Chronograph for bullets

[wannabegeek]

So I looked up the single chip timer/coutners/decoder and some are still made. However, I haven't yet seen one that is sold in small quantities.

ICM7225IPL is an updated version I think and the seller had a 677 min order...

As for microcontrollers, I've never even read about them until now. That's a big field to jump into. I get that they are small computers and a timer/counter is a very basic function for them.

A mc could be a great later stage project, b/c I could write analysis programs in python for the data and have a wifi capability.

But for now, I really want to learn the traditional way.

Does anyone know of a timer/counter/decoder chip still in production that is around $10 range that is sold in small quantities ?

cheers,
wbg

 

[Harald Kapp]

I've found this:
http://www.8085projects.info/post/C...-digital-pulse-width-measurement-circuit.aspx
http://www.8085projects.info/post/Digital-pulse-width-measurement-circuit.aspx
and
http://microcontroller51.blogspot.de/2011/02/circuit-diagram-of-frequency-counter.html

Google search terms: pulse width measurement ic

Harald

 

[wannabegeek]

I finally decided that I want to do this project the old way, using 7490's or rather their modern counter part 74SL90.

I found a cool article on clock circuits designs and a discussion about stability.

My main question now, is how to trigger the timer-counter circuit ?
I thought the counter themselves had a trigger input but it looks like the clock is what is actually triggered ....?

If that's true, I was thinking of using my detector's comparator output to drive a FET switch to kill base current in my 2N3904 driven 20MHz crystal clock. But I suppose I won't be able to use any ripple caps in the configuration I was looking at using. I'd like my clock to be thermally stable since I envision it being used in hot states outside for several hours.

I'll get a schematic of the clock I'm thinking of posted later today.

cheers,
wbg

 

[(*steve*)]

Don't turn the oscillator on and off. Crystal oscillators take time (often a long time) to actually start to oscillate at the rated frequency.

The best approach is to gate the oscillator's output so the oscillator keeps running, but you effectively stop it's output from reaching the counter except during the intervals you're interested in.

 

[wannabegeek]

Don't turn the oscillator on and off. Crystal oscillators take time (often a long time) to actually start to oscillate at the rated frequency.

The best approach is to gate the oscillator's output so the oscillator keeps running, but you effectively stop it's output from reaching the counter except during the intervals you're interested in.

Yeah that was my thought today while at work...thanks Steve...!

I decided to connect the output of the oscillator with a JFET switch, to the counter.
I could also short it to ground and release...

My circuit works by having a comparator throw up hi on detect. Hi stays on until the second detection, which is a comparator with a neg supply, and the two voltages add to drive the JFET down close to zero, turning off the switch again.

 

[(*steve*)]

If you have other logic then you may be able to use a NAND, NOR, AND, or OR gate, or even an inverter or buffer with tristate outputs. I would be looking for a spare gate first as it pretty much ensures that your logic levels and transitions won't be affected.

 

[wannabegeek]

If you have other logic then you may be able to use a NAND, NOR, AND, or OR gate, or even an inverter or buffer with tristate outputs. I would be looking for a spare gate first as it pretty much ensures that your logic levels and transitions won't be affected.

So perhaps I have the clock circuit output go to a NAND gate.

When detector 1 sends a pos Volt then the clock is connected.
When detector 2 sends another +V, then the clock is disconnected.
If no detector is sending, then there is no clock connection.

Does this sound right to you ?

I just realized today while driving home from work, that I don't even need the comparators I was writing about before.

So the only slew rate I need to worry about is this NAND gate. That's a great improvement.

cheers,
wbg


EDIT: I said I would add a schematic for the oscillator circuit I am thinking of using. It's in the PDF below, fig 1(e).
It's a cool looking circuit with two forms of feedback...a really good reason for me to pick up my books and study.
I think the diodes are for thermal stability...?

I'll stick my NAND gate at the inverting output.

I should also add, that I decided to make this clock a 20 MHz clock, not a 1 MHz clock as the thread title says.

http://www.linear.com/docs/4108

 

[(*steve*)]

What you're talking about above is a flip-flop.

Specifically an SR flip flop. You wire your sensors to the S and R inputs and use the Q (or Q-bar) output to drive one input of your NAND gate.

The SR flip flop will take up one state when a pulse comes from one input then remain that way until a pulse comes from the other input.

In your case, it sits there waiting from a pulse from sensor 1, which changes the state so that you counter starts counting pulses until a signal from sensor 2 changes the state back again and turns off the clock. The count that remains reflects the time between the rising edges of the signals.

 

[wannabegeek]

The SR flip flop will take up one state when a pulse comes from one input then remain that way until a pulse comes from the other input.
.

I understand what you mean. However, my detectors are not standard, in that they do not output pulses, but rather a constant voltage.

That's why I skip the flip-flop and go straight to NAND.

However, I feel like I still need to operate a switch to connect /disconnect the clock to the counter...?

How can I do that with just a NAND gate ?

Thanks so much for your input. I'm making a parts order soon and looking forward to all this. I do get my counters on Monday and will work with them while I wait at least.

cheers,
wbg

 

[(*steve*)]

If you pass your constant voltage through a capacitor, you get a nice short rising edge on a positive transition.

It is in fact ideal that your sensors do not then reset as this prevents more pulses arriving later and mucking up your result.

I guess you could also do it with logic. You need something that follows the following truth table:

A B Q
0 0 0
1 0 1
0 1 x (Edit -- this state doesn't matter if 1 it is XOR as described below)
1 1 0

And that function is called XOR (exclusive OR) You can build it from other gates where Q is effectively (A OR B) AND NOT (A AND B). The output of this then drives whatever logic you use to gate the clock.

