op amp knowledge

[alband]

I don’t know much about op amps. I think though they will be suitable for this application.
I’ve got (or I will have in a bit, but I know exactly how it will behave) a 7x7 array of LDR’s, acting as a target, with the intent of sensing a laser.
The way I have the LDR's wired is the crucial bit...
One pin from each LDR is connected to +VCC. The other is connected to two signal diodes. One diode is connected to the common ROW wire of that particular sensor, the other diode is connected to the common COLUMN wire of that particular sensor.
This means the grid has 14 outputs and 1 input (VCC).
If the laser light falls onto the centre target (grid reference 4,4) then output 4 of the row outputs will go high(er) and output 4 of the column outputs will go high(er).

The problem is that these outputs are very analogue, and depend on ambient light levels. No PICAXE has 14 ADC’s so I need to use the digital inputs. This would be fairly easy you might think. “Simply connect each input to a digital input on a 40X. Connect some grounding resistors to each pin and tune their values until it works”. This will however need lots of precise setting up and will need setting up every time, due to changing light levels.

An op amp would oblitorate the analoge problem. As I know an op amp can be used as a comparator. The inverting input is compared with the non-inverting. The non inverting would be set to a fixed, preset voltage so that the system works. I have my own idea of an op amp I need so what I want to know is if there is such an op amp:
It would have one non-inverting input. This pin would be connected (via diodes) to all the output pins and would thus maintain a mean voltage of all the pins and could be compared with each individual pin.
It would have 14 inverting inputs. Each would be connected to one of the sensor outputs.
It would have 14 outputs. Each would correspond to each of the 14 inputs.

Now I know it is slightly unlikely that such an op amp exists . I know very little about op amps and their varieties and was hoping someone could point me towards the closest thing to this.

 

[eclectic]

@David.

I know very little about Op - amps.
But, Dr Acula know lots.
See:

http://www.picaxeforum.co.uk/showthread.php?t=8557&highlight=amp*+tutorial

e

 

[alband]

Very useful, thanks, e & Dr.

I'm gonna need 14 inputs though and at 3 op amps per input that would need 11 quad chips

Is it possible to get chips that have several op amps but the op amps share a + input?

 

[BeanieBots]

An op-amp only has two inputs and one output. The output is the voltage difference between the inputs multiplied by it's gain. That's it.....

The rest is done with resistors and a knowledge of ohm's Law.
If you want 14 outputs which are NOT linked by a mathematical rule, then you will need 14 op-amps. It may well be possible to what you want with 14 op-amps (possibly only 7) but I haven't quite got your idea clear in my own head to be sure.

My approach would be to combine analogue and digital techniques.
Use digital to control (address) the array of sensors and analogue to read the values.

 

[BeanieBots]

You can think of an op-amp in the same way as a variable (say b0).
b0 can be "calculated" from any combination of other variables (inputs) you care to imagine. (eg b0=b1+b2+b3....-b8-b9....)
Addition is very easy, just a simple resistor, multiplication/division is possible but avoid it if possible. (comparable to making a PICAXE do floating point).

 

[sghioto]

Alband,
Should only require 4 quad chips. You need 7 comparators for the rows and 7 for the columns. Something like this connected to 14 digital inputs.

Steve G.

 

[alband]

I'm just trying to suss the best way to channel 14 analogue inputs, which can rise up an down depending on ambiant light, into 14 digital inputs.

Say these are the 14 output vltage levels from the sensor grid:

 [U]2.1[/U]/2.3/2.2/2.1/2.2/3.5/2.1
 [U]2.2[/U]
 [U]2.1[/U]
 [U]2.3[/U]
 [U]2.4[/U]
 [U]2.1[/U]
 [U]2.3[/U]
 [U]3.6[/U]

We can tell straight away that the target 6 along and 7 down has been hit. The LDR's only swing by about 1.5V so it is hard to detect this on a PICAXE. I have tryed using different resistor values, but those values need changing every usage because ambient light levels can swing the voltage by >1.5V.
So if I take an average of all those values (2.39V), this give a reference point to compare the others by which is correct. The two values that have come from the triggered LDR are above this magic value and the rest are bellow.

I think I am right in saying, that if I took these 14 outputs and connect them to diodes and connect the outputs from the diodes together, I would end up with a mean voltage of them. The original 14 outputs can then be sampled (before the diodes) and compared with the mean value with op amps.

