LED Photo studio strobe

Hi all,

A couple of weeks ago I bumped into some LED panels (48 piranha LED) which were cheap enough to consider them for the following project. I was thinking to building a studio strip-light strobe; for whoever is not familiar with the term this is simply a photo-studio strobe but in a shape of a long strip not a square, triggered either by another flash (slave) or directly by a remote (master). I will not get into the details of why building it.
I used the attached schematic, which is fairly simple and so far the test results are ok. The logic behind it is to actually overload the panels with an impulse of max. 0,1s with 10% duty cycle. The panels are rated at 12V/300mA and yes, I know I might burn them, or one of them, by increasing the current, but I'd really like to build them. The characteristics I found on the Internet so far are saying about a ratio between 1/3 - max 1/10, between the rated current and the peak current. While I'm not sure I will peak the panels with 3A individually from the very beginning, I'd like to experiment with 1-2A peak.

0,1s is more than enough for studio shooting, most of the cameras go out of sync after 5ms so I can easily lower the duration of the impulse. The FT is not very sensitive, which is good, right now it can be triggered either by a very close LED flash-light or by the built-in camera flash. In the schematic, only one panel is shown, I will actually use 5 either in parallel or put more MOSFETs. I still have to tweak the MOSFET saturation and the panel currents, this is an ongoing work.

My questions so far are:
1. How can I reset the 555 timer in such a way that after 0,1s (T) is stays off? In other words, if the input impulse is longer than T, let's say 1 sec, then how can I make sure that the output is reset after 0,1s? By this, I would like to prevent burning the panels in case of an accident when there is a continuous and strong light in the FT. Also in other words, I would like the 555 to be triggered on the raising front of the IN-impulse and not to actually care about the duration of the IN-impulse.

2. Just as my regular strobes in the studio, I would like to have some sort of a modeling light. In the classical strobes, this is achieved by having 2 lamps in the head, one regular bulb and the strobe itself. The bulb is continuously lit and it is used not only to actually see what you shoot but also to have an idea about the shadows and help the camera to focus; when the camera is triggered then the strobe flashes.
I was thinking that maybe I can achieve this by having the LED panels normally powered and then trigger only a spike for actual shooting (mimicking the strobe). This idea is in the 2nd picture, in which just a "black-box" is shown; I need some sort of a circuit that can control the current going through the LED panels and in the same time to respond to the impulse given by 555.

I'm opened to suggestions.

Nedix

In response to your question 1, try this:

I have used this solution before to overcome similar problems with other applications using a 555.

JimB

Thanks JimB, I tried the option above but it isn't working; this is because impulse is too weak to get past the capacitor and still fire the TRI. An option suggested on another forum was to use an Op Amp but the basics are still the same. I will put in on a breadboard tomorrow and see the behavior.
I still need to figure out #2.

OK, I was making the assumption that the phototransistor would turn on hard when it sees light.

If the opto was an ordinary switch I am sure the thing would work (he said hopefully from memory!).

JimB

To light the LEDs dimly then simply connect a resistor parallel to the Mosfet. The "flash" will still operate like before.

Measure the voltage at the collector of the photo transistor in bright light and normal light. You may need to adjust the resistor pull up.

I have some leds that I use for driving lights on my off road vehicle and I use a boost circuit and for the feed back, I have an inline resitor that is matched to the current needed, something like this would probably work for you also. Hope you can run this sim, if not, post and I'll post the results. What happens is the lights come on at a percentage of your choice the go full bright of over or under by you settings. What LEDs are you using, what is their voltage and current rating?
PS, this circuit was designed around 24 3 watt bright white LEDs in series and 1 amp of current.

audioguru - could you please sketch a schematic for the resistor

kinarfi - the panels are made of 48 LEDs and one panel is rated at 12V/300mA

How many panels do you have? can you run spice?

Here's a couple more ideas

I have 5 panels, each have the following detailed specs:
Brightness: 200 lumens
Lamp beads: 48
Color Temperature: 5500K-6500K (white)
Operating voltage: 10V-14V
Operating Current: 300mA
Rated Power: about 3.2W
Life :50000-80000 hours
Beam Angle: astigmatism (about 150 °)
Dimensions: 74 * 51mm
Weight: 30g

I worked with Spice 17 years ago, when it didn't even have a GUI, but I'm rusted like crazy with it.

