one from scratch.

The camera’s design is based on a camera obscura – a big box with a pinhole in one side. This is all a camera really needs as far as optics go, but then there’s the issue of digitizing the faint image projected onto the rear of the camera. That’s fine, just build a cartesian robot inside the box and throw a photodiode in there.

There are a few considerations when choosing a photodiode for a digital camera. Larger photodieodes have higher noise but lower resolution. [Artlav] has been experimenting with a few diodes, but his options are limited by export control restrictions.

Even with the right photodiode, amplifying the tiny amount of current – picoamps in some cases – is hard. The circuit is extremely sensitive to EMI, and it’s inside a box with stepper motors pulled from the scrap bin. It’s amazing this thing works at all.

Still, [Artlav] was able to get some very high resolution images across a huge range of wavelengths. He’s even getting a few images in mid-wave infrared, turning this homebrew digital camera into the slowest thermal imaging camera we’ve ever seen.

Simple and Inexpensive Heartbeat Detector

There are many ways to detect a heartbeat electronically. One of the simpler ways is to take [Orlando’s] approach. He’s built a finger-mounted pulse detector using a few simple components and an Arduino.

This circuit uses a method known as photoplethysmography. As blood is pumped through your body, the volume of blood in your extremities increases and decreases with each heartbeat. This method uses a light source and a detector to determine changes in the amount of blood in your extremities. In this case, [Orlando] is using the finger.

[Orlando] built a finger cuff containing an infrared LED and a photodiode. These components reside on opposite sides of the finger. The IR LED shines light through the finger while the photodiode detects it on the other side. The photodiode detects changes in the amount of light as blood pumps in and out of the finger.

The sensor is hooked up to an op amp circuit in order to convert the varying current into a varying voltage. The signal is then filtered and amplified. An Arduino detects the voltage changes and transmits the information to a computer via serial. [Orlando] has written both a LabVIEW program as well as a Processing program to plot the data as a waveform. If you’d rather ditch the PC altogether, you might want to check out this standalone heartbeat sensor instead.

Pendulum Music for Oscilloscope and Photodiodes

Two turntables and a microphone? Try two oscilloscopes and a couple of photodiodes. [dfiction] reinterpreted Steve Reich’s classic feedback piece for more modern electronics. The video is embedded after the break.

The original Pendulum Music is a conceptual musical composition from the heady year of 1968. Basically, you set a bunch of microphones swinging across speakers, making feedback as they pass by. The resulting rhythmic and tonal oscillations change over time as the swinging damps down. It’s either mesmerizing or entirely boring, depending on your mindset.

In the [dfiction] version, the feedback is produced by passing a “light microphone” over an oscilloscope. And since he’s got a pair of these setups, the one microphone also feeds the other ‘scope. The resulting sound is this chaotic and gritty noise-rumble. We dig it.

If slowly evolving “process music” pushes the boundaries of your attention span (or if it’s just not your thing) you can totally skip around in the video. Try around 1:40 and 3:45 into the piece just to get an idea of what’s going on. But once you’re there, you might as well let it run its course.

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High speed circuit design for quantum physics light sensing

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[Limpkin] designs circuits for a living. This board is one of his recent projects, and although his skills are light years ahead of our own experiences, he did a pretty good job of explaining how he put this board together.

He was tasked with measuring the light intensity of two photodiodes. The expected impulses picked up by those components will be less than a nanosecond in duration, putting some special design constraints upon him. To register this signal he’s using three cascading op-amps per input. To ward off false readings from RF interference he also designed in the shielding which you see surrounding the majority of the circuit.

His package choice for the THS3202 op-amps is quite interesting. He didn’t go with the footprint that includes a thermal pad to dissipate heat because he didn’t want to interrupt the ground plane on the underside of the board. To keep the parts from melting he added an aluminum spacer that contacts the top of the package, then a heat sink that covers the entire shield frame. In a future revision he figures he’ll move to a four-layer board so that the can opt for the MSOP package that does the work for him.

Nintendo light gun retrofit lets it play color-based games

This Nintendo light gun, aka Zapper, looks like a stock device. But a peek inside shows that the circuit board has been replaced. [CNLohr] added USB functionality and a few extra sensors that let him write his own games for that use the classic controller.

After cracking open the case he measured the shape of the circuit board so that he could recreate it exactly. This let him design his own board that would drop right into the same plastic support pieces as the original. His circuit uses an ATmega8u2 to provide a USB connection and read the attached sensors. One interesting aspect is the group of four long traces that act as an expandable i2c bus. [CNLohr] went with this so that he could use daughter boards to add in sensors later. In the demonstration seen after the video he’s using a photodiode as a color sensor. It allowed him to write the color-based game seen above where you shoot a different color of target in each round.

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Projectile speed sensor

[Mike] built a sensor rig to measure projectile speed. The setup uses a tunnel with two sensors in it. Each consists of a laser diode on one side focused on a photodiode in the other. The two are monitored by an op amp and measured by an ATmega128 microcontroller. When the beams are broken the elapsed time between the two events is measured in order to calculate speed. There is a setting to adjust the calibration for a range of speeds, which came in quite handy as [Mike] initially tested the device with rubber bands before moving on to a pellet gun and then a rifle.

It seems like he’s tempting fate by shooting a target just a few inches below his exposed circuitry but his marksmanship prevailed. We’ve seen bullet speed detectors in the past, used just for the delight of seeing how fast the projectile is moving, and also to capture an impact at just the right instant.

Light to sound converter

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[Alex] built what he calls a light to sound converter. It reacts differently depending on the type of light: remote controls, light bulbs, TV screens, etc. A photodiode is used with an amplifier to pick up the light change. That signal is dumped through a dual opamp. He swapped in several different types of photodiodes and settled on the BPW34 intended for visible light. He’ll be incorporating this into a much larger project.