Combining 339 Dark Sensor Circuit with Counter Circuit

elec_mech

Joined Nov 12, 2008
1,500
If you're referring to post #81, the 555 pulls down the clock. You don't need a pulldown resistor.
Darn it, that's right, the 555 output is normally low.

General Terms:
Pull-Up Resistor: Resistor placed between device and Vcc to keep device "high" until forced low.
Pull-Down Resistor: Resistor placed between device and Ground to keep device low until forced high.
You are correct, no need for worry. :)

Anyway, either the 555 I have is not working correctly or it isn't able to "pull down" enough, because I added a 10K resistor between the clock pin and ground and changed the 555 resistor back to the 150K. I tested a total of 40 cycles (20 first, reset the counter and did another 20). I used the push/hold for 5 seconds version and the counters worked perfectly.
Hmm, this is promising, but time will tell. Can you try for longer pauses like 10 seconds? Also, use your meter and connect the black probe to GND and the positive to pin 3 of the 555. What is the voltage when:

1) The button is not pressed?
2) The button is pressed (held down)?

It looks promising. I am off to a birthday party, but unless I come back here and read of a better next step, when I get back I think I will hook my LDR back to my 555 with the 10K pull-down still at the clock pin. What do I have to lose???

Does that make sense? It wouldn't hurt anything would it?
Well, it wouldn't hurt to try it, you won't damage the circuit. I pause as you used a pull-up resistor for the switch before and had it jump, but the switch wasn't going through a debounce circuit either which might account for the jumpiness.
 

Ron H

Joined Apr 14, 2005
7,063
In your maze, or whatever you call it, is it possible for any single LDR to be legitimately tripped more than once BEFORE another LDR is tripped? For example, might the participant trip an LDR, retrace his tracks, and trip the same LDR again, before encountering another one?
 

Thread Starter

CoachKalk

Joined Sep 20, 2011
141
In your maze, or whatever you call it, is it possible for any single LDR to be legitimately tripped more than once BEFORE another LDR is tripped? For example, might the participant trip an LDR, retrace his tracks, and trip the same LDR again, before encountering another one?
This will likely occur many times actually - partly becasue I am Mr. Cheapo and trying to get as much life out of a single laser as I can. By that I mean I will use 1 laser to try to get 3, 4 maybe even 5 "reflections" per LDR. For instance, in my prototype room (our family room), the entire maze consists of 1 laser and 1 LDR - the laser is just bounced around the room by mirrors so every time the kids block any of the laser lines, it is the same LDR. This does work for now, but I plan to use all 4 LDR's in the end so all laser lines are strong and visible. But even then, each LDR will have 3 or 4 laser lines each.
 

Thread Starter

CoachKalk

Joined Sep 20, 2011
141
Well, the LDR test went over like a lead balloon!

I am quickly deciding I like problems to happen all the time - not this sometimes business!!!

I went back to the push button so I could take readings on Pin 3 when I held the button down. FYI - Pin 3 show 0V when the buttton was "open" and approx. 9V when held down.

The more painful news is to say the "sometimes" jump has definately not gone anywhere. It still shows up every once in a while.

I did see something that may or may not be a clue for you guys with some experience. After the LDR circuit sucked eggs, I changed the 555 resistor from 150K to 1M. Figured I would see what happens. Just as a reminder, whenever I mention I am using the LDR, that means I am using the laser in an otherwise dark room. Obviously the 1M resistor causes the 555 time constant to noticeably increase. I used the cover/5 sec. and cover/10 sec. and both cases would result in an immediate jump when I uncovered the laser - THEN 1 more with a delayed response. It was like the counts "saved" up while I was covering the laser and as soon as I uncovered the laser the counts would jump from 4 to 7 then ..... 8. Not sure if that means anything, but I wanted to mention it.

Let me throw this out and you guys can shoot it down. I have been using 2 different delays - press/5(hold for 5 seconds and release) and press/10. In the last run, I was able to get 20 cycles of the press/10 to work great. I did not reset the counter and then went into the press/5. I was only able to get 3 good cycles, then on the 4th it jumped. The 555 has an internal flip-flop right? The time of that flip is 1.1xRxC. What happens when the button is pressed and held down for more than the time? And what happens if the button is released at the exact same time as the internal flip-flop is changing back? When I first built the 555 debounce circuit, the original circuit called for a 100K resistor. With that value, even quick press/release action was causing skip counts. As long as the resistor value has been 150K or above, I have NEVER had a problem with quick push button press/release action OR quick LDR/laser cover action for that matter. Will there always be a delay time that interferes with the internal flip flop or does the 555 have some sort of internal corrective action?
 

elec_mech

Joined Nov 12, 2008
1,500
It's late, so I'm not at 100% mental capacity, but let me ask a couple of things.

