Easy quick invertor required

[malc-c]

It's been cloudy for days now, so what better thing to do but to make up a few leads etc in preparation for SGL7

I need a new shutter release for my Canon 400D, and so rummaged through my electronics box and found all the components I needed, plus I had an FT232R 5v TTL brakeout module from Sparkfun, so thought I would use that as the USB - Serial converter.... I've been hours mucking about trying to get it to work correctly, and found the cause.

When initially connected RTS is high (+4.97v) this is thus triggering the LED in the opto-isolator, and thus the camera starts to run a long exposure. Hooking up APT and it basically works in reverse, ie when the plan is running the shutter is closed, when it stops, the shutter opens.

Anyone have any simple idea on how to invert this logic with minimal components. Here's the circuit

Obviously I'm substituting the DB9 for the breakout board.

[Reggie]

Yes, Zero components look on the ftdi website there is an application that will allow you to invert the logic on those pins (rts etc.). I believe it's called FTProg.

I looked at it to look for a less component option for the spc900nc long exposure mod.

[malc-c]

Thanks for the tip... scoured the spark fun site to no avail, and had previously tried the RTS setting under advance settings in the driver config of device managed - no no avail !

[Reggie]

You're welcome Malcolm, if you're having trouble finding the app. or anyone else for that matter, look for ft_prog on this page:

FTDI Utilities

[malc-c]

Something is still not right even inverting that pin... running a plan in APT the rear of the camera displays everything as if it was taking an exposure, but no shutter action, then when the plan stops, APT shows as busy and fails to stop and process...

It would seem that these 5vTTL RS232 boards are not suited for use with shutter releases, either that or the opto-isolator is a duff-un !

[Reggie]

Nah, I'm sure the ftdi is fine, they use rts to trigger reset on the arduino or even for triggering long exposure on spc900nc's.

[Reggie]

I wonder if you haven't updated the eeprom correctly? Also, looking at page 33 of the datasheet http://www.ftdichip.com/Support/Documents/DataSheets/ICs/DS_FT232R.pdf

there is a setting named 'High Current I/Os', perhaps that needs setting?

I'm looking in ft_prog and if you expand the 'hardware specific' option and select 'HighIO' you'll see that set to default (off) the io pins are only giving 4ma.

[malc-c]

Reggie, I'll get the breadboard out and do some experimenting...thanks for the links and tips

[Reggie]

I also wonder about that circuit a little, I'm wondering if it was built around rs232 serial or ttl? I would've thought rs232 as serial was still very common back then. So it's been built around higher voltage.

[malc-c]

I think you are right

[malc-c]

Looking at the datasheet the forward current of the input is 60mA, with max voltage of 6v. I think I simply try a lower value resistor in series with the 4N25 input and see if that switches the output harder ?

[Reggie]

Datasheet on which? the optoisolator? If it needs driving harder perhaps a transistor might be in order if setting the pins to highIO doesn't help?

[Gina]

Yes, I agree that the circuit looks as if it's for the old standard RS232 of plus and minus 9v or such like (I forget the voltage levels used now). The 1N4148 is there to catch the negative voltage when RTS goes negative.

My first thought for inverting the logic assumed that +- 9v were the signal levels, in which case swapping the polarity (leads) of the diode and photocoupler LED would make the Photo-transistor conduct on the -ve RS232 level instead of the positive.

You can increase the LED current by reducing the resistor to say 1K.

[malc-c]

OK, now have it working.

I reversed the RTS line and programmed the chip - then on a breadboard simply hooked up an LED with a 1k series resistor to keep the load down between the RTS and GND - I could then turn the LED on and off via APT (having the camera turned on and connected to the PC) - placed the same 4N25 on the board and connected the 2.5mm socket to the camera - works fine !!

Now to put it on to stripboard and box it up

[Gina]

Great

[Reggie]

By the looks of it you could probably omit the resistor entirely, although you probably don't want it in any state where it might float, so a small 33ohm might be appropriate just as a pulldown to be on the safe side.

Gina, good call on the negative voltage coming from rs232, i hadn't considered that, now the diode makes sense in the original circuit and appears totally unecessary in a TTL based circuit (hence it not making sense originally).

[malc-c]

Well my order for the same converters that are used in the EQMOD for under £4 thread arrived this afternoon and the first thing I tried was a simple serial camera release. As these devices run at 3.3v I didn't use any resistors etc, simply connected the 4N25 LED across the RTS and GND - The PC sees the device and places it on COM16, but the camera is doing some strange things, sort of random mirror lockups and I can't get the lock up to be triggered through APT. Either the connection on the board is wrong (mine did look slightly different to the image shown, but still had the A/B pads as described.) or the 3.3v is not enough to trigger the 4N25 fully.

More experimentation to do. Shame these devices don't have the eprom that the FTDI and functionality that the FTDI chips have.