I will try getting a free version of spice and run the files you provided. Meanwhile, what if I simply put a voltage divider on MOSFET's Gate but in such a way that it is not fully saturated? It should open but with higher resistance thus just dimming the LEDs.

I found this:
Triode mode or linear region (also known as the ohmic mode)
When VGS > Vth and VDS < ( VGS – Vth )
The transistor is turned on, and a channel has been created which allows current to flow between the drain and the source. The MOSFET operates like a resistor, controlled by the gate voltage relative to both the source and drain voltages. The current from drain to source is modeled as:

where is the charge-carrier effective mobility, is the gate width, is the gate length and is the gate oxide capacitance per unit area. The transition from the exponential subthreshold region to the triode region is not as sharp as the equations suggest.
The equation is indeed quite complex but I think it can be simplified. The point is that perhaps I can keep the MOSFET in this stage, working for the modeling light and then, after applying the trigger impulse for it to switch to saturated mode.

I prefer to think of the FET as a switch, on or off, that way you have no heat build up.
What do you have for a power supply for these? May I suggest a cheap 4 amp battery charger and handfull of 25volt caps, say 5 1000 uf caps.
What is your trigger for this? It only needs a spike to trigger, but will only run for what ever time you set it for if its a longer duration than set time, I may have components that need changing for longer time period.

Where in this world are you?

Nedix said:

audioguru - could you please sketch a schematic for the resistor

Click to expand...

Why do you need a sketch of a simple parallel resistor? Here it is.
Add a switch in series with the resistor to turn off the dimmed LEDs if you want.
Use Ohm's Law to calculate the value and power rating of the resistor.

I'm in Germany.
I will power the schematic with a 12v/10A switching power supply, I've already ordered it. The trigger will be either an FT that will respond to the master flash or the receiver of a remote which actually just "grounds" the FT; the TX of the remote is mounted in the hot-shoe of the camera. The provided schematics look nice, except I would like to make use of some IRFZ44N which I have around.

audioguru - "I can see clearly now the rain is gone" . Thanks

A Mosfet that is partially turned on with a divided voltage at its gate is a thermometer. Its current increases as it gets warm and driving all these LEDs it will warm up.
It is an interesting effect but you do not want it in your photo studio.
You would need to add an error amplifier to stabilize the current in the partially turned on Mosfet.

Nedix said:

I'm in Germany.
I will power the schematic with a 12v/10A switching power supply, I've already ordered it. The trigger will be either an FT that will respond to the master flash or the receiver of a remote which actually just "grounds" the FT; the TX of the remote is mounted in the hot-shoe of the camera. The provided schematics look nice, except I would like to make use of some IRFZ44N which I have around.

Click to expand...

IRFZ44N should do just fine, it's much more powerful than the 2N7002 I modelled. Your power supply should do fine, IMHO, the battrey charger idea just gives you a little more overhead to play with if you use a LM317 to control the current, the schematic posted by Audioguru looks good too, your lights can take the full 12 volts, all you need to do is figured what resistor to use, with a the LM317, it's easy to set the current with resistor in the output lead, also you only need one resistor, not 2.

Thank you all for the advises, the resistor in parallel works just fine. However ~!@#$%^&*() I've started with the wrong assumption that the panels work just like a big dumb LED rated @ 12V/300mA - I couldn't be further from the truth. It turns out that the panels have a built-in resistor like in attachment, so it seem that no matter what I do, I will always have a limited current on the panel. Shorting the resistor will get them brighter but I am thinking of "cutting" the contacts on the PCB and connect them like in the 2nd attachment.
What strikes me so far is that even though the MOSFET is saturated, Vgs > 10V, I still cannot draw more current from the source. With all 5 panels, the schematic barely draws 0,8A when it should have been a little over 1,5A (5panels * 300mA/panel * xxmA rest of the components). I tried with other MOSFETs (also IRFZ44N) and I even replaced it with a BJT with the same result. If I connect a panel straight to the power source it does draw ~250-300mA.

PS: these tests have been done by connecting the TRIG pin (2) of the 555 to ground