This could be doen with 14 seperate op amps. Each op amp would be connected to one of the 14 outputs and the mean of all of those outputs.
This would be a lot easier if there was one chip that had multiple op amp that each compaired its inverting input to a common non-inverting. I want to know if there is such a chip?

Thanks for continuing to read my long posts, which will be ful of speling erorrs becose I have no spell chech on this tempory PC.

 

[alband]

I was type while you both were.

Alband,
Should only require 4 quad chips. You need 7 comparators for the rows and 7 for the columns. Something like this connected to 14 digital inputs.

Steve G.

Exactly!
But the + input on each of those op amps is connected to a potential divider. Instead could each + input be connected to a common, mean voltage from all the sensors?

 

[sghioto]

alband,
You could. But I don't think you would need to. How much ambient light will the array be exposed to? Is it shielded or recessed somehow? If ambient light is an issue you could monitor one additional LDR with an ADC input and convert this to the + reference voltage for the comparators.

Steve G.

 

[vttom]

Have you considered a "scanning" approach to the problem?

By that I mean, instead of have all 7x7 LDRs with a common VDD, arrange them so that each row has a unique VDD, which you drive from the output of a 3:8 decoder chip. You "address" a row from the PICAXE by putting a 3-bit value into the decoder.

On the sense side, use an 8:1 analog multiplexor into an ADC input on the PICAXE. You would "address" which column you're sensing from the PICAXE by putting a 3-bit value on the select pins of the multiplexor.

All told, you would only need 6 output pins and 1 ADC input, which a PICAXE-14M will do nicely.

In software, you would "scan" the grid and keep track of which one had the highest reading more-or-less like this:

max = 0
for b0 = 0 to 6 ' Row counter

' Address a row
pin0 = bit0
pin1 = bit1
pin2 = bit2

for b1 = 0 to 6 ' Col counter

' Address a column
pin3 = bit8
pin4 = bit9
pin5 = bit10

' Sample the analog value
readadc val

' Keep the highest value read
if val > max then
hitrow = b0
hitcol = b1
max = val
end if

next b1
next b0

 

[wapo54001]

Won't you have reflected light issues to deal with? The laser hits one device, but the surface of the device reflects the laser onto adjacent devices. You'll have to carefully adjust on/off thresholds, or recess the receiving devices so that the laser only hits one device at a time?

 

[eclectic]

@David.
Adding to posts #9 and #11,
just a few background questions,
c/w half-formed ideas:

1. Is this a real Laser? (As in hacked RED pointer or something similar.) *

2. What are the dimensions of your target board?

3. I'm assuming the LDR's are the “Standard” everyday variety.

4. I'm just thinking about recycling some red, transparent, Christmas wrapping.
It might help.

And it's too late now, and too expensive, for a BPW34 solution

e

* I wouldn't dream of talking about the potential dangers of
a moving, bouncing, RC toy tank, shooting a real laser,
just in case Dippy's listening.

 

[BeanieBots]

Vttom has nicely expanded on my suggestion in post 4.
To take it a little further, if you want to compare against the average of all the sensors, it is very easy to do.
Doc's tutorial shows how to make a summing amplifier. Take this a little further to sum ALL your sensors and then use the op-amp gain to give the average. Use that voltage as your "average" for the trip value of the comparitor. Each sensor can then be fed to the comparitor by use of a multiplexer and read using just one input.

 

[manuka]

I don’t know much about op amps.

I'd hence park that target at this stage & first find out more about them- it's otherwise akin to meandering to a city address without a map or GPS! And like a map, or hard wired logic ICs, they're basically very straightforward devices - " Ah-so this is all they do. That's easy!" being a typical response.

In conjunction with hands on, perhaps one of the best web 741 level op-amp insights is at => http://www.uoguelph.ca/~antoon/gadgets/741/741.html. IMHO however Forrest M. Mims III Engineer's resource booklets better cover classic op-amp applications => http://www.forrestmims.com . You'll learn more in 30 mins with one of these classics than you'd credit!

 

[Jeremy Leach]

I'm still not clear exactly what you are trying to do. Do you want to know where in the 7*7 grid the laser spot is? Is it a grid of LDRs or concentric circles with the central LDR the bulls-eye??!

What is the physical dimension of your target?

 

[moxhamj]

I wonder if the 4051 might be useful? No op amps needed. One 4051 can turn 8 analog inputs into 1 input. It needs 3 lines to drive it.