1) Let's test the 555 again with the switch and an LED. I don't recall what the 555 will do when you hold pin 2 high beyond the RC time constant. So, with just the switch connected to pin 2 (with a 100kΩ pull-up resistor connected), connect pin 3 to an LED with a series resistor - removed from the 4553. What happens when you press and release the switch quickly; press and hold for 1, 2, 3, 4, 5, 8, & 10 seconds?

2) So you're seeing jumping occur whether you use the button or the LDR correct?

If time allows tomorrow, I'll try the 555 circuit with a counter circuit I currently have set up and see what happens.
 

Ron H

Joined Apr 14, 2005
7,063
OK, we need to get serious here.;)
First of all, the 555 monostable REQUIRES that the trigger be removed before the timeout. Your present circuit doesn't do that. If you can't guarantee that the trigger time is shorter than the timeout, the usual solution is an RC differentiator between the stimulus and the trigger pin (pin 2). I don't believe that will solve your problem here, because I think that, with the switch as stimulus, you are getting switch bounce when you close it AND when you release it. With the LDR as stimulus, I believe you are getting "noise" on both edges (light-to-dark AND dark-to-light). Either stimulus will then cause the 555 to trigger on both edges.
I still think hysteresis is the solution. You might still need the 555. Or not.

1. What is the voltage across the LDR when it is illuminated by the laser?
2. What is the voltage across the LDR when it is dark?


I would be helping you a lot more, but we are in the process of packing prior to moving, and we are downsizing, so figuring out what to keep, what to (try to) sell, what to give away, and what to toss is just as time-consuming as packing.
 
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CoachKalk

Joined Sep 20, 2011
141
OK, we need to get serious here.;)
First of all, the 555 monostable REQUIRES that the trigger be removed before the timeout. Your present circuit doesn't do that. If you can't guarantee that the trigger time is shorter than the timeout, the usual solution is an RC differentiator between the stimulus and the trigger pin (pin 2). I don't believe that will solve your problem here, because I think that, with the switch as stimulus, you are getting switch bounce when you close it AND when you release it. With the LDR as stimulus, I believe you are getting "noise" on both edges (light-to-dark AND dark-to-light). Either stimulus will then cause the 555 to trigger on both edges.
I still think hysteresis is the solution. You might still need the 555. Or not.

I would be helping you a lot more, but we are in the process of packing prior to moving, and we are downsizing, so figuring out what to keep, what to (try to) sell, what to give away, and what to toss is just as time-consuming as packing.
Um ... Ron ... are you serious??? I think you are already "helping me a lot more"!!!! If it weren't for you and elec mech (shout out to the others who helped a great deal at the beginning as well), I would be a pile of goo, slobbering in the corner.

Even though I may not understand why, I can now predict the counter jumping. Does any noise from the push button or LDR get "blocked" during any cycle less than the 555 RC Timer because the input is "closed for business" during that time? And when the push button or LDR is covered for greater than the RC Time, the "illiegal" condition causes the 555 to pick up the noise as soon as the input shuts off?

The reason I ask is because I set the LDR circuit back in place and set the 555 resisitor at 4.7M (RC Time ~ 5.2 sec.). Sure enough, the counter worked great as long as I kept it covered less than 5 seconds. I even hooked the siren to Pin 3 again. One quick swipe of the laser and the siren starts screaming for 5 seconds. It shuts off and the counter counts up 1. Same thing with a 3 second cover/release. Siren for 5 sec. then count 1 more. Obviously, when I cover the laser for 10 sec., the siren is "stuck" on past the 5 sec. RC Time and screams away. As soon as I uncover the laser, the siren stops and the counter sometimes double counts. With this setup - and I know it is a bit of a cop out, but I could tell the kids they have X seconds to get out of the laser or they will be penalized additional strikes. It does stink that it may vary between 1-3 extra, but hey, they had 5 seconds to get clear! Right? Yeah, I know it's weak, but I am struggling!!!!

