Fast (simple) beam break detector

On a sunny day (Thu, 2 Oct 2008 05:09:10 -0700 (PDT)) it happened snipped-for-privacy@gmail.com wrote in :

How about setting and resetting a flip-flop. Then, when the FF is high, charge a cap via a constant current source. Buffer and peak detect the voltage on the cap, amplifiy if needed, and drive an old fashing analog meter 'speed'. ?

Good idea about the flip flop. I am unsure about the accuracy when charging a capasitor that way. What kind of uncertaincy should I expect when I am unable to calibrate the setuup with a known high velocity.

On a sunny day (Thu, 2 Oct 2008 07:09:17 -0700 (PDT)) it happened snipped-for-privacy@gmail.com wrote in :

Q = C.U = i.t

For accuracy, calibrate with a signal generator and an oscilloscope.

Cheers, James Arthur

Great, except the meter would be proportional to transit time, or the reciprocal of speed. You could configure some analog multiplier in "divide" mode, or do it with counters or, (gasp) a uC of some sort.

Or, you could draw a backwards scale on the meter, for speed; meter 0 would correspond to infinite speed, of course. ;-)

Have Fun! Rich

On a sunny day (Thu, 02 Oct 2008 21:45:14 GMT) it happened Rich Grise wrote in :

Yes, good observation, so when he knows the distance, he can calculate and write the speed on the scale. If you reverse the meter and bias it, you can have it move to the right for more speed if you must ;-) but it won't be linear. Then perhaps indeed a log opamp circuit. As flip flops come often with 2 in a package, the second one can be used as 'signal detector', enabling the meter and lighting a LED perhaps. Then a reset button to prepare for the next shot.

Let's have a go: + + | | e PNP e PNP b--- b- Q3 c | c | | Q2 | |---- -------| ---| !trigger 1 ------------- set | |-- | 74HC74 c | | | !trigger 2--R7 -------- reset !Q --- R1------b NPN === | R2 | e Q1 | | | a FF1 | | | [ ][ ] meter bias | C2 0 / -- - | | P3 /// / ---------------- | R3 /// 0 | | | | | -----------------| /// | | | 4016 | gain --->[ ] P2 CMOS switch | | gain | /// | | !trigger 1 ----------- set Q - 74HC74 L1 + -- R5------ reset !Q --- R6-------- k LED a ---- + | FF2 shot detected light / reset button | S1 /// shot detector R1 4k7 R2 1k R3 10k R4 depends on meter sensitivity, 5 V for full scale... if 100 uA then 50k R5 10k R6 4k7 R7 4k4

P1 10k P2 10k ? P3 1k

C1 depends on distance (pulse-length and current) C2 1nF poly

D1,2 1N4148

Supply +5V (use CMOS opamps).

Something like this (photo detectors not drawn).

Drawing errors?

trigger1 low will set both FF. Pushing S1 the reset button will reset both FF. When FF1 is set, Q1 stops conducting, and C1 is charged by Q2 with a constant current. Opamp1 charges C2 to the peak voltage, and holds, opamp2 buffers and has some gain if needed. The 4016 switch discharges C2 upon pressing reset. LED L1 indicates a shot was detected.

All values are 'guess values'.

Thanks bt the sbject title was something about simple ;-) The project is not about building a speed detector, it isnot even about measuring the velocity. That is al just small stepping stones along the way. That is why I consider using a sound cards microphone input to make accurate meassurements over time.

The design I consider now is a R/C-flipflop two pullup resistors and two photo sensitive diodes and the sound card. That is the level of simplicity I look for. That way the flipflop goes high when first beam is broken (set) and it goes ow when the second beam (reset) is broken.

I connect set and reset each to a diode which leads to gnd. Set and reset are also each connected to the 5v from the sound card through a pulup resistor. That should do it.

Reset the FF by breaking the rear beam. The diode closes and the pullup raises reset and FF's Q goes low. Break the front beam, the diode closes and the pullup raises set and Q goes high until rear beam is broken again.