So you could have 2 4051s, one for the row and one for the column. Use 3 wires to select, and use 2 ADC inputs on the 14M. I'm thinking you could do the whole project with a single 14M and 2 4051s.

You can use the same three wires to select both 4051s because you will know which one you are reading by which ADC you read. So this only uses 5 pins on the 14M.

BTW I presume you have done the tests on the laser and discovered the beam divergence? A laser pointer diverges to about 25cm wide after 60 metres. (Unless you run it in reverse through a telescope or binoculars with the focus set at infinity)

 

[Dippy]

I think we can assume a 7x7 grid means a square 7x7 grid.

Op-amps. People get muddled. I really think ALBAND is sggesting using it as a voltage comparator. Laser hits, LDR result goes over threshold and therefore output high for that LDR. For something simple, transistors can do a similar job. Having an op-amp for each LDR is pretty clunky.

This could be a fairly doomed approach if there are large ambient light changes and, as pointed out by Drac the problems of long-distance + cheap lasers and even worse if the laser ever strikes from an angle. So, are the LDRs shrouded/shielded?

I very much like the scanned/strobed approach suggested earlier. It makes the circuit smaller, cheaper, easier, but relies on fast code.

It would nice to go via a multiplexed switch to an ADC, but the code may not be able to run fast enough if your project is for one of those laser gun games.

 

[alband]

Tonnes of information there.
I haven't got much time now, so will try and respond quickly to what I can, then do some thorough reading and thinking when I get back.

7X7 square grid. Using miniture LDR's
Final dimentions of target would be about 4x4cm.
The circuit board is already made. The LDR's wouldn't be shielded. I've tested a LDR + laser combo and it work fine at the range I need (about <10m) so the divergance isn't an issue.

The project:
Plastic gun with 08M inside and a laser (same one as in the tank = diode laser). Gun has a sniper scope. 08M simply pulses the laser once when the trigger is pressed: its only function is to stop the gunman "tracking" the laser onto the target. So the 08M just make the laser act more like a bullet.
The target has a 40X (already bought) in it and picks up the shot information (somewhow) and then sends the information to a display next to the gunman. The display has a 40X in (also bought) and turns the information into a 7x7 grid of LED's. This is already doen and dusted and works beautifully.

Ambiant light would vary lots due to different locations, windows etc.

The scanning aproche sound verg good and I would definately use this if I did it again. Unfortunately I have already made the circuit board for the LDR's. I think I would use two of that chip 4051 and a seperate LDR. The 4051's would choose the column and row, those would be fead into adc1 & adc2 and would be compaired with the extra LDR or a mean voltage of all the LDR's, fead into adc0. This would however be very dependant on timing. It would prove difficult scanning the target often enough and sending the information back to the target often enough.

For now though I can't use the scanning aproch (although I think that idea can arguably be names as "The Winner") beacuse the target board is already made.

Reflective issues arn't a problem; those LDR's don't have the glass cover and so are not reflective.

BPW34: looks nice, but as said, too late

BB:

Doc's tutorial shows how to make a summing amplifier. Take this a little further to sum ALL your sensors and then use the op-amp gain to give the average

Not sure. By the summing amplifier: the op amps add all the voltages together. The gain would then be set so that if there were 10 targets, it would times by 0.1 (i.e. it would be set to divide by the number of targets)?

manuka: I'll look at those when I get back.

Dippy:

Op-amps. People get muddled. I really think ALBAND is sggesting using it as a voltage comparator. Laser hits, LDR result goes over threshold and therefore output high for that LDR. For something simple, transistors can do a similar job. Having an op-amp for each LDR is pretty clunky.

Due to the wiring of the target, there are only 14 outputs: 7 vertical and 7 horizontal so only 14 op amps.

The main question is;
If I channel 14 wires into 1 wire, via diodes, will this one wire have the average voltage of all the wires?

Thanks.

 

[hippy]

The main question is;
If I channel 14 wires into 1 wire, via diodes, will this one wire have the average voltage of all the wires?

No. It will give the highest voltage ( less the diode voltage drop ).

 

[Dippy]

I think it might help if you specified EXACTLY how the LDRs are connected. I think you should have posted your question before making the LDR arrays (or are they off Ebay?).

I had 'visions' of a matrix type arrangement which you could scan/strobe.... maybe not?

