LED signal to open 7VDC CKT?

[motleypixel]

Hello everyone,

I've lurked here the past two days, so I've read a bit and feel I needed to register to get some dialog going on this project.

Quickly first (since this may be a long post).

Goal: Electronically open a 7VDC power leg (essentially an electronic switch) when a green LED is energized. The LED is from an independent system. The pwr supply that needs interrupted during the period of voltage drop across the LED is a separate system.

First post, looking forward to leveraging this community. Background: In a nutshell, I have a business degree, but was 6 years US Navy Submarines (reactor operator). I know fundamentals with regards to electricity and some solid state stuff, but really zero experience in design.

I have a one-off requirement and was wondering if this community could help.

First of all, the system: I have built a DIY Time-Lapse Photography Turn Table: **broken link removed**

I'm controlling the camera with a smart device called a Little Bramper: **broken link removed** Essentially this device can bulb-ramp the shutter.

I'm panning the camera with a worm-gearbox that has a 30:1 gear ratio, powered by a 7.4VDC 2-cell lipo battery, speed controlled by a simple PWM controller. The controller is variable from 0-100% modulation and will recall last setting upon pwr interupt.

Problem: As the turn table rotates very slowly (1 rev. per hour) and the camera begins taking pictures (starting out as a shutter open duration of 40ms or 1/40th sec and ~ 3 second delay before the next shot and then after time goes on the shutter open duration increases to the tune of 2+ seconds). I have found that shutter speeds slower than 2 seconds, with the moving turn table, produces motion blur that is too excessive.

Best solution: Stop the power to the speed controller which will stop the power to the DC 1rpm motor, which will stop rotation of the camera.

How to implement the solution?: The good news is that the Little Bramper has a Green LED that energizes for the exact duration of the shutter open cycle, so if the shutter is at 1/40th second, then the illumination period of the LED is 1/40th second, same with a 2 second shutter speed, etc.

I would like to find a way to harness this LED signal (either light from the LED or Vd across the LED) to electronically open the pwr CKT to the motor speed controller.

Any ideas here?

**broken link removed**

Thanks!

 

[diy didi]

use a transistor to power off a relay that switches the motor.

 

[Reloadron]

Cool project. I would start by seeing exactly what is across the LED as to a voltage when it is on. While a relay could be worked into the scheme as mentioned I might think about using a DC SSR. Really depends on the motor current?

Unrelated but what is the camera in the image?

Ron

 

[motleypixel]

Thanks guys, I don't want to seem like I want to be spoon fed, but based on my knowledge of ckt design I'm hoping to find some help on the actual design. I know of a few outfits in the UK (not sure what's up over there but there seems to be a number of small hobby folks that love tinkering with electronics) than can do it for a nominal fee, but I really want to try to pull this off and learn some along the way.

@ Ron, it's a Canon EOS 5D Mark II with a Canon EF 16-35mm f/2.8L USM Mark II lens.

I haven't actually read the voltage drop across the green LED but my presumption is that it's ~ 3VDC. The current draw has to be incredibly low for the turn-table ckt. I know this only based off of one hour of continuous run only pulled 35mAH out of my battery. The gearbox and motor operate extremely slow, but fast enough that I need to turn off the circuit when my shutter speed goes to 2 seconds or slower. I'm fortunate enough to use the Little Bramper because the green LED illuminates for the duration the shutter is open which is when I want to stop the turn-table.

I would like to keep this simple and if possible eliminate the need for another power source. I think a SSR would work but again, haven't a clue where to start.

Thanks,
Roy

 

[Reloadron]

I'll have something for you hopefully this evening. I'll likely use a small relay driven off a small opto-coupler. The idea being to keep the current down. The relay will only be energized when the LED is on. We will use the LED power to drive the opto-coupler and move the existing LED to the relay contacts. This should work. There should not be much to it and easy to assemble with off the shelf parts.

I thought the camera looked familiar and Canon. I have an EOS7D I like and an older EOS10D. I wish I had more time to play around with my photography.

Ron

 

[motleypixel]

... and move the existing LED to the relay contacts. This should work.

Ron

Cool thanks Ron. I'm into photography way more than electronics design. I actually like some metal fabrication too. I enjoy converting old Minolta lenses which are not kind to adapters for canon because of optical registry distance so it takes some machine turning https://vimeo.com/14891384 Just showing that I can do some things on my own

As for your quote snipit above, I'm not following you here, we would move the greed LED? I suppose I could whip up a quick video of how this all works, but for now here's an explanation:

There are two independent systems here which reside w/in 10 and 15cm of each other when in operation. The green LED is on a system powered by 4 AA NiMh batteries (The Little Bramper). This system connects to the camera to fire the shutter and operate the camera under user programmable interface of the Little Bramper. Then this system is fastened to a turn-table that carries this system. The turn table is powered by a 2 cell LiPo 7.4VDC battery connected to a simple PWM speed controller which is connected to a 30:1 gearbox. It's the positive or negative battery leg to the PWM speed controller that I need interrupted when the green LED is energized. The duration of the green LED emitting light is equal to the camera's shutter in an open state, this is when I need to turn table to stop rotating. Ideally I would want this simple ckt powered from the 7.4VDC battery for the turn-table and only have a connection to the Little Bramper's green LED voltage drop state which should be roughly 3VDC when energized. I also have to realize that the connection between the two systems will need to have extra length (signal cable) because the Little Bramper green LED rotates with the camera and the turn table drive system stays put. I generally only see one complete revolution per hour and never really go any further than that so the cable twisting will be minimal.

