Chronograph for bullets

[wannabegeek]

It's hard to conceptualize a mechanical (infinitely reusable) target. This issue always brings me back to a photoelectric solution.

Reusable for 5 shots seems reasonable to me.

The basic sandwhich could work with the right materials chosen.

Also recall my idea about using a sandwhich as a capacitor with external reactive components to form a filter with a transfer function that drops off roughly linear on an interval. Then feeding it a res freq and rectifying and integrating the output, in short getting the RMS voltage.

When a hole is made, the RMS output would change enough to register a hit.

This maybe do-able...

As for LED's I still think they are much harder to implement than it appears when you take into account all the physical conditions.
Plus range masters are conservative about safety, they aren't going to allow me to put a LED frame around a target...I'd bet lunch on it.
It will take some person to person meetings even to allow me to put my electronics box on the ground next to the target.


dpc

 

[wannabegeek]

10 MHz crytal oscillator

I was able to use a 300 MHz digital scope at work and took some measurements of my clock.

I did this using a Proto-board just to get things working and get some preliminary analysis.

The crystal I ordered is supposed to be a series low impedance type, but I got better results using it in a parallel configuration, with a Pierce network.

I used the 74HC quad Inverter chip, with 1k Ohm resistor for feedback, a 480 Ohm resistor for the damping and current limiting, and two 100pF caps (Capacitor Ceramic Multilayer 5% tol. ) for the phase shift.

\Delta(f) ~= [10 +- 0.001] Mhz , when averaged over 512 samples.

The voltage output was 3.7 V peak to peak with a 1 V offset.

The offset bothers me, but the rest is good enough for me unless you all say otherwise.
I still need to buy some DIP sockets then I a gonna put the circuit on PCB and retest on scope.

Cheers,
wbg

 

[wannabegeek]

I put my clock on a perf board to day....I haven't done that too many times and it's bear...

I ordered some rosin to help solder larger parts like wires and I was sure glad I did, it made connecting multiple wires to the DIP socket much easier. And removing solder when I needed to make a change.

The whole thing works, however I think there's some issues from the rest of the counter being on a proto board. I'm gonna put in some 22pF caps to ground in key places on the proto board later.

It's gonna be a big task to put the whole thing on perf board....

I had an issue with my clock running at a 1V offset, so I put a high pass RC network on the output to ac couple it. This is hack is suppose.

I also had to ground the can of the crystal. The tip off was watching how the circuit only worked when I touched the sides of the perf board.

 

[CDRIVE]

Use caution when using solder flux. I've used flux 'past' for many years with no issue. It looks, smells and feels just like the flux in the solder. I didn't have the same happy relationship with a clear liquid solder flux, Nasty stuff that was highly conductive.

 

[Rleo6965]

Place a 100mfd 16V in parallel connection in your oscillator pcb to filter dc voltage +VCC supply and ground to filter voltage fluctuation and 0.1 mfd to filter high noise signal on +VCC. Don't tap +VCC supply in proto board. Connect pcb +VCC directly to Supply connector to avoid voltage drop.

 

[wannabegeek]

Good looking out CDRIVE...this stuff I orded is a gel and it smells just like burning rosin.
I also blew excess flux off with a can of compressed air...which also worked as a great de-solder in non sensitive places

@ Rleo
I did end up putting a 1000uF electrolytic cap in parallel with my clock VCC and a 68pF cap in parallel with ground, after I posted to the thread.
I was kinda proud of myself for remembering how to get rid of the coupling leakage.

But I didn't realize that I should connection the VCC straight to my bench supply, will do that tonight.

Tomorrow I buy rubber cement to make new foil detectors. I think that the light weight rubber cement will help keep the foil from tearing so bad when shot.
Also, I will try placing two sheets of plastic wrap as an insulator to reduce unwanted short circuits.

 

[wannabegeek]

I'm stuck with my clock ....

I thought I had the capacitive coupling figured out and dealt with but that's not the case....

I soldered a 100uF cap onto the perf board of the clock in parallel with the VCC and ground. When that smoothing cap was on the proto board, it seemed to work better. But really, I think that I had to touch the perf board on the corners, until some cap leaked off then it ran OK.

Now it doesn't work unless I am touching the can of the crystal or the output wire.

I don't know what to do....I didn't leave a lot of room on the small perf board to jump pads together to from a ground plane. I need advice. I also need to upload a current picture of the clock circuit which I'll do when I get home today.

thanks,
wbg

 

[CDRIVE]

Please post the schematic. Needing to do what you're describing usually indicates that you need capacitance somewhere or need more feedback. One can be related to the other.

EDIT: I'm not referring to Vcc filter caps. I'm referring to pF values in the Osc.

 

[wannabegeek]

Please post the schematic. Needing to do what you're describing usually indicates that you need capacitance somewhere or need more feedback. One can be related to the other.

EDIT: I'm not referring to Vcc filter caps. I'm referring to pF values in the Osc.

OK....I realized last night that when I had the clock circuit on the scope at work, besides being on proto board, I also didn't account for the cap of the probe etc...

I likely need to take the perf board version to the scope and try to get the full 5 V swing out of it. Recall I was getting 3.8 V out of 5V.

thanks, I'll get that schematic up tonight,
wbg

 

[CDRIVE]

Always use a 10:1 low capacitance probe whenever you can.

 

[wannabegeek]

OK I just put together a schematic using Qucs...the Linux alternative to spice....

Qucs doesn't have a crystal component so used an ac source instead...

The caps are dipped ceramic type I forgot the tolerance but I think it is fair, not those crappy %20 ones...more like %5.

I didn't put in the DC smoothing caps to the +VCC of the inverters or the ground to the crystal can.

