Make Your Own Chronograph New Version

[Nias]

I saw someone had posted a link to my original Chronograph. Here is they original post Make You Own Chronograph Original.

That chrono was made a few years ago and over the last few months I have upgraded it several times.
The original used an Arduino NANO which I found out does not have a quartz crystal but a resonator with a pretty rubbish accuracy I think +-5000ppm.
I tried an Arduino UNO MCU but settled on an ESP32 Dev Module with a clock speed of 240Mhz driven by a 40Mhz Quartz crystal with a spec of +- 10ppm.
It also has WIFI built in which means anyone who wants a simply barrel mount Chronograph without the large display can just adapt my code to link it to their phone or PC.

The image below shows my chronograph with the optional £3.99 plug in GPS module for checking the quartz crystal and removable pellet scales on top and the shoot through HT-X3006 chrono bracket.

This image shows the chrono in live working mode linking to an Excel spreadsheet.
I have included demo sheets for mapping your airgun breaking down best shot groups etc.
Being Excel you can use your own data any way you wish.
The chrono also works in single shot mode off batteries if required.

The design is modular so you can use any part of the design eg just using my sensor module and MCU to build a wieless design with minimal footprint.

Below basic sene module can be used as a building block for you own software and or display.

Calibrating
I don't have access to expensive test gear and can't afford to send the unit off for accreditation.

I have created inbuilt calibration for the following- Sensor length and crystal frequency check.

Sensor Length
The software when is sensor calibration mode shows when the in and out sensors have been triggered electronicaly as if by a pellet.
Using digital calipers in depth mode you just take the differnce between the 2 readings to get the exact electrical trigger points.

Note the distance shown on the LCD display is the original reading this will update once you put the new measured distance into the chrono.

Quartz Crystal Frequency
Using a £3.99 plug in GPS module once an indoor satellite fix has been obtained and a 10min sync pulse from the atomic clock in the satellite has been recived a timer starts on the MCU.

Every 10 minute pulse from the sattelite is then compared to the mS count from the MCU and the culumative differnce is displayed. Over time the differnce is grows until after a day you have a pretty good idea of your Quartz crytal accuracy.

The chrono displays the mS error +or- the PPM error and the frequency error in Hz.

I have spent the last 2 weeks measuring 4 differnt ESP32 modules and repeating the test multiple times.
The spec of the ESP32 is +-10PPM and I measured the 4 at -7.38ppm, -2.97ppm, -3.18ppm and -0.33ppm.

Full details here on my website https://www.brettoliver.org.uk/

Please let me know if you find any errors.
3D files are normally £1.00 to download but any member of airgunforums can get them for free - just PM me for a link.

Edit if you have access to a Nate Chrony or Skan then you can add a correction factor within the chronograph screen.

 

[Buster321c]

Wow !!!!!

It’s going to take me at least 48 hours to even begin to understand what I’ve just read !!
I’m a humble window fitter

 

[kidunknown]

Yep like the above post not a clue what it all means
Glad you put pictures with your description,its nice to see what it should look like,if I'd try it wouldn't look like yours , probably be in the bin

 

[Nias]

I have a whole drawer of failed projects where I have spent so much time and effort trying to get them working I can't face putting them in the bin.

 

[colinb]

I am lost in admiration for this. Way, way beyond my capabilities. But it's an example of poor choices making a simple job kinda complex. You cannot make your own chrono without basic electronic knowledge, but the range of abilities and resources needed to make this blows my brain.
Yet all this clever stuff started by using a poor component as the basic timing device, when a 15p watch crystal could be more than good enough. But even a 5000 ppm error is only 0.5% speed error, and although poor it's not a complete killer.
For a chrono, calibrating a crystal with typical 30 ppm error is unnecessary polishing to reduce an error that is already trivial. You only do it because you can. (OK, I did.)
The calibration of sensor spacing described is simple, but is only really applicable to a single point sensor and fails to check rise times and pellet shape. A general purpose device has to have a row of sensors and usually multiple sources. It may not respond to a slow prod because the circuitry may treat that as an external light variation, eg a cloud arriving. But for the length involved here, if one could not place the sensors within 1/2% your ruler would have to be bent.
My modern TV takes 30 seconds to turn on from zero power. 20 years ago, my transistorised device did that job in about 5 seconds so the tube could start up. Complexity is not progress. In some ways I am a Luddite. Below is my chrono, reading from its calibrator board. Not a processor in sight!. A sink did come apart to provide one of the bits....