Low current on the turn table system too since only 34mAH used in one hour...whimpy...SSR could work here I think?

Thanks,
Roy

 

[Reloadron]

Attached is a rough of what I believe will work. Unfortunately I left out the LED that would replace the existing. There would also likely be a resistor in series with the internal LED of the opto coupler. Hard to say unless we really know that LED voltage.

In a nutshell the motor runs as normal through the normally closed relay contacts. However, when the LED would have been ON the relay pulls in and drops out the motor.

Problem is right now I am jacked up at work as Wednesday I head to Missouri for a Marine Corps reunion and some get away time.

If another member has any thoughts or corrections to my thinking on this please jump right in.

Electronics is bread and butter here. My hobbies are shooting and photography. I wish I had more time to combine the electronics with photography too.

Ron

 

[motleypixel]

Alright, a schematic already!? Well first of all, many thanks for cranking this out so fast and I hope you enjoy your time in MO later this week. I’m down in TX and have in-laws in the Jeff City area.

Couple of quick observations:

1. Not sure what an opto-coupler does (I’ll look it up), but in this situation I see it as merely replacing the std green LED with this component? I guess I’ll still have the visual LED during operation as normal? Looks like this component has 6 pins where pins 1 and 2 (+/- respectfully) hook up to the original landings for the green LED and then only pin 4 is used as an output to the negative side of the motor?
2. Tonight I will measure the voltage drop across the green led just put multimeter on DC and read across the leads or do I need to measure from cathode to negative side of power supply?
3. Hummm now it’s trying to visualize the connection to the turn-table system. See the motor is connected to a simple PWM speed controller, so do I want to open the ckt between the output of the speed controller and motor or open the ckt before the speed controller?
4. So you have this drawn that the relay will open the ckt powering the motor only so then the #4 pin of U1 connects to the negative side of the motor, then the #4 pin of the relay connects to the positive side of the motor. #5 pin of the relay and U1 are common and then #2 of the relay connects to the positive side of the battery. Then if I read it correctly the #6 pin of U1 and #1 AND #3 pins of the relay are ground or the negative side of the battery?

I don’t feel good about my negative/positive layout…I’m confident I have something misunderstood.

Here’s another annotated picture too: **broken link removed**

Thanks,
Roy

 

[Reloadron]

My bad as I forgot the PWM but not a problem at all. Just a minor wiring change with the relay contacts.

1. As simple as possible an opto coupler is like a tiny LED (you don't see) that turns a small transistor on and off like a switch. Not all the pins are always used. Thus you see pins to nowhere in the drawing.
2. Just measure across the LED pins. There is likely a current limiting diode in the LED circuit but just measure the voltage across the LED. No problem if you measure the leads backwards, you will just see a negative number. We only care about the number.
3. Again, forgetting the PWM was my bad. Not going to be a problem.

I will modify the little drawing in the morning here at work. I will also give a better more in depth explanation.

Ron

 

[motleypixel]

Okay Ron,

When green LED off and negative lead of multimeter on the negative terminal of the power supply and positive lead on both cathode and anode of LED reads 3mV.

-1.6mV across the LED when off.

2.01VDC across the green LED when energized...this is my signal and will be a period of anywhere between 40ms and 30,000ms. As a quick enhancement/thought, if the CKT could be tuned to ignore any signal less than say 500ms or user defined that would be ideal. Because really the only need to stop the turn table from moving is for shutter speeds greater than ~ 1000ms. Just a nice feature, but not a necessity.

Again, the original purpose of this green LED is just to signal the user that the Little Bramper is sending a bulb-shutter-hold to the camera i.e. Green LED is lit for the duration of the open shutter, which is the period I want to stop the motor from driving the gearbox which rotates the camera to limit motion blur. replacing it with an optocoupler will give up this tell-tale indicator. Although now I have the power interrupt to the PWM which has a green LED for power so when that green LED goes out will mean the shutter is open. Just thinking about the whole operation.

Hope this helps...really looking forward to working up a proto-type!

-Roy

 

[Reloadron]

OK, the attached should give you what you want. Looks good on paper anyway.

U1 replaces the existing LED. That being the opto-coupler. Anytime the original LED would have been ON (Shutter Open) U1 will be on. When U1 is ON it in turn turns on the relay. When the relay is ON it breaks the PWM motor drive path. The existing LED gets moved to the circuit and is shown as Moved LED in the drawing.

Yes, a delay scheme could likely be worked into the routine. However, being short on time before I get going on that trip I don't really have the time to put into it. The attached should be a simple workable solution. I would try this and see if it works out.