I decided to put the output of the first inverter into the next based on experimentation with the proto board version. I reasoned to myself that the input impedance of the 74HC inverter is really high and therefore it couldn't hurt. Maybe that's a bad thing...

I forgot to bring the circuit into work today, but can do so over the weekend. I look forward to your help !

I'll update with a picture of the actual circuit when I get home to be clear. It might also be that my layout of the parts on the perf board is 'bad' and encourages capcitive coupling.
wbg


EDIT: R on schematic = 10kOhm not 1kOhm

EDIT2: I remade the schematic further down...

 

[wannabegeek]

Here's the picture of the clock circuit.

The output goes to an RC high pass filter on it's way into the 74HC AND gate.

When I had the clock on the scope, I had this 1 V dc offset. I didn't know how to get rid of it, so I stuck the high pass network on the gate input.

High pass:

C = 22pF, R = 1 kOhm

f_3db = 7.2 MHz

PS: It's hard to see the 10k resistor right above the +VCC smoothing cap. That cap is 100uF.

 

[(*steve*)]

I was playing around yesterday and managed to connect the output of a schmitt trigger to its input on a breadboard.

It was a 40106 and the voltage input was low (possibly only a couple of volts). I observed it oscillating at 4MHz. The frequency was almost certainly due to a combination of the capacitance of the (solderless) breadboard, and the output resistance of the gate.

I mention this because there was some mention yesterday (perhaps earlier) in this thread (I think) about the maximum frequency on a solderless breadboard.

The proto board has a high capacitance by construction and enables the tendency to longer jumpers and leads which makes circuits vulnerable to capacitive coupling.

Incidentally I also have the output of my 10MHz Rubidium standard connected through a solderless breadboard at the moment and I see no difference to it's output when it is...

Note for young players... output impedance plays a major part in this too..

And a crystal oscillator involves devices with a fairly high impedance, so the capacitance will have a larger effect.

 

[wannabegeek]

I didn't get to scope my clock yet but did play with it more....

I did some testing ( w/o scope and a variable cap which was kinda dumb ) and decided that I
needed to increase the capacitance of the input side from 100pF to 122pF.

This did't fix my problem which sucks....BUT, now when I touch Rfb ( feed back resistor )
I get a solid output. In the past touch various things made it work but this time the touching of Rfb makes it snap into oscillation.

I suspect that by touching Rfb I am adding a capacitor in series and thereby reducing the total C of the input side enough to make the circuit vibrate.

@Steve ....I'm still rereading and thinking about what you wrote...

wbg

 

[CDRIVE]

Earlier I mentioned using a low capacitance probe. Are you? Also,.. You said that touching the xtal can made the osc kick in so you grounded the xtal can. Crystal cans shouldn't need grounding. When you touched the can your entire body became part of the field. And your body isn't grounded. My point is that this could be more of an indication of needed feedback or it could be an indication that the circuit broke into (unstable) self oscillation, unrelated to the crystal frequency.

Regarding your spice...I don't see how replacing a crystal with a voltage source simulates a crystal oscillator.

Chris

 

[Rleo6965]

Have you tried connect another 74HC gate inverter in parallel to Y2 to increase output voltage? You still have 2 extra gate.

 

[wannabegeek]

Earlier I mentioned using a low capacitance probe. Are you? Also,.. You said that touching the xtal can made the osc kick in so you grounded the xtal can. Crystal cans shouldn't need grounding. When you touched the can your entire body became part of the field. And your body isn't grounded. My point is that this could be more of an indication of needed feedback or it could be an indication that the circuit broke into (unstable) self oscillation, unrelated to the crystal frequency.

Regarding your spice...I don't see how replacing a crystal with a voltage source simulates a crystal oscillator.

Chris

I don't know my probe's cap off hand I'll try to dig the instructions.

I'll take the clock to work and look at it on scope instead of guessing...I'd hate to get a harmonic and trick myself into thinking I had 10MHz and been using 20MHz....

I didn't mean to imply that I'm simulating a crystal with an oscillator, just using it as a stand-in symbol since I don't have a xtal symbol. i suppose I should just make one with paint...that would have been smarter.


Is there a way to delete attachments in a previous post...? Id didn't see anobvious way to do that. I need to clean up my schematics....

cheers,
wbg

 

[wannabegeek]

Have you tried connect another 74HC gate inverter in parallel to Y2 to increase output voltage? You still have 2 extra gate.

I didn't think to connect them in parallel. I did run the output of the first inverter into the other in serial...

I'll play with that configuration when I get the clock on the scope. Of course it's tricky b/c I'll have to make changes like that on the fly w/o soldering.

There's no bench anymore at work, that was left out of the design for the lab when it moved....kinda dumb really since it's an fMRI lab. New guy coming is a physicist and uC programmer so he'll have some kind of setup I can't wait to meet him.

EDIT: I think I may need some trimming caps like these.http://www.jameco.com/webapp/wcs/st...0001&freeText=trim capa&search_type=jamecoall

 

[wannabegeek]

At work now and the scope says f = 153 MHz .....

The proto board version was 10MHz and the perf board version is a high harmonic, I fail to understand how the capcitance of P board made this large difference.

My understanding is that the two caps are just there to get a Pi radian phase shift since the inverter is also giving a shift of Pi rad, which sums to something close to 2*Pi and in-phase feed back.

I don't know what to do at this point...if I just need to match the feed back phase better than I can use a trimming cap...if it's more than that it's gonna be tough w/o a scope and tedious.

wbg

 

[(*steve*)]

153 MHz almost certainly means that the oscillator is not being controlled by the crystal. If it was, you'd be seeing 10MHz, 20MHz, 40 MHz, 80MHz, etc.

I'm surprised, but a little more capacitance between the gate's input and ground may help.

However I'm not going to profess any super powers when it comes to crystal oscillators.