 

[Gazpopper]

Hi, this is a cool project which I'm going to have a crack at. I have some D1 Mini dev boards, do you think they would work? I'll check out your Web pages for details and may take you up on your kind offer of 3D files to get the bits printed up. Great work, thanks for this

 

[Nias]

I went for the ESP32 as it has a fast and accurate quartz crystal and loads of pins to play with, I think faster than D1 Mini. You would have to check how many digital pins on the D1 mini and the resolution of the analogue input. You may have to change my code a bit.

If you are going to develope WIFI only with no display then the number of pins would prob not matter and it would be a whole lot easier hardware wise to build.

Apologies if my coding is crap as until a few years ago I never knew microcontollers existed and I did everything with logic chips. Please feel free to correct me on any errors and ask any questions on why I have done things.

 

[Nias]

I am lost in admiration for this. Way, way beyond my capabilities. But it's an example of poor choices making a simple job kinda complex. You cannot make your own chrono without basic electronic knowledge, but the range of abilities and resources needed to make this blows my brain.
Yet all this clever stuff started by using a poor component as the basic timing device, when a 15p watch crystal could be more than good enough. But even a 5000 ppm error is only 0.5% speed error, and although poor it's not a complete killer.
For a chrono, calibrating a crystal with typical 30 ppm error is unnecessary polishing to reduce an error that is already trivial. You only do it because you can. (OK, I did.)
The calibration of sensor spacing described is simple, but is only really applicable to a single point sensor and fails to check rise times and pellet shape. A general purpose device has to have a row of sensors and usually multiple sources. It may not respond to a slow prod because the circuitry may treat that as an external light variation, eg a cloud arriving. But for the length involved here, if one could not place the sensors within 1/2% your ruler would have to be bent.
My modern TV takes 30 seconds to turn on from zero power. 20 years ago, my transistorised device did that job in about 5 seconds so the tube could start up. Complexity is not progress. In some ways I am a Luddite. Below is my chrono, reading from its calibrator board. Not a processor in sight!. A sink did come apart to provide one of the bits....
View attachment 863145

When I start a project I start by making a list of things in my head I want it to do.

I wanted a digital LCD display and a barrel mounted design and be able to use batteries and must be able to upload to Excel.

I found the basic design in 2 other existing projects both of which used NANOs. I picked the bits I liked in both then added my own bits I wanted.

Yeah it worked OK and the main advantage of the resonator is it is very shockproof compared to a quartz crystal. The results seemed a bit high and hit and miss (turned out I had 2 major problem with my rifle).

Then I started reading about the NateChrony and although for commercial reasons he can't give exact details about how they are made so accurate I picked up little bits and pieces from his comments.

Things I picked up - on chronos all errors are cumulative and little bits of errors here and there all add up.

Starting with my NANO's resonator. OK 5000 ppm is not terrible but it's still +- 5fps at 1000fps.

Choice 1 ditch the NANO and fit rubber mounts on the MCU to cut back on any shock from firing the rifle. I chose an Arduino Uno design on a custom PCB where I could fit a quartz crystal.

On testing I found yes it worked a lot better but I was getting some identical readings which at first I thought was good but on thinking about it I was probably only reading a resolution of a couple of fps hence the identical readings. I looked into it and yes when reading in microseconds the 16Mhz Arduio's missed a few readings due to overheads.

I switched to the ESP32 as it had a 240Mhz processor and a specifies 10ppm quartz crystal it also has WIFI so I or other people could build in WIFI connectivity in the future.


The other lesson I learnt from the NateChrony was sensor positioning and how important it was.
I downloaded the data sheet for the sensors and when you start reading the few fractions of millimetres errors for case size a few fractions for the semiconductor location within the case then a huge 2mm semiconductor with a non specified sense area I realised using a ruler alone won't work. That's why I use electro mechanical measuring with a flat ended probe.

As I use only 2 sensors the rise and fall times are between 25nS to 45nS so I could be at worst 45nS out. It they were both slow at 75nS that is 0. Once you start adding multiple sensors how do you know what sensor has tripped etc etc.

I keep my shot tube small and my sensor tube tiny and sense from the front of the pellet my thinking being no matter what pellet shape any delay by eg a pointed pellet will be the same on both sensors.

