Sensor connection

erniehatt · Apr 4, 2011

Hi,
I am new here, and just a hobbyist in Electronics.
I have an electronis Camera Triggering unit, and am trying to to put together a multi-laser device, the laser side is working fine, I am having a problem with the photo transistor side. I have been thinking along the lines of an AND GATE, which if I am reading the truth tables correctly, should, trigger the camera unit, when either of the inputs go low,i.e the beam is broken. If so how do I connect this, do I need pull up or pull down resistors.
If I am wrong in my assumptions then what to I need to make it work.
The image shows how I would connect a single transistor. Ernie

erniehatt · Apr 5, 2011

I mst say this is a great introduction to the forum.
Ah well plenty of other places to look.

Reloadron · Apr 5, 2011

First, welcome to the forums!

I'm sorry. I saw your post early this morning on the way out to work. I was going to reply and simply lost it. Lost as in forgot.

So what is it exactly you are wanting to do? Trigger the shutter when a beam is broken? Do you know what voltage the trigger unit works at? Generally, I know on my camera it is a matter of pulling a line low to common. Can you explain a little more as to what you have?

Ron

erniehatt · Apr 5, 2011

Thanks Ron,
My camera works the same as yours.
My triggering unit works like this, it reads a value from the sensor, I set a threshold just below that reading if the value drop below the threshold the camera will trigger.
What I am trying to set up is a multi laser, photo transistor sensor. I have the Lasers up and running, but the transistor side is giving me problems.
I tried daisy chaining them, but that only seemed to work with two, I am trying to set up five. What I am after is if one or more beams are broken, the camera with trigger. Will the AND gate work. I want to use it to photograph birds flying to my feeders. Ernie

Reloadron · Apr 5, 2011

Hi Ernie

Let me think about it.

I had done some experiments with my cameras last year (Canon EOS 10D and a 7D). I was using some photo transistors at the time. However, I was messing with a single beam, then I did some triggering off sound. I am not sure how to stack things up but a several input NAND may work with each input driven by a photo transistor.

I do remember I used a preset manual focus so there was no need for auto focus. Let me think about it tomorrow.

Maybe till then someone else may have a thought.

Ron

erniehatt · Apr 5, 2011

Thanks Ron,
I have used them singley they work fine, also sound and water drops work fine. Ill await your return. Ernie

Reloadron · Apr 6, 2011

I put the attached together this morning at work real quick. I have no clue if it will work or how well it will work. However, the old classic prevails as it looks good on paper.

First, I am not quite sure how well the 2N5777 responds to LASER light? It should work. A better choice might be a BPX 43-4 (Osram) detector. Next with all the transistors in saturation I am guessing we will get a good enough logic low for the 8 input NAND gate in there. The resistors I labeled 10 K could likely be lower around 2.2 K and should help set the sensitivity of the phototransistors.

There are other ways to do this using comparators but I figure start simple as possible to keep the parts count down. Using the 74LS30 there could be as many as 8 inputs, I just grounded the unused inputs. With all inputs low the output should be a logic high and if any input goes high the output should go low. It will remain low only for the duration of input change but should be enough to trigger the camera.

Anyway, my best guess is to start with something like this, real simple and get more complicated as needed.

Ron

erniehatt · Apr 6, 2011

Thanks Ron, looks good to me, will give it a try, The BPX43-4 would be more suitable for my project. It will take me some time to round up the bit and bobs, letyou know how it goes, thanks again. My main concern with this is the ambient light, will it effect the result after the beam is broken, I am not sure how these gate work, do they react on a small state change or on a complete shut off. Ernie

Reloadron · Apr 6, 2011

The gate I used (74LS30) responds to TTL logic levels. The output is either high or low. There is no inbetween.

I would think about shielding the sensors a little. Small piece of dull black tubing if possible. The latter sensor is designed more for LASER light so hopefully ambient won't over effect it. I would start with and try just a few and see how it goes.

Ron

erniehatt · Apr 6, 2011

Sounds good I have already designed a board from your circuit, See if I can round up the transistors, I do have a number of Unknown Photo transisters may try them out first just to see whas happens. Ernie

erniehatt · Apr 6, 2011

Using the 74LS30 there could be as many as 8 inputs, I just grounded the unused inputs. With all inputs low the output should be a logic high and if any input goes high the output should go low. It will remain low only for the duration of input change but should be enough to trigger the camera.

