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Show HN: A high-speed laser camera trigger for under $2

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Re: Show HN: A high-speed laser camera trigger for under $2

#21

Post by omegant »

glibgil wrote: post #14
omegant wrote: post #8

Earlier quoted context omitted.

Find that lately there are less of this kind of Hacking articles and those talking about interesting and inusual things, and much more about New frameworks or libraries.

I went back into the archive and tried to run into a hardware hacking post before a framework post. It went, startup, startup, Google buys YouTube, startup, startup, then this https://news.ycombinator.com/item?id=1981 (2694 days ago). I couldn't find evidence that this site cut its teeth on hardware hacking.

It not specific on hardware, it's general about all things not about programing and new products. Also it's not about new posts, at the new section I keep finding them. It's that they hit the front page less frequently and when they make it the fall to the second or third page in a mater of one hour, compared to the framework or product posts that stay very high with relatively less points. Maybe it's not the case, maybe it's just the habituation to HN contents after three years of daily reading. If it happens I don't know if it's due to the new curation team (for example now I find that conversations are not growing so much into long rants about small details as before) or to more people flaging non "programmer" stuff more than before.

Re: Show HN: A high-speed laser camera trigger for under $2

#23

Post by ascorbic »

joe_bleau wrote: post #3

$2 sounds really cheap--did I miss the laser on the bill of materials? Anyone else hoping for a modern and inexpensive replacement for the legendary classic EG&G microflash?

A cheaper, safer replacement for the EG&G air-gap flash is exactly the final goal of my project, but it will take a few months. Expect a Kickstarter. The BOM doesn't include the laser. I'm using the laser sight on the air pistol, and I tested it with a cheap eBay laser pointer.

Re: Show HN: A high-speed laser camera trigger for under $2

#24

Post by ascorbic »

aceperry wrote: post #11

Gotta wonder about the schematic, looks like it's wrong. The battery and the cathode of the opto is not connected correctly.

You are correct. Good spot. I added the power supply after making up the rest of the stuff in Fritzing, and screwed the the polarity. I'll add a note!

Re: Show HN: A high-speed laser camera trigger for under $2

#27

Post by oldmanjay »

michaelmior wrote: post #13
sp332 wrote: post #6

Earlier quoted context omitted.

I picked up a 1 mW pointer for £1.98 on eBay. That's about $3.38 USD.

> I’m only including the cost of components, and not things like cables, breadboards or the laser pointer (or flash).

I like that! I'm going to start giving estimates like that

"I'll do this for free, not including my hourly or the cost of materials"

Re: Show HN: A high-speed laser camera trigger for under $2

#28

Post by kwantam »

aceperry wrote: post #20
kwantam wrote: post #12

Earlier quoted context omitted.

Looks correct to me. The idea is to trigger in the absence of the laser---i.e., when the beam is broken by a projectile. When the laser shines on the base of the phototransistor, the transistor pulls current from the node between the two resistors. Assuming the laser causes sufficient electron-hole pair generation in the base, the transistor can pull a bit more than 1mA (5V/4.7k) from the battery; the photodiode insi…

>the transistor pulls current from the node between the two resistors The battery is connected incorrectly for this scenario to work. Node 2 of R2 should be connected to the positive battery terminal for correct flow to the LED. >the phototransistor's collector won't pull much closer than a couple hundred millivolts to its emitter while conducting that much current The transistor won't operate like that given the way…

See my comments below. The only error is that the OP reversed the symbol for the battery. Look at the breadboard; he/she built it with the power supply correctly installed.

You're probably right about the datasheet for the opto, but that neither clashes with my statement ("most optocouplers") nor invalidates the broader point (Vcesat is ~300mV; the diode is most definitely off).

Aside: I spent several years as the lead engineer on a line of digital isolators (modern replacements for optocouplers: CMOS integrated circuits with much higher speed, better immunity to common-mode transients, and no need to use expensive and generally boutique processes supporting LEDs). Most of the time, opto manufacturers oversell the capabilities of their optos at low voltage, and generally speaking in industrial applications optocouplers are driven much harder than the currents at which forward drop numbers are quoted in datasheets.

Aside #2: bipolar transistors absolutely do operate upside-down! Reverse beta is usually far worse than forward beta, because the collector region tends to be much larger than the emitter and as a result there is substantial carrier recombination.

In fact, when you want to use a bipolar transistor as a low drop switch, and if you don't care how much base current you have to use to do it, running the transistor upside-down tends to result in lower drop and faster operation. There are a few reasons for this, but it boils down to this: the speed of the bipolar transistor as a switch is limited by charge storage in the base when the device is saturated, or more precisely, the amount of time it takes to remove this charge when trying to turn the device off. The base-collector diode behaves differently than the base-emitter diode because of the difference in emitter and collector doping. As a result, when upside-down, the base-collector junction stores much less charge, the lower beta actually means that the charge is removed faster, and the higher forward drop of the base-emitter diode means that reverse Vcesat is lower than the forward Vcesat.

The downside, to which I alluded above, is 10x or more base current to actually drive the transistor. Depending on the quantum efficiency of this phototransistor and its reverse beta, were the OP to use the phototransistor upside-down in this circuit, it might not be able to shunt sufficient current to keep the optoisolator off.

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