When I measure I wind the probe in a fraction of a millimetre at a time and do multiple repeat checks.


I don't know why you think my circuit is complex it has 2 sensors going to 2 pins and the processor is doing nothing but checking the 2 pins in firing mode. All the other menus happen outside of the firing loop. The chrono takes a fraction of a second to power up. I have added power up delays so I have time to read menus or set it into calibration modes.


GPS calibration. My cheap imported ESP32s crystals specify 10ppm but how do I know what they are running at? As I use microprocessors I spent £3.99 on a GPS module and connected 2 wires and I was away.
At least I now know these things are well within spec. If you sent a chrono off for calibration yes they could tell you withing a few seconds what you crystal is reading but do they leave it on for a whole day to get an average over different temperature. Maybe they do maybe they don't.

How do they check sensor alignment and spacing? I presume they have some way of checking it I suppose you could get a microscope out and measure from the semieconductor?

I have tried to make the most of the things I can easily get hold of, yes it takes me a whole day to check my crystal is working but it seems very accurate and very repeatable.

If all else fails I have added in a correction factor menu so if you have access to a Nate or Skan you can at least shoot through both, compare and correct.

I do appreciate and admire your design - I spent most of my life working with relays then transistors and logic ICs (I was an electromechanical exchange engineer ). I have taken the hugely uncomfortable step at my time of life moving over to microcontrollers and programming so I can I think appreciate both.

In the end doing this project I have learnt how to strip down my rifle, learnt about ballistics and fired 100s of rounds and had loads of fun..… got to be better than retiring and watching TV all day?

 

[colinb]

When I start a project I start by making a list of things in my head I want it to do.

I wanted a digital LCD display and a barrel mounted design and be able to use batteries and must be able to upload to Excel.

I found the basic design in 2 other existing projects both of which used NANOs. I picked the bits I liked in both then added my own bits I wanted.

Yeah it worked OK and the main advantage of the resonator is it is very shockproof compared to a quartz crystal. The results seemed a bit high and hit and miss (turned out I had 2 major problem with my rifle).

Then I started reading about the NateChrony and although for commercial reasons he can't give exact details about how they are made so accurate I picked up little bits and pieces from his comments.

Things I picked up - on chronos all errors are cumulative and little bits of errors here and there all add up.

Starting with my NANO's resonator. OK 5000 ppm is not terrible but it's still +- 5fps at 1000fps.

Choice 1 ditch the NANO and fit rubber mounts on the MCU to cut back on any shock from firing the rifle. I chose an Arduino Uno design on a custom PCB where I could fit a quartz crystal.

On testing I found yes it worked a lot better but I was getting some identical readings which at first I thought was good but on thinking about it I was probably only reading a resolution of a couple of fps hence the identical readings. I looked into it and yes when reading in microseconds the 16Mhz Arduio's missed a few readings due to overheads.

I switched to the ESP32 as it had a 240Mhz processor and a specifies 10ppm quartz crystal it also has WIFI so I or other people could build in WIFI connectivity in the future.


The other lesson I learnt from the NateChrony was sensor positioning and how important it was.
I downloaded the data sheet for the sensors and when you start reading the few fractions of millimetres errors for case size a few fractions for the semiconductor location within the case then a huge 2mm semiconductor with a non specified sense area I realised using a ruler alone won't work. That's why I use electro mechanical measuring with a flat ended probe.

As I use only 2 sensors the rise and fall times are between 25nS to 45nS so I could be at worst 45nS out. It they were both slow at 75nS that is 0. Once you start adding multiple sensors how do you know what sensor has tripped etc etc.

I keep my shot tube small and my sensor tube tiny and sense from the front of the pellet my thinking being no matter what pellet shape any delay by eg a pointed pellet will be the same on both sensors.

When I measure I wind the probe in a fraction of a millimetre at a time and do multiple repeat checks.


I don't know why you think my circuit is complex it has 2 sensors going to 2 pins and the processor is doing nothing but checking the 2 pins in firing mode. All the other menus happen outside of the firing loop. The chrono takes a fraction of a second to power up. I have added power up delays so I have time to read menus or set it into calibration modes.