 

[wannabegeek]

kickass...thanks

I've never really used gates and I'm into it...I am guesing that they have really quick
switching rates. Way faster than a JFET to open and close again.....?

wbg

 

[(*steve*)]

Your problem is not so much speed as symmetry. You want to ensure that it takes just as long to turn on the clock as to turn it off.

If you're using CMOS, the gates are symmetrical and that should ensure similar propagation delays from 0 to 1 as from 1 to 0. The same may not be true of another configuration. Any switching where you drive a signal low, but allow a resistor to pull it high (for example) will have one transition faster than the other.

There are always many ways to skin a cat. If you're confident in your design abilities you should be able to calculate (or at least estimate) the worst case behaviour of a circuit and use that to determine the likely precision of the result.

 

[wannabegeek]

hey again...

You know, I was or am a little confused...

I need to switch my clock output to the counter input.

I have two detectors that output 0 or constant 5 V.

XOR logic is great, it keeps everything neat and simple.

However, I still need an actual electrical switch between the clock and the counter...
I thought that there was an XOR gate that had a third contact that would do this for me.

Meaning, I have two XOR inputs and a pass-thru that is active IF, the gate is ON.

I don't see such a part...am I not looking for the right part ?

I tested using a 2N3904 which switches at 300MHz ( not really good enough ) and I cold use a transistor with a base or gate powered by the XOR output.

EDIT: 300 MHz is the small signal freq range not the switch speed from 0 V. I also realized, that TLL logic defines LOW = (0 - 0.8 )V and HI = (2 - 5)V. So I should bias the base of the transistor switch to ~1.4V (with diodes ?), if there' no better way.


EDIT: And I've revised my uncertainty criteria so that I can use a 10 MHZ crystal instead of 20 MHZ.
TIA,
wbg

 

[(*steve*)]

You can use an AND gate (or a NAND gate) to turn the clock signal on and off. you connect the clock to one input, and your "enable" signal to the other.

With an XOR gate, the enable signal will be active high (it will be high when you want the clock signals to go through)

When the enable signal is high, the clock signal will pass through (inverted for NAND). If the enable signal is low, then the output will be low (high for NAND).

If your enable signal is active low (i.e. it is low when you want the signals to go through) then you would use an OR or NOR gate.

 

[wannabegeek]

I would still use an XOR gate as I see it.

This is an elegant solution.

Since the detector throws a constant voltage, the AND gate will pulse with the clock.

When the second detector throws a constant voltage, the XOR goes LO, and the AND gate
stops relying the clock.

Super COOL...!

Now, to make sure the delay is OK on the gate.
The XOR I ordered has a time delay graph based on the input capacitance of the next device.

I couldn't find the input cap of the counter (74SL90) but my guess is that it is lower than
10 pF, giving me the nominal delay as 80 ns, which is plenty quick.

You are such a gigantic help Steve.....I'm learning a ton. I can't wait to get the parts in the mail.

wbg

 

[(*steve*)]

These days I'd be suggesting you use 74HC or 4000 series CMOS rather than LSTTL unless you have some specific requirement that CMOS won't suit.

 

[wannabegeek]

These days I'd be suggesting you use 74HC or 4000 series CMOS rather than LSTTL unless you have some specific requirement that CMOS won't suit.

Since this is new to me, I can't always be sure what led me down the 74LS road...ahh I remember now...I was afraid of the counter-decoder thing and wanted a 7490 and foudn the 'modern' 74SL...now that I've been reading and asking questions for a week, I'm not afraid anymore and want the 74HC..!

I ordered parts already, but this is just a dry run. I'll do it all again with 74HC once it all works.

 

[wannabegeek]

First time printed circuit: Thin copper trace on adhesive

Hi all,

I'm getting ready to prototype my project which has been worked out in a few places in this forum... : ) thanks again to the regular repliers for all their help.

An important part of my project is having an easy to use and easy to apply, breakable copper wire detector. The wire will be a trace on a thin adhesive sticker, that can be applied to surfaces that will be impacted by a projectile. Breaking the wire or trace, will count as a detection. For now, I just want to count one break in an area roughly 6 cm^2 ( size is arbitrary for now) .

I have my reasons for this design and not interested in changing the method of detection. People seem to find this idea a little repulsive and un-elegant. But without getting into a bunch of details, it is truly my choice of detection method.

I suspect this can be done in a factory cheaply, since I see RF merchandise tags that have copper spirals on them. Now all I want is the spiral so to speak. But really, I want the two traces together, to snake out a pattern that covers a lot of surface area.

I have never used a CAD program at all but this is likely not the hardest first project to get into it with. I'm not sure where to start.

I run Linux at home and might be able to use my work computer that is XP. I do have a media server running Windows 7 that could be used for this. There are also those free web based software that are on the factory web pages.

I'm open to advice and suggestions from the community.
TIA,
wbg

PS Perhaps there is already a product like this on the market...? If you have an idea of where to look, that's also welcome.

 

[wannabegeek]

found this ....

http://http://hlt.media.mit.edu/?p=1376

I searched like crazy a week ago...

 

[CocaCola]

If you want to keep cost down you use over the counter 'conductive security foil', if you want to get fancy you find a supplier of it by the sheet and have a stamping die made or possibly have it cut on a plotter into the shape you need...

http://alarmsuperstore.com/store/in...ducts_id=311&zenid=qqk3ea35tah6pbf2k7lij26q34

If you want to prototype in low numbers you can use adhesive to bond copper 'leafing' foil to the substrate and then etch the foil into the final design like you would a PC board...

*** Edit you can also cut a stencil from sheeted vinyl paint release/stencil, this can be cut on a vinyl printer an the applied to the surface, it's a very low tack vinyl that will life right off with little effort like masking tape... Apply this to the surface and simply airbrush, or even manually brush on a conductive paint, remove stencil sheet...