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Topic Title: Timing Circuits
Topic Summary: Tips needed in building a timing circuit
Created On: 29 July 2013 11:11 PM
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 29 July 2013 11:11 PM
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ryannorrington

Posts: 4
Joined: 29 July 2013

First post on the IET

I am attempting to measure the velocity of the crack front of a prince ruperts drop, cool video link here (Link removed/watch?v=xe-f4gokRBs). As I don´t have access to a high speed camera, I have decided to use two glass break detectors, one on the top of the drop and one on the bottom, and measure the difference between the detectors recorded time. I´m not using microphone based detectors, rather am hoping to place two fragile wires within the glass which will break as the glass breaks, one at the top of the drop and one at the bottom. Does anyone on the IET know how I would approach this and actually recieve meaningful output from the two wires breaking, Thanks
 01 August 2013 12:41 PM
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amillar

Posts: 1918
Joined: 28 May 2002

Hi,

If you can reliably break the two wires then measuring the time difference between them is easy - as long as you have access to a counter/timer unit. If you're still around (sometimes posters give up if they don't get an answer straight away) reply here, ideally adding whether you're doing this on your own in your bedroom or as e.g. a college project, and let's see if we can help - sounds interesting!

-------------------------
Andy Millar CEng MIET CMgr MCMI

http://www.linkedin.com/in/millarandy

"The aim of argument, or of discussion, should not be victory, but progress." Joseph Joubert
 01 August 2013 12:53 PM
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amillar

Posts: 1918
Joined: 28 May 2002

I'd never heard of Prince Rupert's drops before - having been looking them up over lunch I really want to try making some now!

-------------------------
Andy Millar CEng MIET CMgr MCMI

http://www.linkedin.com/in/millarandy

"The aim of argument, or of discussion, should not be victory, but progress." Joseph Joubert
 02 August 2013 05:07 PM
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ryannorrington

Posts: 4
Joined: 29 July 2013

Awesome. I'm doing the IB and I have chosen to research the prince rupert drop for my extended essay. If I can time the difference between the two wires breaking using the length of the drop I can find the velocity of the crack front, which is about 0.25 Mach. Don't know where I will do it though. Supposed to do it in the 7 week summer holiday but I'm in Bolivia for 4 weeks and Los Angeles for 3 weeks. So I'll probably end up doing it in LA. Can you recommend the components I need? Like a shopping list that would be amazing. Thanks for your help. Wasn't expecting such a quick reply.
 04 August 2013 11:58 PM
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kengreen

Posts: 400
Joined: 15 April 2013

hello Ryan,
please excuse occasional spelling mistakes - I am writing with the aid of a machine. I was approached several years ago with a similar problem by a man a man who repaired cameras and he wanted to measure the exposure time setting of the iris.I roughed outfor him a quick sketch in block form of a system which should serve his purpose. His knowledge of electronics was minimal yet when I next met him he had not only built it but was using it.
Briefly as a timing device he used a 1 Mc/S (sorry - 1MHz) crystal controlled oscillator and then put that through a chip that turned it into a square wave and then differentiated it. That was then fed into a gate circuit in which a gate was opened to allow the pulses separated by one microsecond to be sent to a counter; the next pulse closed the gate again. when the gate is suitably triggered so the counter displays the interval in microseconds.

these logic gates are readily available off-the-shelf at a very low cost. The above arrangement will give you time intervals at 1 µs which can be scaled by varying the driving oscillator. Afraid however that I have no immediate suggestion as to how you will develop the gate-trigger pulses at the beginning and end of each sample drop. if you can rig a device to generate Sharp transitions between (say) light and dark then again a differentiator will yield the necessary triggers.
Ken Green
 05 August 2013 01:28 PM
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amillar

Posts: 1918
Joined: 28 May 2002

Hi Ryan,

Errr..you're not making it easy for yourself! Measuring this in a school / college lab would be easy (if they've got the right equipment, which they should have), you would feed the connections into a piece of equipment called a counter/timer and it would just time the delay for you. You can (as Ken said) build such an instrument reasonably easily from scratch if you know something about electronics and/or have someone around to guide you, but it doesn't read like that will be the case. Much easier (although more expensive) would be to use a PC interface like "Analog Discovery" or "PicoScope" (each around £150, rather less if you're lucky): you connect each of your break wires to its own resistor (value doesn't matter too much, but about 1kOhm would be about right) and then to a battery, and connect each of two channels of the PC interface to one of the two wires. I expect if you do a web search on using a PicoScope / Analog Discovery as a timer you'll find examples.

Bigger problems, having thought about this over the weekend:
a) how are you going to emdeb the wires in the glass (since it needs to fall freely)?
b) isn't the act of embedding the wires going to affect the stresses within the glass?
If the wire's thin enough I guess it may work with the wires attached (superglued?) to the outside of the glass?

-------------------------
Andy Millar CEng MIET CMgr MCMI

http://www.linkedin.com/in/millarandy

"The aim of argument, or of discussion, should not be victory, but progress." Joseph Joubert
 05 August 2013 05:11 PM
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amillar

Posts: 1918
Joined: 28 May 2002

P.S. I am assuming here that you have a PC or similar available...

-------------------------
Andy Millar CEng MIET CMgr MCMI

http://www.linkedin.com/in/millarandy

"The aim of argument, or of discussion, should not be victory, but progress." Joseph Joubert
 06 August 2013 12:27 AM
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ryannorrington

Posts: 4
Joined: 29 July 2013

I've got a PC available. I was going to attach the wire to the outside. Got the inspiration sitting in Starbucks looking at their alarmed glass. Same concept. I'm allowing a margin of error in regards to the embedded wire affecting the final value. As I'm doing it for school, the process is more important than the final value. If I was going to go down the counter/timer route, would that be cheaper than hooking it up to a PC. Thanks for all the help. Amazing response
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