I am in need of help to complete a home made laser beam photo cell detector timer, which I will use to time my kids horses in an arena for barrel racing. I have the major components but need to figure out how to change (very simply) from the photo cell being always on (while sending no signal) to sending a simple ground to the timer when the beam is broken. I have thought of a hold open switch, that when the beam is broken closes, but the response time has to be in the nano second range (my timer measures out to the .000) so I was wondering if anyone knew of a simple electronic chip that would do the same thing. (Commercial timers of this type retail for 800.00 bucks) any help is appreciated.

 

I replied to your other post in another thread but here it goes again.... use an NPN type transistor to switch your load to ground.... here is a simple circuit used in a similar fashion so you can see what I am talking about......

 

I was incorrect in my earlier description of the needed result, I only need to complete the curcuit (with no voltage) basically closing the switch. I know how to do it by turning the beam on to close the switch, I just don't know how to complete the switch in absence of the beam

 

is there anyway you could post what circuit you have now? It would be really helpful in finding a solution to your question....

 

Hi, sorry I was gone, I really don't have a curcuit built for the detector yet, all I have is the timer that has two leads coming out (external input stopwatch kit from hobby engineering) so I have the two leads that have to make contact to start the stopwatch, then the same two leads have to make contact again to stop the stopwatch. I wanted to use the laser for the light source for its stand alone ability, (like I said, 125 feet from the detector and I can't cross the arena with wires) but the detector can have leads travelling to the timer (because it sits at the arena fence). The photoeye is encased in a plastic garage door sensor shell (that was what I was going to try to use origionally, but the IR didn't have the range I needed outside) So what I need is a electronics guru who can lay out a schematic to cause contact between the two leads when the beam is broken (photeye normally closed, but not sending signal, signal sent when photo eye curcuit opens) Prefer to keep any electronic curuitry under 6 v so I can use AA batteries (portability, this thing stands on a camera tripod.) If I can make one work I am going to have my 14 year old daughter build one as a 4-h project and donate it to our county 4-H program when she is done.

 

PS. I did draw the curcuit out that I need but don't know how to get it to post. Any help posting diagrams to this forum? Thank you

 

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Ok, now I have a better understanding of what you need, basically you just need something that will trigger a "dry" contact switch to start/stop counter....... so, your photocell, will have to actuate a relay to close 2 contacts, and the counter module wires will have to just connect to the common and normally open terminals on the relay..... I'll have to do some research to see if I can locate the right components for you, is there a Radio Shack near you??

 

There is a Radio Shack, and my need for this device isn't immediate (I'm in MN, pretty darn cold for outdoor arena work) so online parts are fine too. The Cadmium sulfide photo cells that I have (radio Shack pt No 276-1657)have 18 ohms of resistance when exposed to light, and go infinite when darkened. If only I have a backwards photo cell (ifinite resistance when exposed connected when dark. Isn't that the curcuit that dusk to dawn uses? Open when exposed to light? (But I have to operate this on 4 AA BAtteries) Thanks

 

Don't know if I spelled that quite right, but one of the parameters of this curcuit is speed of the action. We are timing to the one-thousanth of a sec. so the curcuit needs to be quite quick. (But if all parameters are the same, there would be no variation in different timing runs, I guess the curcuit wouldn't have to be that fast.)

 

OK, here is a quick circuit I put together that just might do what you want, It can runn off of 4 AA's and it will use minimal current when the Laser is illuminating the photocell, the only time the relay will activate is when the light source is removed or blocked, which in turn will trigger your counter module to start/stop.

This circuit is basically what you where referring to as being "dark" on..... if the photocell and resistor were swapped in the circuit, it would make it a "light" on trigger..... and this is small enough to where the battery holder will be the bulk of the circuit....

You can substitute the fixed resistor for a variable one to adjust sensitivity...

I hope this helps....

 

Hello,

@BMorse, How fast do you think your relay will be?
The TS wants to trigger in mili second range.
Also contact bouncing could be a problem

Replacing the ralays with an optocoupler would overcome these problems.

Greetings,
Bertus.

