High speed circuit design for quantum physics light sensing


[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


[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.