GPS calibration. My cheap imported ESP32s crystals specify 10ppm but how do I know what they are running at? As I use microprocessors I spent £3.99 on a GPS module and connected 2 wires and I was away.
At least I now know these things are well within spec. If you sent a chrono off for calibration yes they could tell you withing a few seconds what you crystal is reading but do they leave it on for a whole day to get an average over different temperature. Maybe they do maybe they don't.

How do they check sensor alignment and spacing? I presume they have some way of checking it I suppose you could get a microscope out and measure from the semieconductor?

I have tried to make the most of the things I can easily get hold of, yes it takes me a whole day to check my crystal is working but it seems very accurate and very repeatable.

If all else fails I have added in a correction factor menu so if you have access to a Nate or Skan you can at least shoot through both, compare and correct.

I do appreciate and admire your design - I spent most of my life working with relays then transistors and logic ICs (I was an electromechanical exchange engineer ). I have taken the hugely uncomfortable step at my time of life moving over to microcontrollers and programming so I can I think appreciate both.

In the end doing this project I have learnt how to strip down my rifle, learnt about ballistics and fired 100s of rounds and had loads of fun..… got to be better than retiring and watching TV all day?

We actually have different objectives. All I want is to spend nothing, be legal and check rifles for reasonable consistency. I have no requirement to be buried in data when I don't have a degree in statistics- they are a black art to me. Each to his own!
If you really would like to check your crystals, there are several methods, but it all comes down to comparisons. Your counter has to be much more accurate than the crystals! Or you display ref and check unit at the same time on a 'scope and look at the drift rate. In order of accessible levels:
1. An off air standard. BBC radio 4 is exactly 198kHz. There are others of course. One pulse per second is difficult to use as your crystals have to be divided down. Temperature at the ppm level needs to be reasonably steady- draughts are your real enemy.
2. Borrow a checked counter. Mine was checked out at only 0.3ppm error. It's 50 years old, and I found that figure hard to believe!. But that counter is available if you want to use it.
3. Just get a simple crystal oscillator checked at a given temperature and then use that as a reference. I did that then added my calibrator circuit to it. I have several self contained 14 pin (corners only) crystal modules at around 4MHz that run from 5V, and am happy to check one for you and send it as a gift. PM me if you want to take up either option 2 or 3.
But in reality virtually any fundamental mode crystal will not cause us any significant error unless the user gets too clever. I saw this happen just once when a not really required filter was added. It was so good that when the crystal circuit failed, the filter struggled on manfully and acted like a poor resonator, so the fault was not spotted until rather too late!
4. I admire your efforts at getting sensor distance correct. I very much liked the linear probe idea and I'm going to try that with a long edge glued onto the probe so as to get all 4 of my sensors turned off at once. (My device actually works with 1/2 a sensor turned off, and I can disable my AGC that rejects ambient light changes).
Waisted and pointed 0.177 pellets can easily cause a double pulse; you have to ensure the circuit handles that correctly, or all efforts result in a nonsense. A processor is actually a great help here. Not essential, but rather easier than the discrete solution. It drove @Nate nuts to get that right! I think the actual opto components are better than you think. Really, only the sensor is the most important. The rise times you quote are extremely good, but I suspect that is after you have "squared up" the raw signals. The pellet edge has to pass through and gradually stop or open a finite width of beam, and that time alone has to be rather bigger than your figures. The whole thing is complicated by lenses often built into the sensor to...get more light in.... the last thing we actually want it to do.

 

[blklab]

Fascinating guys I love this type of projects

 

[Nias]