 

That looks really good, thank you so much, but I don't have an electronics background, are t1 and d1 resistors and their respective part numbers? And is R2 a trimmer? Could you label each part in your diagram for me? If you could build a couple of sets of these (just the electronic emmitter/detector and timer part) for a reasonable price ($200) you could sell them all day long at horse shows. The raw material I have in it so far is about $45.00 (And That includes the camera tripods, used of course) I might try to transmit the signal wireless on RF(after I make this work on a wire of course) Then you could have the timer anywhere within range of the detector. (Such as up in the judges booth). There are plans out there to open up an electronic digital stopwatch ($6.00)and solder leads to the switches. Don't know how thats done but it would have lowerd the cost of my timer about 25 bucks. Your help on this was excellent and I appreciate it greatly.

 

Hello,

@BMorse, How fast do you think your relay will be?
The TS wants to trigger in mili second range.
Also contact bouncing could be a problem

Replacing the ralays with an optocoupler would overcome these problems.

Greetings,
Bertus.

I have been looking at that a bit, I see me and the op had cross posted.... I am looking for an alternate for his requirements, but his needs call for a "dry" contact closure, and at the moment the relay sounded good until he presented the "speed" issue.....

 

are t1 and d1 resistors and their respective part numbers? And is R2 a trimmer? Could you label each part in your diagram for me? If you could build a couple of sets of these (just the electronic emmitter/detector and timer part) for a reasonable price ($200) you could sell them all day long at horse shows.

Your help on this was excellent and I appreciate it greatly.

They are actually labeled.... T1 is a transistor (2N222), D1 is a diode (1N4148), R2 is a resistor, I had labeled it 10K to 100K (I was meaning that you could use one of those values, or anywhere in between, I would start with a 10K .... sorry I assumed you had some knowledge of this already....) or you can use a 100K potentiometer so you can adjust the sensitivity ("trimmer").....

I am building a similar circuit on a breadboard to test speed of the relay to see if it will meet your requirements, if not I will find a suitable one for you and provide part numbers.....

Do you have a part number for the stopwatch module from hobby engineering?? I cant seem to find it on their site.... I just wanted to double check the input for the module so I can see what will work best at the speed you need...

 

Hi again, I have the part number for the timer module and the part number for the stopwatch firmware kit They are HO1256-01a (timer kit) and K148T2 for stopwatch firmware. One of the problems with the hardware of the timer is that it uses an input for start and a separate input for stop. BUT, if you input a signal into the start button (same lead) after the first start, it will give a display on pause, (Display shows frozen time while the internal counter continues to run) which in effect is all the result I need. You would then have to do a hard reset at the timer for the next run through the light.

 

That definitely simplifies it even more now that I had a look at the specs for the counter module, you do not actually need a relay or dry contact closure, an open collector transistor setup will do just fine..... I will re do the circuit and have it up here for you tomorrow....

 

Hello,

Perhaps take a look at this FET optocoupler, see datasheet.

Greetings,
Bertus

 

The random jitter present in a relay is enough to affect the accuracy at .1 ms, and voltage variation will shift it +/-30%. you will need a regulated supply if you do use a relay, such as a 9 volt battery and a 7805 regulator, using a 5 volt relay. also its possible the contacts will bounce, though this isn't normally the case.

The photocell rise and fall time should be in the microseconds, so no need to worry there.
I'd imagine one of those leads from the timer is connected to the battery (-) directly, and the other will have up to the voltage of the internal battery across it (3 volts maybe?) its safe to connect those leads directly to the timer, provided they are close together, if not then you risk blowing up the timer, as it may be sensitive to ESD and such.

Edit: didn't notice there was two pages.
An optical coupler is the solution.

 

Ok barelracer, I think this should do it for you, the turn on/turn off times of the optocoupler I chose is in microseconds, so the switching times should be pretty fast.....

You can get the optocoupler from here >> http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=751-1342-5-ND

the CDS Photocell I used in my test setup was this one >>http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=PDV-P5001-ND, you might have to double check your laser output to see what it is rated for, this particular CDS is in the 520nm range....


here is the circuit....

 

^Thank you , that looks to be a very easy curcuit to build, quite simple. The only question I have is the wiring at resistor 3, does that indicate an inline on/off switch? And the ground points for the battery and the curcuit (are they a common ground?) Thanks for all your help BRD