Submitted by RCFisher on Wed, 08/11/2010 - 21:31
Posted in
I am setting up box to trigger 4 cameras at the same time and I just wanted to run this by someone. My OK is a 4 channel device with a forward current of 60mA per channel. I am going to feed 5v through a offboard switch to all 4 channels in parallel. So I add all 4 channels current say 50mA that would be 200mA (.2). So now I divide 5 by .2 and I get 25 ohms. Is that correct? I guess it doesn't matter which side of the OK the resistor goes on either.
Thanks
Also would a 1/4 watt
Also would a 1/4 watt resistor be right or would a 1/2 watt resistor be better?
You're missing one piece of
You're missing one piece of information: LED forward voltage, need that to calculate R value.
Here's a short-cut:
R = (V-F)/A
So, if V is your circuit voltage, and F is the forward voltage of the LED, and A is the forward current: (presuming F is 1.2V)
(5-1.2)/0.05 = 76 Ohm (for each channel, do not wire all channels through one resistor)
The resistor can be on the GND or the +V side of the LED, doesn't matter.
Now, to calculate resistor wattage requirements, we know that W = VI, so to calculate the amount of heat dissipated through the resistor: (5-1.2)*0.05 = 0.19W, so you'll need at least a 1/4 watt resistor. You can do half-watt, but I don't see the need.
!c
Alright the OKs I'm using are
Alright the OKs I'm using are NEC PS2501L-4. I found the forward voltage it says 1.17 typ.-1.4 max. and the forward current is 80mA. I am also using a regulator to bring the 9v battery down to 5v, it's running through a switch and sometimes through a radio trigger so I guess the 5v would be a better bet. Ok so I use say 1.2v of the OK and the 80mA, (5-1.2)/.08 = 47.5 or 48 ohms. I tried using 1.17v but it really didn't a huge difference .3 ohms. So each channel would get a 48 ohm resistor. I also just heard from the guy with the cameras that you have to trigger both focus & Shoot lines for consistent results. They are using Canon T1i cameras. I just calculated the wattage and I guess I need 1/2w resistors I come up with .28W using the 80mA current rating of the OK. I think I get this now, kinda. could I use less current say 50mA for a smaller current draw on the battery?
It's been a long time since I had electronics classes, 30 years and counting. I haven't done much with it over the years so it's starting over again!
Thanks Church
The problem with reducing the
The problem with reducing the current, is that you're feeding a transistor with light. If you reduce the light, you reduce how much current flows through the junction, and with some cameras, it can cause it to not recognize the signal (my K10D can trigger off of half the voltage that my K7 refuses to accept). I'd target 80mA for proper function (no resistance on the CE side of the transistor).
You could probably use a switch or jumper to disengage the focus lines for cameras where it'd cause problems to have both high. (Of course, there's no real need to if they really want it, but you never know... =)
At over a quarter watt of dissipation, you'll definitely want the half-watt resistors, better safe than sorry and there's not a huge price difference (sometimes none) between the two.
Always glad to be of assistance!
!c
Well it seems most cameras
Well it seems most cameras need focus high to fire the shutter, I know Nikon does and according to my friend so does Canon. For my OpenMoco rig maybe I should set up a 2 transistors to fire both OKs at full current since now they are both in parrallel on the same lines of the Arduino. Will the 5v on the Arduino provide enough current?
thanks Church
The 5V line can provide
The 5V line can provide enough current, I think =) I use transistors on the dollyshield for tripping the dual OK and the LED backlight control. I notice there is quite a bit of current drain when both OKs are firing (sometimes the LED bkl dims, even though I've restricted its current quite a bit).
Odd about Canons - I know Milapse and others are using them, and the only people who've ever asked for focus high w/ shutter were Nikon users - Milapse seems to have no issues w/ his Canon and just using the shutter line. Even other designs I've seen only add that function for Nikons. Perhaps he had some issue with another intervalometer, or its that specific model of Canon?
!c
He has a 4 camera rig for
He has a 4 camera rig for doing panoramas and has tried a ton of ways to trigger the 4 cameras mostly with highly mixed results. I suggested total isolation and using relays or OKs 2 years ago when they came up with this but they went another way and in the end came back to me. Shock! Any way when the cameras come in, I am hoping he sends me 2 cameras at least, I will be able to test that.
What would be a good transistor to use? I figure 2 one for focus and on for shutter with resistors for the output. I will draw a schematic this afternoon.
Hi Church Would this work? I
Hi Church
Would this work?
I am using this with the OMTLE this is just the shutter the focus would be the same except for the pin to connect to.
Robert, You should have a
Robert,
You should have a resistor between the Base and the Pin, to keep it from drawing too much current, and stabilize the transistor. See http://www.kpsec.freeuk.com/trancirc.htm#npn -- or you can just copy the design from the DollyShield that uses the 2N4124 and a 5K resistor between the Arduino pin and the trans. Base.
Also, when you finish your 4-way shutter trigger, would you mind doing a tiny write-up? Just the circuit design, etc.? I know this has to be a common topic =)
Thanks!
!c
I was wondering if I should
I was wondering if I should have a resistor between the pin and the transistor. How do you figure that or is it an eyeballing experience thing? The data sheet has a ton of info that I guess is useful to someone.
I will do a write up when I am done but the 4 camera trigger is pretty simple. A switch, a regulator, a pair of OKs and some connectors. Now the other day the guy also asked to be able to plug in a radio remote to trigger as well as a pickle, not at the same time. I am sending out raw 5v 1A from the regulator to the pickle but do I need to limit the current to the radio receiver? It's a cheap Chinese set up but I would think there is an OK on the output plug to the camera.
My more complicated project is the OMTLE I am working on which is what the transistor driving the OK is for. This after your reference to having the correct current driving the OK for max output. Driving 1 OK is easy but to get correct current for a pair of OKs driving 2 cameras now I need a transistor to drive the pair of OKs so they each get their 60mA of current. Now they only get 40mA for the pair so they are operating at 1/3 less output. This may have been the cause of a few issues I had. The main thing is the OKs are on a 25-30 ft cable to the cameras shutters so on the desk it may be ok but may loose enough current to not work properly in the final rig. Is this a reasonable assumption?