We actually have different objectives. All I want is to spend nothing, be legal and check rifles for reasonable consistency. I have no requirement to be buried in data when I don't have a degree in statistics- they are a black art to me. Each to his own!
If you really would like to check your crystals, there are several methods, but it all comes down to comparisons. Your counter has to be much more accurate than the crystals! Or you display ref and check unit at the same time on a 'scope and look at the drift rate. In order of accessible levels:
1. An off air standard. BBC radio 4 is exactly 198kHz. There are others of course. One pulse per second is difficult to use as your crystals have to be divided down. Temperature at the ppm level needs to be reasonably steady- draughts are your real enemy.
2. Borrow a checked counter. Mine was checked out at only 0.3ppm error. It's 50 years old, and I found that figure hard to believe!. But that counter is available if you want to use it.
3. Just get a simple crystal oscillator checked at a given temperature and then use that as a reference. I did that then added my calibrator circuit to it. I have several self contained 14 pin (corners only) crystal modules at around 4MHz that run from 5V, and am happy to check one for you and send it as a gift. PM me if you want to take up either option 2 or 3.
But in reality virtually any fundamental mode crystal will not cause us any significant error unless the user gets too clever. I saw this happen just once when a not really required filter was added. It was so good that when the crystal circuit failed, the filter struggled on manfully and acted like a poor resonator, so the fault was not spotted until rather too late!
4. I admire your efforts at getting sensor distance correct. I very much liked the linear probe idea and I'm going to try that with a long edge glued onto the probe so as to get all 4 of my sensors turned off at once. (My device actually works with 1/2 a sensor turned off, and I can disable my AGC that rejects ambient light changes).
Waisted and pointed 0.177 pellets can easily cause a double pulse; you have to ensure the circuit handles that correctly, or all efforts result in a nonsense. A processor is actually a great help here. Not essential, but rather easier than the discrete solution. It drove @Nate nuts to get that right! I think the actual opto components are better than you think. Really, only the sensor is the most important. The rise times you quote are extremely good, but I suspect that is after you have "squared up" the raw signals. The pellet edge has to pass through and gradually stop or open a finite width of beam, and that time alone has to be rather bigger than your figures. The whole thing is complicated by lenses often built into the sensor to...get more light in.... the last thing we actually want it to do.

I understand your spend nothing approach I love your bottle cap legs and waste pipe shot tube. My DIY silencer uses waste pipe. Why spend all that time 3D printing a tube when you have a 2m length of ABS tube sitting in the garage.

I used to have access to endless bits of high end test equipment but I like the challenge of finding other solutions. My main interest is clocks as you can see from my web site and the best way of checking a clock is over a long time period and comparing it to a known time source. GPS has a few nS of jitter over short periods but long term it's amazing. All I am doing is comparing my Chrono clock over a long time with GPS atomic time and measuring the drift.

It was not until I measured my quartz crystals I realised how accurate they were. I had one that kept showing zero PPM so I had to display fractions of PPM to check it. I also added Min Max errors and I could see it would run a tiny bit fast then a tiny bit slow over the course of a day. As you rightly say I can ignore it but I only had confidence once I knew.

My sensors rise/fall time have a range and looking into it I could have spent a few more pennies and got one with 20nS rise and fall. I think the manufacturers test batches and any very good ones they put to one side and add a premium on top.

My chrono senses dark and sits in a matt black tube and once the 1st sensor triggers it is locked out until the 2nd triggers. I run off the assumption the pointy end of a pellet will trigger both sensors at the same point even if it is a fraction of a millimeter in. I notice lots of commercial chronos have shiny metal tubes and must have infrared light bouncing all over the place.

 

[Nias]

Fascinating guys I love this type of projects

I must admit I love the little HT-X3006 chronos from China, 2 for less than a tin of pellets! if you get a good one they work quite well.
I have 2 of them and hope to run some experiments to see if I can easily tweak them to get them to perform better.

My project if for those people who like to tinker and it's a good starting point for a much improved version especially the code.

I have also tried to make it so people can easiy change the 3D design as it is very much made for my needs.

I never use the battery option now - just plug it into my PC or plug in a USB battery pack.

 

[colinb]

I understand your spend nothing approach I love your bottle cap legs and waste pipe shot tube. My DIY silencer uses waste pipe. Why spend all that time 3D printing a tube when you have a 2m length of ABS tube sitting in the garage.

I used to have access to endless bits of high end test equipment but I like the challenge of finding other solutions. My main interest is clocks as you can see from my web site and the best way of checking a clock is over a long time period and comparing it to a known time source. GPS has a few nS of jitter over short periods but long term it's amazing. All I am doing is comparing my Chrono clock over a long time with GPS atomic time and measuring the drift.

It was not until I measured my quartz crystals I realised how accurate they were. I had one that kept showing zero PPM so I had to display fractions of PPM to check it. I also added Min Max errors and I could see it would run a tiny bit fast then a tiny bit slow over the course of a day. As you rightly say I can ignore it but I only had confidence once I knew.

My sensors rise/fall time have a range and looking into it I could have spent a few more pennies and got one with 20nS rise and fall. I think the manufacturers test batches and any very good ones they put to one side and add a premium on top.