Ron - I did try the hysteresis circuit you provided. I did not have much luck and I have a few questions/comments to throw out. The circuit you provided is not much different than my current setup, so it wan't too painful.
1. Is the fact that in my circuit the LDR and R2 are swapped a problem? I left my circuit as is during my test. If that is a show stopper, that may explain my problem.
2. My circuit currently has a 10K at the R4. By switching to 100K (everything else in my circuit unchanged), I could not even get my circuit to register anything on the counter. So I added the R5 100K resistor, but kept R4 at 10K.
3. When I had the LED in place, the LED was lit on Power Up and did not change when the LDR was covered.

So, I must have messed something up because I could not get the counter to respond at all with the R5 in place.

One other thing I came across ...

My first experience with this site was to go through some of the lessons/experiments involving the 555. I was trying to remember some 555 info and did a search. I found the info below and it looks exactly like what I need. I included the entire page, but the bottom looks like the best match. Do I understand the details - NOT AT ALL! It doesn't even specifically say a 555 can do the things described, I am just hoping I guess.

http://www.allaboutcircuits.com/vol_4/chpt_10/8.html

I guess I need to sleep sometime so I will go to bed and try to shut my brain off ...

Thanks again everyone.
 

thatoneguy

Joined Feb 19, 2009
6,359
Ok, I am a bit confused on the desired outcome (fast counted or slow counted or both?)

I am guessing what you have either:

A) A Non-Retriggerable One-Shot in an application where you need a retriggerable one shot.

B) A Retriggerable One Shot in an application where you need a non-retriggerable one shot.

Short descriptions:
Non-Retriggerable one shot is a way of configuring a 555 so that when you press a button, it sends a pulse out of a duration, no matter how long you hold down the button, while the retriggerable will continue sending pulses as long as the button is held down.

Wiki says it better, but uses a multi-vibrator for the example (you are using a 555 timer instead, which can run in either mode)
When triggered by an input pulse, a monostable multivibrator will switch to its unstable position for a period of time, and then return to its stable state. The time period monostable multivibrator remains in unstable state is given by t = ln(2)R2C1. If repeated application of the input pulse maintains the circuit in the unstable state, it is called a retriggerable monostable. If further trigger pulses do not affect the period, the circuit is a non-retriggerable multivibrator.
 

Ron H

Joined Apr 14, 2005
7,063
Coach, you figured out the 555. If you hold pin 2 low past the timeout duration, the output will stay high until pin 2 goes high. If pin 2 has bounce when it is returning high, then the 555 will trigger and time out normally. With the switch, this is caused by switch bounce. With the LDR, even a few millivolts of noise will cause full-amplitude pulse(s) to appear at the 339 output, because the 339 has very high gain.
When you tried my hysteresis, you had to follow the schematic EXACTLY, or it wouldn't work. Having said that, I believe I have a better hysteresis idea, but it depends on the answers to the questions I asked in my previous post. If my idea works, you won't need the 555. It would be independent of how long the LDR is blocked.

Attached is what I believe is the root of your problem.
 

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Ron H

Joined Apr 14, 2005
7,063
Ok, I am a bit confused on the desired outcome (fast counted or slow counted or both?)

I am guessing what you have either:

A) A Non-Retriggerable One-Shot in an application where you need a retriggerable one shot.

B) A Retriggerable One Shot in an application where you need a non-retriggerable one shot.

Short descriptions:
Non-Retriggerable one shot is a way of configuring a 555 so that when you press a button, it sends a pulse out of a duration, no matter how long you hold down the button, while the retriggerable will continue sending pulses as long as the button is held down.
A retriggerable one shot only sends out one pulse each time the button is pressed. If you release it and press it again before the one shot times out, the one shot will reinitiate the timeout period from that point. It does NOT "continue sending pulses".

Wiki says it better, but uses a multi-vibrator for the example (you are using a 555 timer instead, which can run in either mode)
 
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Thread Starter

CoachKalk

Joined Sep 20, 2011
141
I still think hysteresis is the solution. You might still need the 555. Or not.

