Sunday, 17 February 2013

Chronograph part 2

Having sorted out most of the code and my tube having arrived from Ebay, I started to work out the mechanics of the sensors.  I decided to mount the emitter /sensors 4 inches apart on an aluminium tube, the internal diameter big enough so that it would slide an inch over the end of my air rifle barrel.

Because the emitters and sensors are side looking I decided to drill small holes only just bigger than the small lenses and secure them to the tube using some heat shrinkable tubing.

Section cut from Google sketchup
Once the whole thing is complete I intend to place the whole length in a piece of heat shrink.

I used some Veroboard to mount the sensors and I'll use the copper tracks to feed all the power and signals, I should be able to use a 4 core cable, VCC, GND, output1 and output 2.
Sensors secured to tube with heat shrink tubing
My plan is to use the cable to feed the signals into a Jeenode with display first and then later, if all works well, add a Arduino nano and RFM12B breakout PCB to the tube and transmit the data wirelessly.



That's all for now.

Sunday, 10 February 2013

Air rifle chronograph

For some time now I've had an old Diana air rifle in the garage gathering dust and it's been a long time since I used it but I recently got interested again and even bought myself an air pistol.

As usual I like to have a tinker with anything I can, including my air guns.

One important tool you need when tinkering with airguns is a chronograph, this lets you check the power of the gun to make sure you don't go over the legal limit allowed for the maximum energy.

To buy a small clip on chronograph was going to cost me £49..... "hang on" I thought, I've got all the parts I need to make my own chronograph using the Jeenode.

So, the basic idea is to put two IR beams about 100mm apart in a tube and use the jeenode to time the duration between the first and second beams being broken by the pellet.

If you know the weight of the pellet, the speed and distance travelled then you can calculate the energy used to fire the pellet.

Some googling had come across a few projects form other people and given me a few good tips and even some basic code to start with.

Where my chronograph would be different was that I was going to use the RFM12B to transmit the timing information to a remote receiving station using my "alternate jeenode" and displaying the information on the GLCD.

I chose IR sensors that had a buffered logic output, this would give a clean square trigger signal.  They're also quite fast at 25nS rise/fall times.


OPTEK TECHNOLOGY - OPL550A - PHOTO SENSOR

I Chose the matching IR LED to go with the sensor, OPTEK TECHNOLOGY - OP240A. - IR EMITTING DIODE

So, I started the prototyping whilst waiting for the IR sensors to arrive, to test the code I used a couple of switches to simulate the trigger sensors.

And used an oscilloscope to get an idea of what the real timing was and sorted out the code until it matched the oscilloscope measurements.

Cursors placed on the leading edges on channels A (trigger 1) and B (trigger 2)

When the sensors arrived I used some plastic waste tube I found in the garage to mock up the triggers at 4 inches apart.

I tried out the sensors by dropping some objects down the tube and reading the results via the serial link to my PC, all seemed well and the times were in the ball park when compared to some calculations I'd done earlier.

..... Next steps, write the receiver code for a 4x20 LCD and also the GLCD and maybe look at a VB programme to receive the data on a PC and display the results.



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