My chrono senses dark and sits in a matt black tube and once the 1st sensor triggers it is locked out until the 2nd triggers. I run off the assumption the pointy end of a pellet will trigger both sensors at the same point even if it is a fraction of a millimeter in. I notice lots of commercial chronos have shiny metal tubes and must have infrared light bouncing all over the place.

Yes, the pellet sensor is your weakness here. Using the pointy end is reasonably definite to detect. but carries the highest timing uncertainty. From your remark re risetimes, I guess you are using a module rather than making your own opto sensor amp. Unless you have insane amounts of power, a typical rise time here is extremely unlikely to be much better than 1 usec or so.
It's a case of turning relative rubbish in into something decent. And you have to dive into the guts of it.
@Nate and I have come to the firm view that the best method that gives fastest sensor edges uses the back edge of the skirt. Because that is always sharp and all pellets are similar in that respect. The rear edge is also better because it turns the sensor ON. Turning a light sensor off is usually a relatively slow business. Here are the sums:-
Assume a 0.177 pellet travelling at 1000fps = 300,000 mm/second.
Assume that the sensor, with lense, is seeing a light beam just 1mm wide. (optimistic!)
A perfect edge will take 3.3 usec to enter, then completely cut off that light beam. And the input signal may approximate to a linear ramp somewhere in the middle, depending on the gains. It's actually better to hold that circuit in a linear, area, because anything saturated (Stuck at one end) is intrinsically slow and it's difficult to match 2 to be the same. (Sensors also vary hugely and have to be selected or have a circuit that copes with variations- it ain't easy!)
Using a pellet with a shaped head, say a pointy job 2mm long, will increase the total ramp to more like 10usec. That's why the back edge is better. Your ramp time significantly decreases. Slower pellets of course makes this ramp time increase. You can improve things a bit by having the sensor in a recessed tube and add a narrow slit for the light.
You can see why I find your 20-45 nsec rise time somewhat dubious. It must have been pre-processed, as it is 50 times less than an optimistic ramp time.
At some point in that ramp, the circuit has to make a go-no go decision, using a comparator with hysterisis to give a good edge into the logic circuit. Even this needs care, as the comparator usually can give one (falling) fast edge, and another relatively slow.
It's possible (I tried) to use some logic families( biased linearly) as the comparator to save bits, but the rise times suffer somewhat.
So you have to have 2 matched circuits. It's actually possibly better to have just one circuit that detects both sets of sensors- you still get 2 sets of detection data at the expense, maybe, of a reduction in sensitivity.
Using the back edge then adds the complexity that a waisted skirt can give a false trigger that has to be ignored...

 

[Nias]

I don't use a module it's a single component IR detector with a matched pair IR transmitter. The sensor IR diode has a schmit trigger and output transistor built in. The rise time is the same as the fall time and includes the schmit trigger etc.

Even if the rise or fall time was 1 second as long as they are the same it will still work.

Using 1 sensor at each end and very long narrow IR sense tube cuts down on spurious parts of the pellet being detected in error. The cost is the pellet must be traveling down the exact middle of the shot tube or the pellet will be missed. My method can only be used on barrel mounted chronos.

If you are using multiple sensors then they must be spread over a wide area. How to you detect which sensor has triggered so you can apply error correction? For example you have 6 sensors each end of the shot tube. The top sensor triggers going in and the top sensor triggers going out that's fine. But what if the top sensor triggers going in and the botom sensor triggers going out. The pellet is travelling further so the time will be slower. Also as the pellet is traveling at at angle the top of the skirt will trigger on one and the bottom on the other. Do you apply an error correction factor? I guess this is where the warnings come from the Nate?

Very interesting all this.

 

[Nias]

I don't use a module it's a single component IR detector with a matched pair IR transmitter. The sensor IR diode has a schmit trigger and output transistor built in. The rise time is the same as the fall time and includes the schmit trigger etc.

Even if the rise or fall time was 1 second as long as they are the same it will still work.

Using 1 sensor at each end and very long narrow IR sense tube cuts down on spurious parts of the pellet being detected in error. The cost is the pellet must be traveling down the exact middle of the shot tube or the pellet will be missed. My method can only be used on barrel mounted chronos.

