Electronic Circuits and Projects Forum

30th April 2012 02:36 PM

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: http://www.motleypixel.com/forum/ind...ic,1121.0.html

I'm controlling the camera with a smart device called a Little Bramper: http://www.thewhippersnapper.com/Lit...Site/Home.html 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?

Thanks!

30th April 2012 03:01 PM

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

30th April 2012 03:42 PM

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

30th April 2012 03:52 PM

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

30th April 2012 05:13 PM

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

30th April 2012 05:28 PM

Originally Posted by Reloadron

... 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

http://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

30th April 2012 07:23 PM

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

30th April 2012 07:47 PM

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: http://www.motleypixel.com/public/po...able_setup.jpg

Thanks,
Roy

30th April 2012 08:19 PM

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

1st May 2012 02:42 AM

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

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Electronic Circuits and Projects Forum

LED signal to open 7VDC CKT?

LED signal to open 7VDC CKT?