The F output goes to the signal in pin on the card and I should get a high signal while the projectile is between beams. With a distance of 1m and a velocity of 300m/s it will be a duration of 3.3ms or 147 samples at 44kHz

On 2008-10-02, snipped-for-privacy@gmail.com wrote: Newsgroups: sci.electronics.design From: Jasen Betts Subject: Re: Fast (simple) beam break detector References: Organization: Dis (not Dat) Organisation Followup-To: X-Face: ?)Aw4rXwN5u0~$nqKj`xPz>xHCwgi^q+^?Ri*+R(&uv2=E1Q0Zk(>h!~o2ID@6{uf8s;a+M[5[U[QT7xFN%^gR"=tuJw%TXXR'Fp~W;(T"1(739R%m0Yyyv*gkGoPA.$b,D.w:z+ I need to measure a 1cm diameter ball traveling at arround 300m/s.

hmm, smaller than a paintball, faster too.

light emitting diodes could be used instead of unless you need the extra range that the lasers get you.

phototransistors have a higher output amplitude than the photo diodes.

that should make a sizable dip on the wave trace. using a divider between the sensor and the soundcard so that the trace stays inside the acceptable limits may lead to the ability to more accurately determine the time of the event.

soundcards don't read DC, with the diode illuminated you'll get a zero signal, when it is obscured the signal will dip.

it's more than enough.

handled properly they will respond to signals 100 times faster than that.

Bye. Jasen

Would a shooting chronograph work for your purposes?

RogerN

purposes?

It certainly would. I did not know the name of those devices, so never looked them up. Not even that expensive. I do however only need the measurements for a few shots and after than I have no real use for it. I will keep it in mind if ad hoc solutions fail.

They dont read dc? Are you saying that they can not measure a constant voltage of for example 0.4v?

Good to know. I think this could be a fast cheap and simple solution

On a sunny day (Thu, 02 Oct 2008 21:45:14 GMT) it happened Rich Grise wrote in :

Oh, yes, I forgot that late last night, we will use a second 4016 switch (it has 4 after all) to disconnect the meter when no shot is detected. And an other one to disable the current source if no shot detected to save power :-) So, now 1 74HC4016 switch left, do not want to use it for Q1, as Ron can be as high as 150 Ohms... OK, save more power: Switch of P3 :-)

Corrected version:

• + | | e PNP e PNP b--- b- Q3 c | c | | Q2 | |---- -------| ---| !trigger 1 ------------- set | |-- | 74HC74 c | | | !trigger 2--R7 -------- reset !Q --- R1------b NPN === | R2 | e Q1 | | | charge a FF1 | | | [ ][ ] meter bias | | C2 0 / -- - | | | P3 | /// / ---------------- | R3 | /// | 0 | | | | | | | -----------------| | | /// | | | | | 4016 | gain --->[ ] P2 | | discharge switch | | gain | | | /// | | | | | | | | !trigger 1 ----------- set Q--^----------------------------------------------------------------- 74HC74 | L1 + -- R5------ reset !Q --- R6-------- k LED a ---- + | FF2 shot detected light / reset button | S1 /// shot detector R1 4k7 R2 1k R3 10k R4 depends on meter sensitivity, 5 V for full scale... if 100 uA then 50k R5 10k R6 4k7 R7 4k4

P1 10k P2 10k ? P3 2k2

C1 depends on distance (pulse-length and current) C2 1nF poly

D1,2 1N4148

Supply +5V (use CMOS opamps).

Something like this (photo detectors not drawn).

Drawing errors?

trigger1 low will set both FF. Pushing S1 the reset button will reset both FF. When FF1 is set, Q1 stops conducting, and C1 is charged by Q2 with a constant current. Opamp1 charges C2 to the peak voltage, and holds, opamp2 buffers and has some gain if needed. The 4016 switch discharges C2 upon pressing reset. LED L1 indicates a shot was detected.

All values are 'guess values'.

Move the beams further apart.

-- Paul Hovnanian snipped-for-privacy@hovnanian.com

----------------------------------------------------------------------- Have gnu, will travel.

Thats right, the capacitor in the input circuit will block that. you'll get a reading of a dip or peak if the DC voltage changes but the DC itself will look like 0V.

So it will work for your ballistic chronograph because it will see the changes.

Bye. Jasen

Thanks for the confirmation. By now I have also read that there is a cap on both the input and the output... makes sence for a sound card, but is anoying when you want to "misuse" it for other things.