I've just finished the Camera Axe 4. It's the forth generation of my high speed photography trigger. It is a custom PCB, but I open source that and all the software. It uses the Arduino bootloader and the main advantage of that is it makes it easier for others to update the software if they have a special need.
If I may ask, what kind of circuit are you using to drive old style flashes (you know, the kind that can go up to 300V where just the contacts are shortened)?
Very impressive! (both the design of the project itself, and the photos you show from it)
EDIT: having looked at the Buildguide, ...wow! That is the most carefully, completely, and also beautifully photographed electronic project that I have ever seen. VERY impressed.
Korman, currently the Camera Axe only supports flashes up to 30V. If you are using an old flash with high voltage you need to drop the voltage before sending it to the Camera Axe. I'm actually working on an external circuit to do this. Doing it with a circuit and power is very easy (just an SRC), but I tried to do it without external power and it didn't work. I'm sure it's possible, but I'm not very good at analog circuit design. If anyone has a suggested circuit to drop a flash's trigger voltage that doesn't require a separate power source I'd really appreciate it.
Would one of the SCR circuits shown below work? They are shown with a 9V battery, but I'd think 5V might work also. All you really need is the high-gate sensitivity SCR. You can apparently get them with 400V standoff rating, which covers any (small) flash unit.
Or did you want to completely isolate the flash unit (no ground connection between circuit & flash)? You can get an opto-isolated triac with 400V standoff and 5000 V isolation, for example Fairchild MOC302XM, http://www.fairchildsemi.com/ds/MO/MOC3010M.pdf
In either case, it seems to me the only power supply you need is the 5V you are already using to run your CPU. For the MOC3010M, you just need to drive 15 mA into the IR LED inside the part, easy to do directly from the processor pin with one series resistor. You can use this to drive an external SCR that carries the trigger current, see this schematic:
just to clarify, that schematic I included assumes you have a plain switch closure or relay in the camera hotshoe, so it's connected through a 6V battery. For your circuit, you would just drive the LED in the optoisolator (pins 1,2 on OPTO1) directly from a processor output port pin through a resistor, to ground.
Thanks John. This is a different approach than I took last time so it might work. Here is a circuit for the Camera Axe(http://www.cameraaxe.com/files/CameraAxe4/CameraAxe43_schematic.png). You will notice I already have optoisolation for the camera/flash triggers. The reason I didn't go with a high voltage optoisolator like you suggested is I need 4 of them and I couldn't find a quad optoisolater at the higher voltage. You will also see that I don't have any 5V signals on the flash cable. I could use a splitter cable off a sensor port and use 5V from that, but I'd rather avoid this.
Your circuit seems promising, but I would need to reduce the voltage going to the optoisolator. Do you think I could use a zener diode to drop this voltage?
I am currently doing a similar project. Looking at your layout it appears that the camera axe code might run on my hardware as well even though it is quite different. At least after some modifications. I will have to try this once my hardware is ready
Modifications that I would have to do:
Modify for running @8MHz / 3.3V.
Modify for software driven back light step up converter.
Modify for different key layout.
It's great to hear that you might be able to use some of the Camera Axe software! I like it when my software gets reused. Keep us updated.
I had the case milled by a company called polycase. I really like their work, but it only makes economic sense if you're having a bunch of cases made. I did make a prototype case myself. It was fully functional, but it didn't look nearly as nice.
With regard to the optoinsulator issue discussed above: this circuit will not work with some low voltage flashes since they they will not stop sending current through the thyristor. Has anyone here ever found a circuit that can deal with both types of flashes?
Note- not my schematic, I just copied it from the linked web page. It was designed for trigger by a mechanical switch, which is why it calls for +6V. You don't want or need that if you are driving it from a port pin on the Arduino, which can supply up to 40 mA. The MOC3010 datasheet says you only need 5 mA, you control that with the resistor R1. So you drive pin 1 of the MOC3010 from your processor, and you want something like this:
Now, it has been pointed out that once it is triggered, the SCR will not shut off until the external load current drops to zero. Apparently the new low-voltage flash triggers keep supplying current, so you cannot retrigger unless you disconnect the flash- obviously not ideal. In that case I guess you'd want a high-voltage transistor, instead of an SCR. I don't have a circuit handy for that, but I think it's possible.
I made one of the circuits above and it does a good job at protecting the Camera Axe from high voltage flashes. However, as mentioned the SCR doesn't work with newer flashes. John suggested a high voltage transistor might work. I can certainly make a circuit with the SCR replaced with a transistor, but I'm wondering if anyone has addtional input on this idea. I've always seen people using SCRs for flash triggers and I wonder if there is some hidden issue with using high voltage transistors.
Just as an experiment I tried a low voltage transistor with my low voltage flash and that worked so that's promising. I'd need to order a high voltage transistor from mouser to try it. Any suggestions on what transistor to use? Otherwise I'll just pick one...
If you use a HV transistor you will not opto insulation anymore. Unless you get a separate power supply for the HV side. This is possible but will complicate your circuit somewhat. So in the end - in order to keep cost down - it implies that you will get a common ground connection and the risk that high voltage is input into your circuit. It follows that your circuit might under unfavoruable conditions damage a possibly pricy camera.
I have a similar issue because I found that opto insulators are either to slow or require additional power from somewhere. Hence I decided that I will not support anything above 30V. Works for all new cameras. In order to keep HV out I added suitable TVS diodes. The idea being that I want to short circuit any HV device and sacrifice my circuit if necessary. Notice that any HV transient with to much power would blow
the TVS at the corresponding outport
my circuit
the TVS at the power supply and all other outports
before it would start to damage other devices. I doubt that trigger voltage of any old flash would kill other devices. However there is a chance. So bascially the question is: which of the following stuff do you want:
When I suggested a HV transistor, I meant that would be just the output device replacing SCR1 in the circuit I showed. It would be used in addition to the opto-isolator OPTO1 for circuit protection. I think you would need separate power on the HV side providing base current for the HV transistor, which might be as simple as a CR-2032 coin cell, that would last many years (current is drawn only for the duration of your logic signal; that is, when the flash actually fires). However I have not thought more than a minute or two about this circuit idea, so I'm not sure if there's something I'm missing.
If you want to isolate the different channels from each other you need a separate power supply per channel.
There is another issue as well. The Miller effect Miller effect - Wikipedia. If you want to switch high voltage with a low voltage you will require a high gain. This in turn will make your circuit slow.