1. What is the voltage across the LDR when it is illuminated by the laser?
2. What is the voltage across the LDR when it is dark?
I looked through my pile of papers and here are the measurements I took. I took these measurements because I was trying to get a better idea of the LDR resistance under the glass fixture - hoping to get away from needing to have a direct hit by the laser.

I now have the LDR wired to 2 4ft leads so I can just drop the glass fixture over the top of the LDR - away from the protoboard.

These readings were measured in a dark room.
Laser Hitting Bottom of Fixture (Nearest LDR): LDR Resistance ~ 300K
Laser Hitting Middle of Fixture: LDR Resistance ~ 500K
Laser Hitting VERY Top of Fixture: LDR Resistance ~ 800K
Laser Blocked - LDR DARK: 5M

I used these numbers to select my resistor values for the 339 + and - inputs. The "ref" voltage uses a 270K and 1M. With the 12V supply, I "think" this calculates to 9.5V.

I am not sure if my next calculations are what you want, but these are the calculations I worked through.
LDR "Brightest w/Laser" - 270K and 300K, 12V = 6.3V
LDR "Darkest w/ Laser" - 270K and 800K, 12V = 9V
LDR DARK - LASER BLOCKED - 270K and 5M Plus = 11.4V

Let me throw this out here as well. After I took these reading, it occurred to me to just look up the photocells I ordered and check the specs there. Well, I must say the "specs" noted and my actual readings are drastically different. The Futurlec site has the following info for the photocells I ordered: Light - 3K, Dark - 300K. Is it just an issue that my application (starting with Laser light as the "Lit" condition already would normally be classified as a "Dark" condition in typical applications? Hopefully it will not cause issues, but make no mistake, these LDR's go MUCH higher than 300K!

Let me know if this is what you were asking. Obviously if not, you will probably be able to calculate it with the resistance values I provided.

But hey - if you run into any problems, I am here for you!!:D:D:D:D:eek: Sometimes I crack myself up!!!!!
 

Thread Starter

CoachKalk

Joined Sep 20, 2011
141
Ok, I am a bit confused on the desired outcome (fast counted or slow counted or both?)
First, let me say I appreciate the suggestion of getting an oscilloscope. I have not ignored your suggestion, just trying to keep the wife off my trail with any "bigger" ticket items. I am in no way shape or form an experienced electronics hobbiest and daily I am thinking I already have a pile of components that may just end up in the corner. But, Christmas is coming up and the mother-in-law is always asking what I want - so it may happen.

As far as the outcome I desire - you mean other than going to sleep and having an electronics fairy visit me?

My main issue is the kids "could" keep the LDR blocked for any given time. That is why I am not sure if I am making this a bigger problem that it has to be. In real life, if it only takes them 2 seconds or less to get clear, a one shot with a RC Time of 4 seconds would never pick up the noise - everyone is happy. BUT - if the 8 yo panics and keeps the laser blocked for 5 seconds, now the noise jumps the counter. Also, even if the kids do clear the laser in 1 second, the siren would keep sounding for a total of 4 - so they may think they are still blocking it.

Definately trickier than hit a button and watch the green LED stay on for 4 seconds, then the red LED comes back on!:rolleyes:
 

thatoneguy

Joined Feb 19, 2009
6,359
In that case, I'd strongly suggest building in the hysteresis circuit Ron H posted a page back.

Then the one shot timer could be eliminated, and the buzzer would only be on for the duration of blockage, rather than a set time. What his circuit does is make the 339 less sensitive to change when the input is right at the trigger point.

What hysteresis means in your circuit is:
Once triggered high, the comparator needs a lower voltage than the fixed set trigger voltage to set it low, and once triggered low, it takes a higher voltage than the fixed level trigger voltage to toggle the output high.

Hysteresis would prevent the 339 from giving multiple output pulses as the the addition or removal of light causes the input to vary barely above and barely below the trigger point when being partially covered or uncovered.
 

Thread Starter

CoachKalk

Joined Sep 20, 2011
141
In that case, I'd strongly suggest building in the hysteresis circuit Ron H posted a page back.

Then the one shot timer could be eliminated, and the buzzer would only be on for the duration of blockage, rather than a set time. What his circuit does is make the 339 less sensitive to change when the input is right at the trigger point.

What hysteresis means in your circuit is:
Once triggered high, the comparator needs a lower voltage than the fixed set trigger voltage to set it low, and once triggered low, it takes a higher voltage than the fixed level trigger voltage to toggle the output high.