So, you could set one column high, then via a 4051 or similar, connect each row to the ADC and measure.
.. then do the same for all columns one at a time. And then compare the numbers and look for a significantly big one.

But, as your 'laser bullet' may be brief, I'm not sure whether PICAXE is fast enough.

(So, basically, exactly how you'd strobe a matrix keypad but use ADC instead.)

Also, as I guessed it would be a game, I thought that the position on the LDR array of the 'hit' was important.
If the position isn't important then it's easier as you are simply looking for a signal significantly-over-ambient to confirm a hit.

 

[eclectic]

@Alband

From post #18
"7X7 square grid. Using miniture LDR's
Final dimentions of target would be about 4x4cm.
The circuit board is already made. The LDR's wouldn't be shielded. I've tested a LDR + laser combo
and it work fine at the range I need (about <10m) so the divergance isn't an issue."

Hitting an 5mm LDR at 10 metres, with a handgun.
Blimey, are you training for 2012?

But, in the meantime,
Something to aim for.

http://www.lasercomms.org.uk/france.htm

http://www.lasercomms.org.uk/76km.htm

And, somewhere on this forum, there's mention of an Oz -Tassie version,
which went even further.

e

 

[BeanieBots]

I imagined the "array" same way Dippy did.
If position is not important, then very simple, just series/parallel or combination of both and look for 'change'.
If PICAXE can't scan quick enough, then scan with hardware into 'sample & hold' and use PICAXE to read at leisure.

 

[alband]

From Dr's tutorial post:

Consider the circuit in figure 2, which is a practical amplifier for many sensors. Inputs are fed through two op amps configured as voltage followers. The output is simply the same value as the input, but these op amps act to make the inputs high impedance. This is important if the sensor were say a resistor bridge with 10k resistors. If this sensor were fed directly into a differential amplifier then the amp itself would change the value of the resistor bridge.

I don't understand this rolleyes: typical), is it acting like a photocouple or relay?
sghioto: I see you've use an op amp like this but I can't understand why you've connected the output to the adjustment POT.

Attached is a 2x2 grid to show how the LDR's would be attached (currently the LDR's are labelled as PTR's because I first tried to to use phototransistors but they didn't respond to red laser light. Also, the outputs wouldn't go straight into the PICAXE as explained below). Also attached is the code for the 40X target chip (this is only for a 5x5 grid just because I couldn't be bothered and I was still testing).

Hitting an 5mm LDR at 10 metres, with a handgun.
Blimey, are you training for 2012?

I wish . It's going to be a "lying-down-sniper-with-a-scope" type gun.

Very cool channel crossings. If only we were small enough to catch a lift on those photons .

I think basically I will be as sghioto's picture shows, except I will have an average of all the LDR's fed into the +'s so that it doesn't need adjusting. I think BB, you mentioned that averaging can be done with op amps. I understand how the divide bit can be done by setting the gain, but how does the op amp "add them up" first?

 

[jglenn]

A couple of ideas for you. TI has a light sensor that has a digital output, a freq. I think. Wide dynamic range. Parallax sells them, probably a hobby dealer somewhere. This eliminates the analog signal conditioning.

But that is fun, so consider just using analog switches, 4066 or something with more inputs, a multiplexor perhaps, to select and feed one light sensor output at a time to the PICAXE for a/d input. Then it becomes a software problem!

 

[sghioto]

alband,

The pot is not connected to the summing amp output. It is a completely seperate circuit as an alternative way of "manually" adjusting the reference voltage bus. You would use one or the other.
I'm not sure if you're asking me this but the reason for using a voltage follower is the high input impedance. Meaning it will not load down the sensor outputs or affect the voltage from the sensors.
My circuit uses a dual opamp to monitor the whole array. This output, which will always be about .7 volts higher then the array, will need to go through a sample and hold routine before each "shot" so a "hit" will not affect the reference, which will automatically adjust to ambient light levels. I had to correct the previous schematic because the logic levels were reversed. The "sample and hold" routine can be done using code or a dedicated circuit.

 

[alband]

I'm still not quite sure how this works.

The Dual opamps; they create a mean voltage of all the sensors?
Why does the output of the dual opamps go to a PICAXE ADC pin? Surely, it would make more sense to put that output into + of the other opamps?

When opamps are use as comparators, can't they be made to swing either low or high, depending on which input is at a higher voltage?

When the sensor is hit, the average of all the sensor outputs would be lower than the hit targets (resulting in a high) and the other sensors would be lower than the mean (resulting in a low)?

