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HOW TO: Phototransitor Chronograph

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Unread postAuthor: brplatz » Sun Jul 20, 2008 11:49 am

Yes Yes i do believe it 180 not 180k sorry it was late when i drew up the schematics. And yes the 2v is a capacitor. Sorry for the confusion.
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Unread postAuthor: starman » Sun Jul 20, 2008 11:54 am

brplatz wrote:Yes Yes i do believe it 180 not 180k sorry it was late when i drew up the schematics. And yes the 2v is a capacitor. Sorry for the confusion.


That's better! Would that be a 2 uf cap?
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Unread postAuthor: jimmy101 » Fri Jul 25, 2008 1:25 pm

The cap can probably be omitted since most sound card MIC inputs have a cap in serires already (technically, the input is said to be DC uncoupled and to have a high-pass filter.) In the schematics below the MIC input portion of the sound card is shown on the right, the wiring to the plug on the left.
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Line inputs are often either DC uncoupled, via a cap, or coupled using a 1:1 transformer. In either case, the cap on the chrony isn't needed.
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Unread postAuthor: rp181 » Fri Jul 25, 2008 4:56 pm

not much of a how too =)

The cap is pointless, especially with a IR LED, with me, the line is flat, and theres huge peaks, with very little noise. On mine, i only made it 6in long to save space, but it still works well. here are some pics:
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This design lets me configure it to any bore size i choose (well up too 6in :roll: )

Cost: 0$ =p
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Unread postAuthor: Bonkin » Tue Jan 06, 2009 11:42 am

This may be a dumb question, but why does Jim's circuit run only on the power from the mic jack, but this one needs the 3v battery? Is it to give extra power due to the IR LEDs or because of something else?

Thanks!
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Unread postAuthor: jimmy101 » Tue Jan 06, 2009 1:26 pm

Bonkin wrote:This may be a dumb question, but why does Jim's circuit run only on the power from the mic jack, but this one needs the 3v battery? Is it to give extra power due to the IR LEDs or because of something else?

Thanks!

It depends on what kind of input on the PC you are using. And there are really two reasons for the battery.

A line input is unpowered. You have to supply power to the circuit so there is a signal for the soundcard to detect.

A MIC input is powered. The electret microphones that the MIC input is designed for require a power source. Instead of adding a battery to the microphone the power is often supplied by the amplifier (the PC in this case).

Desktop PCs usually have both MIC and line inputs. Laptops usually only have the MIC input.

If all you need is enough power to make an audio signal then the MIC input supplies plenty of power.

My circuit uses ambient light so I don't need to supply the ~25mA or so that the LED requires. The MIC input won't supply that much power, it's typically limited to about 1mA. More than enough to create an audio signal, not nearly enough to actually light up an LED.

So,

1. If you use ambient light the MIC input supplies more than enough power to create a signal.

2. If you use a line input then you need a power source even if you use ambient light. (You might be able to use a solar cell as both the detector and energy source but solar cells respond pretty slowly.)

3. If you use your own light source (i.e., LEDs) the MIC input can't supply enough power so both the line input and MIC inputs will need a battery to light up the LED.

In my experience ambient light works fine. The only time you have a problem is when the ambient light is flourescent light bulbs. They blink on and off 120 times/second. Incandescents brighten and dim 120 times a second but that usually isn't a problem. Of course, the sun doesn't blink at all and presumably most people shoot outdoors during the daytime anyway ...
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Unread postAuthor: rp181 » Tue Jan 06, 2009 3:18 pm

I prefer to use a infrared LED rather then rely on ambient light due to changing conditions. The detector i use has a filter that only lets infrared light in, and matching emitter (Fairchild semiconductor tells you the matching pair).

On side side note, @jimmy:
Im trying your scheme to get the voltage vs. time on the mic input. Does a 30kohm resistor sound good for 400v peak? I forgot my emitter's part number, so i can't find the operating current.
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Unread postAuthor: Bonkin » Tue Jan 06, 2009 4:54 pm

Thanks very much for the explanations. I was wanting to try to build the one with IR LEDs to the MIC input, and just trying to figure out the differences between the two different circuits. Your explanation has answered my questions for sure. Thanks again.
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Unread postAuthor: jimmy101 » Tue Jan 06, 2009 7:33 pm

rp181 wrote:On side side note, @jimmy:
Im trying your scheme to get the voltage vs. time on the mic input. Does a 30kohm resistor sound good for 400v peak? I forgot my emitter's part number, so i can't find the operating current.


scurries off to look up the circuit ...


Typical LED (visible or IR) use 20~30mA max., above that and it'll turn into a puff of smoke.:D

For a 400V supply, V=IR,
400V/25mA = 16K ohms
Add in some "windage" and 30K sounds OK. The LED will tolerate a pretty hefty over current for the fraction of a second that it'll actually see the 400V so you could probably get away with a smaller resistor, even less than 16K. As you've probably already figured out though, the sound card has a pretty wide dynamic range (2^15, roughly 90DB) so you have a lot of "space" to work with.

Going conservative on the resistor (like your 30K resistor) is probably a good idea and won't affect the data much. The only problem I can see with a resistor that is way too large (say 100K or bigger) is that the 1.4V drop across the LED starts to become significant.
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Unread postAuthor: rp181 » Tue Jan 06, 2009 10:09 pm

I used a small 1/4W resistor, and shoved it in a pen tube with detector and emitter at opposing ends, and i plugged it up with epoxy, no changing it now =p
I was thinking, what if i made a identical one, with the detectors in parallel, but the LED's are connected anti parallel (anode of one LED connects to cathode of other, and the cathode connects to anode of other). This would give me 2 consecutive peaks on the graph, and the second peak would represent negative voltage ringback. Would this still work with a anti-parallel diode?
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Unread postAuthor: jimmy101 » Wed Jan 07, 2009 1:14 pm

rp181, very clever. Yep, two diodes should give you both the positive and negative voltages. They'll be offset a bit since the LEDs have about a 1.4V drop, so voltages across the LED pair between -1.4V and +1.4V will be invisible. Other than that it should work great.

You can buy LED pairs wired this way. They come in two flavors; (1) two LEDs wired as you describe in a two wire package. (2) Two LEDs that each have their own input lead, both LED share a common lead, for a total of three wires. Often the pair of LEDs have different colors, e.g., red and blue. For the first type you can only turn on one of the colors at a time. For the second type it is posible to get both LEDs on at the same time.

You're using this to monitor the voltages in a coil gun's cap as it fires? What kind of switch do you use to fire the gun? If you use an SCR then there shouldn't be any ringing of the RLC circuit since the SCR won't conduct backwards. A TRIAC will conduct backwards as long as there is still gate voltage.
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Unread postAuthor: rp181 » Wed Jan 07, 2009 2:00 pm

es, its to monitor the voltage pattern in the load (the coil) Right now im using a mechanical switch, but i am working on getting SCR's. This would be helpful because i am shutting off the SCR's mid pulse with a reverse voltage on the SCR. The projectile speed/optical detection will tell me when to shut it off. I know what LED's you are talking about, theres a visible light filter on the detector though.
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