Hysteresis would prevent the 339 from giving multiple output pulses as the the addition or removal of light causes the input to vary barely above and barely below the trigger point when being partially covered or uncovered.
I "sort of" tried Ron's hysteresis, but there were a few components in different positions (270K and LDR) and the value of the pull-up? resistor 10K (mine) vs 100K his. I was hesistant to rebuild my circuit completely because I am new at all this and I could see me causing more problems. Ron has since answered a few questions I posed and let me know it would have to be build exactly per his circuit - but I was having some trouble getting the LDR to respond with my "partial" build.

One of the main differences I noticed was the switching of the 270K and LDR. Wouldn't that small change completely throw off the calculation (voltage divider) and keep the trigger voltage always way to low?

Also, I know this is repeating some of the questions I posed to Ron, but my current circuit that works fine (minus the noise business) has a 10K out of the 339 output. What would be the reason that a 100K in that position would cause the circuit to not count at all (everything else left alone)?

Now, I am going to hang out a bit and let Ron finish his moving stuff before I jump in again. It sounds like he has something else bouncing around that he is going to throw out ... I can't wait ...

If his new idea or some sort of other hysteresis works, I hope to be able to implement elec mech's idea of a one shot to keep the siren going for a few seconds. What I mean is that if Ron can help me solve the LDR noise, the counter will respond to each laser block, regardless of duration. But, if the kids really only block the laser for .5 seconds, even though the counter would count, the siren would sound more like a sick chicken as .5 seconds isn't enough time to rev that baby up. I hope to be able to use Ron's idea for the counter and maybe a separte one-shot for a siren blast to keep the kids on their toes.

So - Ron - if you read this - hopefully your life saving solution would still work if I tagged a one shot on for a free ride afterward. BUT, like I have said before - if not I can live with it - it just won't be as fun to not see the kids JUMP when they block a laser.
 

Ron H

Joined Apr 14, 2005
7,063
The idea I had doesn't work with those voltages, but that doesn't mean hysteresis won't work.
Meanwhile...
I'm just wondering if an LDR is the optimum sensor.
Do you know the wavelength of the laser? If not, what color is it? is it a laser pointer?
Do you know if the LDR is cadmium sulfide, or ???
What is the diameter of the active area of the LDR? You may have mentioned a part number, but I can't find it.
How big (diameter) is the laser spot on the LDR? Is it difficult to keep the lasr spot aligned on the LDR?
 

Thread Starter

CoachKalk

Joined Sep 20, 2011
141
The idea I had doesn't work with those voltages, but that doesn't mean hysteresis won't work.
Meanwhile...
I'm just wondering if an LDR is the optimum sensor.
Do you know the wavelength of the laser? If not, what color is it? is it a laser pointer?
Do you know if the LDR is cadmium sulfide, or ???
What is the diameter of the active area of the LDR? You may have mentioned a part number, but I can't find it.
How big (diameter) is the laser spot on the LDR? Is it difficult to keep the lasr spot aligned on the LDR?
Laser - Green, 5mW
LDR - http://www.futurlec.com/Photocells.shtml Part #: LGPHOTO
Difficult to Light LDR - Until I switched/got rid of the 10K pot AND added the glass fixture from the bathroom (Shhh ... don't tell my wife), it was a big pain in the @$$ to keep the LDR lit. PLUS, I use mirrors balanced around the room. So not only did I have to hit the LDR dead on, but any room vibration would cause my laser to move. Now, I have dialed in the resistors so when I cover the LDR with the glass fixture, the LDR stays "lit" anytime the laser hits the fixture - anyplace at all. It is a much bigger target.

Eventually I will have solid mirrors in place, but I still plan to have the LDR covered with some sort of reflector so I do not have to have perfect alignment.
 

Thread Starter

CoachKalk

Joined Sep 20, 2011
141
OK, so I decided to setup another trial run with mirrors and the fog machine. I figured I would take some time to figure out how much life I can get out of 1 laser. This time I had to break out the big guns - I used Duct Tape and taped smaller pieces of mirrors to the walls! It didn't make the maze incredibly diffucult because all the laser lines were pretty much level, so I just made the kids go over/under. Anyway, a few observations:

1. After cleaning the mirrors, I was very happy with how many laser lines were visible from just the 1 laser. If I max out the 339 (4 LDR's), the maze should be pretty darn cool.