Ron,
My reading of this seems a little confused, if the transistors are covered by the lasers, would they not be high.
The truth tables state that if all inputs are low then the output is high if all inputs are high the output is low, so with the five transistors and lasers on, the out put would be low, and with the beam broken the output would be high, perhaps I am reading this all wrong. It does not matter whether the output is high or low, I can set that in the Camera trigger, I just need to clarify what I can expect the output to be. Ernie

Reloadron · Apr 6, 2011

Hi Ernie

When the LASER is striking the photo transistor the transistor will be ON. When the photo transistor is ON the output will be close to common or ground so a low level. View the transistor as a switch that is on so a straight shot to common. When the LASER beam is interrupted the transistor will turn OFF and the output will swing high on our output wire. Normally with LASERs striking all the transistors all the outputs will be low thus the NAND gate output High. When any beam is interrupted the gate will swing low. Think of it this way. When the transistor is on all the VCC (5 Volts) is dropped across the 10K resistor, however, when the transistor is off (no light striking it) the output will be about 5 volts. Does that make sense? I know it can be confusing.

Ron

erniehatt · Apr 6, 2011

Ron,
I appreciate what you are saying here, I am trying to compare this with the truth tables and they do not seem to concur.
with respect to what you are saying zero's on the truth table are low and 1 is high. so lets say we have 3 transistors if all beams are on (0) the output is high (1) I am happy with that part, so if the beam is broken the transistor turns off and will go high (1), according to the truth table the output remains high, it will only go low if all the the inputs are at level 1 . so here is where I get lost, the part that puzzles me here is if the out put stays high how can my trigger work, there has to be something to invert that signal to low for that to happen. Maybe I am reading something entirely wrong. Ernie

Reloadron · Apr 7, 2011

Damn, now I see what you are saying. Time to rethink this a little and yes, you are indeed correct. Let me see what I can think up.

Ron

erniehatt · Apr 7, 2011

No Worries Ron, being retired I have more time to think about it.
Did a bit of a search, and found this little circuit to tag on behind the Nand gate will this do what we want

Reloadron · Apr 7, 2011

Retired?

Give me a few more years and I'll be there. Planning a warmer more mild climate too!

OK, looked at what you have. I just made another circuit which is attached. I used switches to simulate the inputs from the phototransistors.

What has been added is a 7404 hex inverter, meaning six inverters on a single chip. If you look at my cartoon I have omitted a few things that should be covered. The power pins of any chips should have a .1 uF capacitor going from VCC to common (ground) to eliminate noise. I also don't show the power pins on the chips. The .1 uF caps shopuld be as close to the power pins as possible for each chip. With the 7404 hex inverter I went ahead and grounded the inputs to the unused inverters just to keep them from floating.

I believe this modification should work but look it over. Early morning at work.

Ron

erniehatt · Apr 7, 2011

Thanks Ron,
lets see if I have this right.

All switchs closed lasers on logic 0 at the 7404 inputs, they get inverted to logic 1 at the gate inputs, so the gate output is 0, giving us a final output of 1, switch off one of the switches i.e break the beam then the final output will be 0.I think you may have nailed it this time Ron.
I understand about the VCC pin etc, but does the circuit need any pullup or down resistors.
I found the warmer climate 46 yrs ago. Ernie

Reloadron · Apr 7, 2011

Hi Ernie

Yep, that about sums it up.

Come May I generally take what Kathy (patient wife) calls the "Ron Trip". I take a few weeks and head south to see old friends with my cameras in tow. I enjoy getting out in rural and country areas and getting pictures. As we move more into summer up here I want to make more time to get into my photography, maybe get another lens. Anyway with work really busy I look forward to the end of May and some time away from it all.

Ron

erniehatt · Apr 7, 2011

Ok Ron,
you didn't aswer the question about resistors and they were not in the schematic, I assume there are none. Thankyou for the great help you have given. Enjoy your time away. Regards Ernie

Reloadron · Apr 7, 2011

No resistors, sorry I missed that. When you get this going let me know how it works out. I mean no resistors for the gates. There are the series resistors used with the photo transistors.

Ron

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