 

[Dippy]

I haven't looked at Steve's example so can't comment.

It was just a reminder about comparators used in light sensing.
"When opamps are use as comparators, can't they be made to swing either low or high, depending on which input is at a higher voltage?"

- yes, you compare two things; your signal from LDR(s) and a reference.
That's fine except where you have two other variables;
1. Varying ambient light
2. Non-linear perfromance of sensor.

If you compare LDR to a simple fixed voltage reference you may have trouble, especially if you are adding/averaging multple LDR outputs.
Mix into the brew the resistance change of an LDR when ambient is very bright compared to LDR+laser hit (esp dispersed and/or brief) then you may have trouble.
It may be fine on a cloudy day and awful on a sunny day.
The LDR response could be a real pain without good planning and construction.

If I were using comparators I'd compare each LDR level to an averaged level.
But I wouldn't, i'd have designed the LDR array for fast scan, but as your wiring precludes this then move to Plan C.
All the best.

Oh and then there is the pretty lousy tolerance of LDRs too.

 

[moxhamj]

I'm going to go back to Jeremy Leach's question and quote it; "I'm still not clear exactly what you are trying to do"

I'm not sure either.

Are you trying to detect a hit?

Are you trying to detect a hit in the centre? Is a centre hit better than a hit near the outside of the 7x7 grid?

If you just want to detect a hit, then you could use a simple lens and focus it onto a single LDR. Have a look at the cheap flexible fresnel lenses - only worth a couple of dollars, light, and the size of an A4 piece of paper. You can cut them to size. A hit anywhere on the lens will focus on the LDR.

If you want to weight inputs so that centre ones are worth more, then you can simply scan all the LDRs in sequence and work out which one is highest. Ambient light will give a reading the same for all LDRs. A 'hit' will make a few higher.

But you will need to make the transmitter pulse longer than the time it takes to scan all 49 LDRs.

 

[alband]

Dippy: I don't see the problem. It will always be used in darkish conditions but the level may vary slightly. Surely taking a continuous average of all the LDR's, using that to compare the other LDR's to by feeding it into one input of each opamp and it would work.

I can't use the scanning technique because of my circuitry.

Take a look at the code in previous post.

I want to know whether it has hit the target and where.

Once I've got the target sensor outputs to work with the PICAXE's inputs, It will all work.

The centre may be worth more to the user, but that bit is doen in the user's head. The system will simply tell him where in the 7x7 grid he hit. The laser dot only covers one LDR at the range required so lenses aren't needed. However, I keep coming a cropper when needing lenses and have never known where to get them. Those sheets of lens; I've never heard of them but they sound great . Are they made by "fresnel"?
Thanks.

 

[Dippy]

I don't understand your drawing from earlier....

That shows a pile of LDRs connected in parallel by diodes to the two big blue blobby things and the 2 blue blobby things connected at the bottom by black lines.
And the red +ve line connected to the bottom blue blobby thing marked 'PIC MICRO'.
Eh?

Whats all that about then??
Too confusing for me post-lunch I'm afraid.

 

[alband]

Perhaps you may understand after reading the accompanying post??

Attached is a 2x2 grid to show how the LDR's would be attached (currently the LDR's are labelled as PTR's because I first tried to to use phototransistors but they didn't respond to red laser light.

I go again FTHOI:
That is an old diagram made on powerpoint. Each LDR is connected to +V. The other pin of each LDR is connected to its specific row and column line via a diode. These lines in the old diagram are connected to the PICAXE (PICmicro) but we are currently discussing what these should be connected to (e.g. opamp).
Hope this helps.

Edit: Is this going to be possible using 16 opamps (however it is done exactly can be discussed later) because if it can I will order four of these (LM324 QUAD OPERATIONAL AMPLIFIER (RC)) now.

 

[Dippy]

Ah, I see. The sketch wasn't clear.

So, if the LDRs along top row are marked 1 to 7 and so forth, the resistance of the row is:-
(ignoring diodes)

Row1 : 1/LDRRtotal = 1/LDR1 + .... 1/LDR7
Row2: 1/LDRRtotal = 1/LDR8 + .... 1/LDR14

and

Col1: 1/LDRtotal = 1/LDR1 + 1/LDR8 +1/LDR15 ... etc.

I think, without a lot of help. you're going to end up in a big pile of confusion with op-amps.