2. I had the circuit set-up for the 4 sec. one shot. I am NOT just saying this because the noise issue won't go away nicely - but after seeing the kids go through the maze and seeing the angles that the lasers will most likely be at (just imagine horizontal V's going across the room), it was a very workable option. It would have to be an EXTREME case for the laser to be blocked longer than just a second - maybe two when that siren starts screaming. I am very interested in workable hysteresis circuits, but if that proves problematic, the current setup is definately an option.

3. I hate to even bring up other questions when you guys are hot on the noise trail, but this is just a general question. Regarding my LED Count display - how visible should the segments that are off (based on the number that is displayed) be? I know elec mech highly suggested getting a "covering" over the display so maybe that would make the faint "off" segments not so visible. Is it normal in 7-seg displays to have some faint off segments or do I need to dial in the resistors better? Is it a sign of another issue?
 

Ron H

Joined Apr 14, 2005
7,063
Here is a circuit I would like you to try. It has hysteresis, which causes the comparator to ignore non-monotonic changes in resistance when the LDR goes from light to dark and vice-versa, up to a point. If the fluctuations are too large, the circuit will still put out spurious pulses during light transitions.

In the attached circuit, when the LDR is illuminated, the voltage Vldr will be somewhere between 2.8V and 5.4V, depending on the amount of illumination (this is using your 300k and 800k measurements). The threshold at this time will be Vth(hi)=8.9V, because the comparator output is high.
When the LDR goes dark, Vldr=10V. This is considerably above Vth(hi), so the comparator output will go low, causing the threshold voltage to change to Vth(lo)=6.5V. When the LDR is next illuminated, the voltage will have to go below 6.5V before the output can again go high, which, of course, makes the threshold go back to 8.9V.

Another way of looking at it:
When the LDR resistance starts low and is rising, the comparator will switch when Vldr=Vth(hi). This will happen when the LDR resistance rises to about 2.9Meg.
When the LDR resistance starts high and is falling, the comparator will switch when Vldr=Vth(lo). This will happen when the LDR resistance falls to about 1.2Meg.

Therefore, variations in LDR resistance between 1.2Meg and 2.9Meg, up or down, will cause no change in the comparator output. Without hysteresis, changes in LDR resistance which cause the LDR voltage to cross the threshold multiple times during light transitions will cause spurious clock transitions.

If you use multiple LDRs and comparators, you will need a separate reference divider (R2, R3, R6) for each comparator, due to the feedback.

I have to say that I am not comfortable running the LDR at such low illumination levels. I seem to recall that it takes a long time for the LDR resistance to reach its final value, and I am concerned about consistency between units. You might need selected resistors R2, R3, and R6 for each comparator.
 

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SgtWookie

Joined Jul 17, 2007
22,230
Radio Shack carries an assortment of CdS photoresistors; 5 for ~$3.20.
http://www.radioshack.com/product/index.jsp?productId=2062590

I used to have some of those. When within a foot of a lit 60W bulb, their resistance was quite low; just a few Ohms if I remember correctly, and at least several hundred K when dark. If you're lighting them up with just a spot laser, they would probably be quite a bit higher in resistance.

CdS cells are really not very fast to respond to changes in light.
 

Thread Starter

CoachKalk

Joined Sep 20, 2011
141
Radio Shack carries an assortment of CdS photoresistors; 5 for ~$3.20.
http://www.radioshack.com/product/index.jsp?productId=2062590

I used to have some of those. When within a foot of a lit 60W bulb, their resistance was quite low; just a few Ohms if I remember correctly, and at least several hundred K when dark. If you're lighting them up with just a spot laser, they would probably be quite a bit higher in resistance.

CdS cells are really not very fast to respond to changes in light.
When I first jumped into this project, that is exactly what I started with. My concern was the different sizes included in the assortment - and the small face of the cells.

I decided to order LDR's as large as I could - and all with the same "specs" in order to hopefully get repeatable results as I will be using 4 per 339.

I know several people have commented on the reaction time being slow, but I have not noticed that with my trial runs. In my dark room with just the laser on the glass fixture, the 339 triggered when I swept by hand through the laser as fast as I could. But, maybe you guys are referring to "electronics" fast, not my "fast"!:D
 
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