Muzzle Velocity Chronograph

[gth629e]

Hello All,

I am currently working on a chrono project to measure muzzle velocity. The rang will be 0-800 fps or so. Primarily for paintball and airsoft use and also just another project to learn and explore new ideas. I have searched the forums a lot and found a few relative topics but the topic seem to die and are more than a few years old.

The setup,

I am using a BS2 module for computation and control of the LCD panel etc. Compiling the code was not a problem and I can post it here if anyone would like to follow the project and build the unit. The primary setup is that of a simple speed trap which has been discussed several times on the forum but never in this application.

The setup requires two sensors at a set distance apart. The first sensor starts a counter and the second sensor stops it. The counter can them be related to velocity, pretty simple.

My first setup I used two push buttons to test the code and LCD. Everything worked well so I switched to using two IR LED's and two 38.5 Khz IR receiver modules. I chose those module because of being less sensitive to ambient light. The problem was that modulating the LED at 38.5 KHZ uses a lot of resources from the processor making the code very buggy. Also, this setup was very unstable, tripping every few milliseconds.

What I would like to do

I would like to replace the IR receiver module with a simple IR LED and IR photo transistor. I realize that I would need to use a small signal NPN transistor in conjunction with the photo transistor to get the crisp on/off signal to emulate a push button type setup. I am also aware that I would need to protect the sensor from other light sources. That will not be a problem.


I currently need help on getting a working circuit using this LED, Photo transistor, and NPN transistor. (links below)


Infrared Phototransistor - RadioShack.com
High-Output Infrared LED - RadioShack.com
2N3904 NPN Small Signal Transistor - RadioShack.com

By all means other components will be needed and I am not limited to these components. I also realize pullup/down resistors will be needed.

Any help would greatly be appreciated. Thank you all.

Regards,

 

[gth629e]

Any takers?

 

[crutschow]

RadioShack has no data on these devices at their website so I can't give resistor values or voltage levels. They are really terrible in that respect.

You many be able to get by without the transistor if you have sufficient signal.

Use a resistor to bias the IR LED with it's rated nominal current from your supply. Calculate the resistor value based upon your power supply voltage and the stated LED voltage drop, (R = (Vps-Vled) ÷ Iled).

The fastest response from the Phototransistor should be in the emitter follower mode so put the load resistor from the emitter to ground. You may have to experiment with different resistor values depending upon the amount of detected signal, try 10kΩ to start. A larger resistor will make it more sensitive but slow it down. Connect the Phototransistor collector to your positive supply (supply voltage less than the maximum voltage rating of the Phototransistor).

 

[gth629e]

crutschow,

You are right about the radioshack not having much data, but they are the most convenient for me. I did try the setup you suggested and SUCCESS! Well, sort of. I had to play around with the BS2 to set the rang for the I/O states.

I posted the circuit I am using to test below, if anyone is interested.

I am playing with the value of R1 and R2 now to get the desired response. I a having some difficulty with it detecting fast objects. Would there be any real advantage to using another transistor?

 

[Mikebits]

If I did the math right, your paintball will be moving at 4us/mm. This may be a bit short time to detect with a photo device.

 

[gth629e]

Mikebits,

You are right, at about 800 fps that is about 4 us/mm. The actual limit is closer to 500 fps which is around 6 us/mm. I am open to suggestions. I know that the chronos available use an optical switches but I am unsure of what configuration. Do you think a photocell might be more fitting? I would think that a photocell would be far slower. I also know that photo diodes have a faster response time. Seems like there should be a way to use a photo transistor.

 

[Mikebits]

I have not worked with many photo devices, but I would think a wide angle receiver would be needed.

 

[Mikebits]

You may be in luck.
**broken link removed**

 

[crutschow]

I am playing with the value of R1 and R2 now to get the desired response. I a having some difficulty with it detecting fast objects. Would there be any real advantage to using another transistor?

Another transistor will not speed up the circuit. But if you make the phototransistor emitter resistor low enough to get the desired speed, and the signal level is then too low, you can add a transistor to amplify the level.

What value of phototransistor emitter resistor are you using?

Any photodiode will likely be fast enough for you needs. But you will need to add an amp to get sufficient signal level (Google "photodiode amplifier" for some examples).

 

[gth629e]

Now I am using anything between 68k and 1M ohm as the phototransistor emitter resistor, above or below that falls out of the range for the bs2. That's why I had imagined using another transistor to amplify the signal as you said. I need at least 3.4 volts for the stamp to consider the circuit "on", below that is considered "off".

I did find this today (link below), the only difference is he is using a 3 pronged phototransistor. I do not have enough knowledge as of yet to know if I can adapt this circuit using the components I have.

6.9 GigaHertz: DIY Airsoft Chrono [PC-based]

 

[gth629e]

I did get the circuit working with a small signal transistor. My best combination so far is a 22 ohm resister on both the em miter and on the IR led. It can detect up to about 200 fps above that I am still having difficulty. I guess the smart thing to do would be to buy some pots so I could potentially zone in on the correct values. It would also help if I had more information on the shack stuff I am using.

A larger resistor will make it more sensitive but slow it down

crutschow,

Did you mean a larger resistor would slow the response to change in the ir light?


Also,

This looks very interesting,

https://www.electro-tech-online.com/custompdfs/2009/03/getfileaspxtypepressfiletsl250r-e30.pdf (and cheap like 3 bucks a piece)

 

[crutschow]

crutschow,

Did you mean a larger resistor would slow the response to change in the ir light?

Yes.

You can use the circuit you found in the link. He's not using the base connection to the phototransistor, so you just ignore it.

 

[gth629e]

yeah, I figured as much and tried and it did work. I am still trying to zone in on the best R1 and R2 values. Did you see the link above?

 

[Mikebits]

Look at the risetime spec for your phototransistor. Some are slow. I have seen 40us rise times for some devices. This would be too slow for your app. Use one with fast rise time spec.

 

[crutschow]

Look at the risetime spec for your phototransistor. Some are slow. I have seen 40us rise times for some devices. This would be too slow for your app. Use one with fast rise time spec.

That's the problem with Radio Crap parts...no specs.

 

[deepfreeze]

is anybody run this chrono and who can find windows based program?? Some one can help me