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Re: [Paparazzi-devel] 5v pulse required for CHDK on Lisa/M2

From: Christophe De Wagter
Subject: Re: [Paparazzi-devel] 5v pulse required for CHDK on Lisa/M2
Date: Fri, 2 Nov 2012 13:51:18 +0100

The opticoupler looks nice. the end stage is exactly the transistor described in previous mail and the optocoupler is very fast for a reasonable amount of current (<=10mA).

DC cam module can do both by define the DC_PUSH and DC_RELEASE to either LED_ON or LED_OFF. (Note that LED_ON is a low voltage since LED's are hanging between the IO pin and the 3.3V.)

But: if you use just the MosFet or direct optocoupler approach you will WANT to keep the default: 0V when triggering -> Transistor Open -> pullup 5V on camera, 3.3V IO -> transistor closed -> 0V on camera

With the optocoupler board from Sparkfun, it has an extra transistor which inverts things again so it must be the opposite. (or use the signal at R3 and R4)

3.3V -> Light in optocoupler -> internal optocoupler-output-transistor closed -> pull down Gate of transistor2 -> transistor2 open -> pullup of output -> high
0V -> no light in optocoupler -> internal optocoupler-output-transistor open -> pull-up of gate of transistor2 -> transistor2 closed -> output is low impedance to ground


On Fri, Nov 2, 2012 at 1:36 PM, Gerard Toonstra <address@hidden> wrote:

Thanks for the help. So this breakout would probably work?

I've also noticed that the default signal for the dc cam module is (3.3V) by default and switches to 0V when triggering, where I'd expect the opposite.
Can you elaborate on the reason for this?



On Fri, Nov 2, 2012 at 5:30 AM, Hector Garcia de Marina <address@hidden> wrote:
Agree with the last solution of Christophe. It would be desirable using an optocoupler for keeping cleaner the GND for the analog signals especially.

On Fri, Nov 2, 2012 at 9:07 AM, Christophe De Wagter <address@hidden> wrote:

Use e.g. a 2k to 20k Ohm resistor connected from USB+ to 5Vusb to pull it up. Use any N-MOSFET transistor (e.g. bss138) with its Gate to any paparazzi IO pin, its Source at Ground and it's drain at the USB+ pin, pulling it to zero when the io pin in paparazzi is higher than 0.7 volt.

If you have no Fet but have a bipolar NPN transistor around (like a bc547) then you just need 1 extra resistor of about 10x more ohm in between the paparazzi board and the NPN.

Another solution we use most of the time is a bipolar optocoupler switch instead of a MOSFET. Then there are no copper connections between the noisy camera ground and the autopilot/GPS/modem ground for optimal telemetry and gps signals.

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