Ron

 

[motleypixel]

Okay, this is great! Maybe my local Fry’s will have U1 and the SSR, else I’ll have to hunt them down on the interwebz. Okay, I’ve printed it out and then scribbled a diagram as if I was to actually lift the existing LED, solder wires, etc., trying to visualize it all.

Alright so the U1 part is pretty simple…pwr source is already available, so it’s just a matter of de-soldering the existing LED and using short wire runs to connect pins 1 and 2 (keeping polarity correct). Looks like pin 6 is open and that’s all for that component.

Now just a couple of quandaries on the other side of the CKT you built. My power source will be that 2-cell LiPo 7.4VDC pack. I measured the pack at no-load with my multi-meter and it actually read 8VDC…opps…not sure how much that will bias resistors in this CKT?

Looks like pin 3 of the SSR is open?

Now, power common will be my negative leg of my battery correct? This is sort-of an isolated system w/o any type of chassis ground.

And finally, my PWM speed controller, trying to visualize how it’s hooked up now and I’m not quite translating it per the new CKT. The PWM speed controller has a P/S input (+/-) wires from the battery and the O/P of the controller has +/- wires that connect to the motor. I made the O/P connectors of the speed controller non-polarity connectors so I can swap the polarity to the DC motor to reverse the direction.

So would it look the “crude” edited attached image below?

Thanks,
Roy

 

[Reloadron]

You pretty much got it. The relay in my circuit is just a tiny mechanical relay. The opto coupler is not an SSR but a very common opto coupler. The relay I used has a 6 volt coil. Just about any common off the shelf SPDT (Single Pole Double Throw) relay will work. If you have a local Frys they should have no problem fixing you up. I took it that your motor common (-) lead is circuit ground so drew it that way. Your change should be fine. The unloaded 8 volts will be fine, I used a 22 ohm resistor in series with the relay coil which should be fine between 7 to 8 volts and even when the battery voltage drops a little below 7 volts. Pin 3 of the relay is intentionally not used. The 1N4002 across the relay coil is there to act as a flyback diode and snub the inductive kick when the relay coil field collapses. It should work as drawn with your changes just fine.

Ron

 

[motleypixel]

Excellent, thank you Ron, it's easy to visualize that relay as the switch to open the + leg to the motor. I'll make a non-polarity plug pig-tail off of pin 4 of the relay and the negative lead from the output of the speed controller and that way I can swap the polarity to the motor and reverse the direction.

I assumed that the relay was a SSR, like a IC pack of sorts.

It's off to component shopping now...this should be fun. Thanks for all of your help and I'll be sure to post results.

-Roy

*Update:

For the relay, here's the data sheet: https://www.electro-tech-online.com/custompdfs/2012/05/az8.pdf Wouldn't the Light Duty 3 Amp work as well (AZ8-1C-6DE)? I will have to measure the amp draw of the system under load and see. The only data I have is that the system took ouT a mere 35mAH after an hour of continuous operation.

This is U1 correct? http://search.digikey.com/us/en/products/4N25/160-1300-5-ND/385762

 

[motleypixel]

I’ve found the American Zettler relay on-line where Fry’s doesn’t carry it but does carry this one from NTE (R46-5D3-6): https://www.electro-tech-online.com/custompdfs/2012/05/R46.pdf Differences being it’s 5A and the coil resistance is 70ohm.

I could use this instead right? If so, then I see the hook-up per your CKT design you provided me to be identical correct?

Thanks,
Roy

 

[Reloadron]

OK, as to the relay. There are a few considerations. First I wanted a high coil resistance so the relay coil current would be low. There are two reasons. The most important was how much current the opto coupler driving the relay coil could handle. I had to work within the limits of what the opto coupler could drive. That was also why I went with the more sensitive relay. The 4N25 Opto Coupler has a max forward current of about 50 mA and I wanted a margin below that max. So I wanted a coil resistance above 120 Ohms. Even though the duty will be intermittent I didn't want to push my luck. Like I mentioned any SPDT relay will work but we can't let the relay coil draw too much current. The rest of the logic was based on battery longevity.

Now Fry's may know of a better opto coupler to use with a relay drawing more current. Maybe an opto with a "Darlington Output". As long as the opto used can turn on into full saturation (full on) with the about 2 volts you have to work with and handle the relay coil current.

I suggest you show the sales rep at Frys the schematic and seek some advice. Hopefully a sharp rep will see and know what they are looking at. I don't have much time to start researching optos and other relays right now.

Ron

 

[motleypixel]

Roger that...I can find everything on-line just as you spec'ed-out. I'm not surprised to hear about resistance changes and current effects. Thanks!

 

[Reloadron]

Let me know how it goes.

Ron

 

[motleypixel]

Parts on order, had to get the 22ohm resistors at Frys. Did up a wiring diagram, now just need a breadboard to test or I may learn how to dress that copper clad board and make my own runs etc.

 

[diy didi]

I like the cct, but dont understand why an opto isolater is needed??