If you are using multiple sensors then they must be spread over a wide area. How to you detect which sensor has triggered so you can apply error correction? For example you have 6 sensors each end of the shot tube. The top sensor triggers going in and the top sensor triggers going out that's fine. But what if the top sensor triggers going in and the botom sensor triggers going out. The pellet is travelling further so the time will be slower. Also as the pellet is traveling at at angle the top of the skirt will trigger on one and the bottom on the other. Do you apply an error correction factor? I guess this is where the warnings come from the Nate?

Very interesting all this.

 

[colinb]

I don't use a module it's a single component IR detector with a matched pair IR transmitter. The sensor IR diode has a schmit trigger and output transistor built in. The rise time is the same as the fall time and includes the schmit trigger etc.

Even if the rise or fall time was 1 second as long as they are the same it will still work.

Using 1 sensor at each end and very long narrow IR sense tube cuts down on spurious parts of the pellet being detected in error. The cost is the pellet must be traveling down the exact middle of the shot tube or the pellet will be missed. My method can only be used on barrel mounted chronos.

If you are using multiple sensors then they must be spread over a wide area. How to you detect which sensor has triggered so you can apply error correction? For example you have 6 sensors each end of the shot tube. The top sensor triggers going in and the top sensor triggers going out that's fine. But what if the top sensor triggers going in and the botom sensor triggers going out. The pellet is travelling further so the time will be slower. Also as the pellet is traveling at at angle the top of the skirt will trigger on one and the bottom on the other. Do you apply an error correction factor? I guess this is where the warnings come from the Nate?

Very interesting all this.

My sensors are in lines across the path of the pellet. So 4 emitters in the top, and 4 sensors in the bottom, to give a detection width of about 15mm. Repeat for second set, in my case at 4.8 inches. That was a convenient distance to allow no processor, a 2 byte memory, a carefully chosen clock speed and an analogue 2 simple components divider - necessary as Speed = Distance/Time, and we are all more familiar with Speed. One of the divider components can be tweaked to compensate if my 4.8 inch gap was proven to have a significant error. A great advantage is that there is no boot up time - and it never crashes...

If you calculate it, sensing the wrong part of the pellet by 2.5mm on just one sensor pair will introduce a speed error of around 2% minimum for my sort of gap.- 4% on ft lbs, so 0.5ft lb uncertainty. That would seriously worry those who are close to the limit; its an error that is unacceptable. A big sensor spacing reduces the error, but slowing of the pellet along a big spacing is the next factor to worry about. Put 2 identical chronos in line then swap them round and you'll see what I mean. The law wants the speed at the muzzle, tho that is almost impossible to establish unless you put the sensors in the moderator.
For legality reasons, if one has to be wrong or uncertain, it's best but still poor to be reading fast rather than slow. One reason I don't like radar chronos as you don't know where the pellet was when the measurement occurs.

If you are using either a computer or a spreadsheet, Time is actually all you need, but that for me that would need a counter with display and a decimal based 10 MHz clock. So the divider and a cheap DVM was perfectly OK and I used the DVM for fault finding when I built it. All expense spared.... If I'd known about cheap Chines chronos, I admit I would not have even started.
Angles would give slight errors, as you say , with the speed reading a tad low. My device can be barrel mounted or free standing. A single sensor almost has to be barrel mounted.

I hear what you say about your sensors. The internal electronics has actually improved and balanced up the rise times in the final output stage, but be assured that deep in its bowels, relatively slow and different waveforms are being seen. You can only hope they are identical, (and they possibly are) but you could prove and quantify that by swapping the sensor modules around in your rig and seeing if the answers shifted a bit.

Nate found (to his despair and disgust) that sensors and photodiodes in the same batch varied by such a large amount that he could not easily guarantee to just take a random pair and use them in a production environment. One more reason why a bought chrono can be more expensive than we'd really like. There are an awful lot of factors just ready and waiting to bite.

 

[Nias]

Interesting about sensors in the moderator. I always presumed the police take off any moderator to measure at the muzzle. I am waiting a freedom of information request fom the Met to find out exactly how it's done.

"Nate found (to his despair and disgust) that sensors and photodiodes in the same batch varied by such a large amount that he could not easily guarantee to just take a random pair and use them in a production environment."

Now you have me thinking................... Mk6 use my proccessor to calculate the difference in fall time of my sensors.

I think that will have to wait. Forum members have given me some ideas on improving my DIY silencer- I have some testing to do.

Thanks for taking the time explaining the details of your chrono and the use of multiples sensors.