How about using a couple of these and 2 ADC channels on your PICAXE?
ISL84051

http://www.farnell.com/datasheets/113263.pdf

Will that help?

 

[sghioto]

Sounds like I need to explain the basic operation of the circuit again.

You have 14 comparator circuits, one for each row and column.
When a LDR is hit by the laser the output from the comparators corresponding to that row and column will switch low which is recorded and processed by the micro.
To monitor the ambient light level of the array requires only 1 dual opamp as shown in the schematic. This opamp (IC1) is connected to each row and column through a 1 meg resistor. It's basically a 14 input buffer circuit.
Opamp IC1 will "monitor" the voltage from each row and column. The output from IC1 will be equal to which ever row or column has the highest voltage reading. IC2 takes this voltage and adds .7 volt so the reference is always higher then the array.
You could connect the output of IC2 to the + inputs of the comparators but it will still require somekind of "sample and hold" circuit before the laser is fired. When a LDR is "hit" the voltage on the corresponding row or column will increase which will then increase the reference so the "hit" may not switch the comparator. Just before the laser is fired the reference voltage needs to be locked. This can be done several ways by the micro.
For components I would use the LM339 chip (4) for the comparators and a LM358 (1) for IC1 and 2.

Steve G.

 

[eclectic]

David.
Coo, your project is getting seriously complicated (and expensive).

A few cheap and cheerful suggestions.

Either use a small fresnel lens, as per Dr-Acula's suggestion.
e.g.,
http://cgi.ebay.co.uk/ws/eBayISAPI.dll?ViewItem&Item=280297319023&Category=31416&_trksid=p3907.m29&_trkparms=algo=LVI&its=I&otn=1

Or something like this
http://www.rapidonline.com/Educational-Products/Science/Microscopes-Magnifiers/10x-Magnifier-with-tripod-stand/71119/kw/magnif

with one Unfiltered BPW34 detector.

I'm using a BPW34, without a lens at the moment.
It's happily receiving Serin data at N1200, from a cheap laser pointer.
It also responds to “gun” Pulsout values of 10 (100 &#956;s)

I've attached the receiver circuit.

e

 

[Dippy]

Mmmm... just a couple of questions.
(I confess I haven't got time to study Steve's approach).

"Just before the laser is fired the reference voltage needs to be locked. This can be done several ways by the micro."
- how does a 'remote' receiver know when the laser is going to be fired?
- or more precisely, how the remote receiver (LDR array+Op-amps+PICAXE) know when it's just about to be 'hit'?

Ecelectic,
Alband already has this LDR array, how can he easily change it to photodiodes?

And how will a lens help?

A very intersting project, all the best.

 

[alband]

Dippy: I'm not keen on multiplexing because it will make the code longer, slow down the chip and possibly muck up communications

sghioto: That all sound great, I found the LM339 on rapid. I will probably get this.

eclectic: I wouldn't use the lenses for this but thanks for the links. I think I would use the BPW34 if 49 of them didn't cost £53.90.

 

[eclectic]

Ecelectic,
Alband already has this LDR array, how can he easily change it to photodiodes?

And how will a lens help?

Don't use a slow/complex LDR system. Recycle them for other projects.

Use a cheap simple SINGLE target, either at the focus of the lens
(As Doc's post #28 suggestion)

OR, have the photodiode inside a curved reflector (as in satellite dish)
e

 

[alband]

I need to have position sensing though. That's why I'm using a 7x7 grid. The target gets a grid reference of the hit and then send the grid reference to the display next to the gunman.

 

[BeanieBots]

As mentioned by Dippy, you will encounter a few problems trying to determine "just before" a hit.
However, it should not be too tricky to design a self adjusting ambient compensation trip point. This is how PIR sesnors work. It's not much more than an op-amp integrator whos output is used as its own reference. Rapid changes cause a "trip", very slow changes just gradually change the trip point.

 

[alband]

"Just before the laser is fired the reference voltage needs to be locked. This can be done several ways by the micro."
- how does a 'remote' receiver know when the laser is going to be fired?
- or more precisely, how the remote receiver (LDR array+Op-amps+PICAXE) know when it's just about to be 'hit'?

The target constantly tells the display whether there has been a hit and what grid reference; it's in the code.

Perhaps I could just use an independent LDR, separate from the target, and use that as a self adjusting ambient compensation trip point.
That way, nothing will need to be locked, plus the wiring should be simpler.