Advice on MCP23017 with dual-optocoupler - Page 1
Forum section: electronics / beginners

author: gcardinal
author_rank: Unknown
timestamp: June 04, 2014, 08:02:30 am
content: Would appreciate feedback on this schematics. It is for dual camera release, here is a bit about camera cables:http://www.doc-diy.net/photo/remote_pinout/MCP23017withTLP504A-2and for resistor network I haveBourns 4608X-102-331LF.

author: Paul Price
author_rank: Super Contributor
timestamp: June 04, 2014, 09:13:37 am
content: Why not just use 4 wires directly to the dual optocoupler inputs?Why not replace the MCP23017 with just a simple 74C164, 74HC164, or 74HCT64 shift register, cheaper, less pins, simple Clk and data wire interface? Also works with very long cable length.Why not a 1-wire interface?Use a single wire to autoclock a 74C164 clock and data inputs. Pulse width determines 1's and 0's with a simple R-C discriminator with diode fast discharge of data input with clock level.

author: gcardinal
author_rank: Unknown
timestamp: June 04, 2014, 11:02:18 am
content: Quote from: Paul Price on June 04, 2014, 09:13:37 amI dont quite understand what do you suggest, but I want to trigger cameras independently.Quote from: Paul Price on June 04, 2014, 09:13:37 amMCP23017 will be connected to Raspberry Pi and is well supported by the libs on RPi and is part of the overall schematics (almost all I/O pins will be used).

author: nsayer
author_rank: Regular Contributor
timestamp: June 04, 2014, 01:23:22 pm
content: I have experience with this chip - it's used for the RGB backlit LCDs I use all the time.The wiring of the chip itself looks exactly like what I do.The use of the optocoupler looks ok to me. My only potential concern would be that the camera and/or flash might exceed the collector current spec. You might consider using reed relays instead if that's a concern. Reed relays could be driven with a 2N4401 instead of the optocoupler. Don't forget a reverse biased diode across the coil.

author: Kjelt
author_rank: Super Contributor
timestamp: June 04, 2014, 02:25:32 pm
content: small addition: if this should run for longer periods of time, connect the reset line of the MCP23017 to an I/O pin of the raspberry incase the I2C bus gets stuck.

author: jlmoon
author_rank: Unknown
timestamp: June 04, 2014, 02:34:40 pm
content: QuoteJust curious, why would you employ yesterdays (originally 1940's) technology reed relays when one could use fast / high current MosFets with protection devices? They make some fantastic parts today with built in suppression and they are orders of magnitude faster and reliable.

author: Paul Price
author_rank: Super Contributor
timestamp: June 04, 2014, 06:47:11 pm
content: I suggest using a 74C164 because you can shift into it any combination of 0's and 1's you want to turn on each and every optocoupler you want. You show only 4 outputs of the MCP23017 being used not almost every pin. The 74C164 has 8-outputs at your shifty command.

author: gcardinal
author_rank: Unknown
timestamp: June 05, 2014, 10:25:25 am
content: Quote from: Kjelt on June 04, 2014, 02:25:32 pmGreat idea, but RPi has logic 3.3V so for MCP23017 I'm using P82B96P - but it has only 2 channels so I would have to upgrade to something else. But P82B96P works so great for me since its a DIP package and is great to daisy-chain several I2C with a bit longer traces... I guess I would have to find a better chip then P82B96P, Im just not sure which one to pick...Quote from: Paul Price on June 04, 2014, 06:47:11 pmYes, I did remove other parts of the schematics to keep it simple. But all but 2 outputs will be used on MCP23017. Im pretty much limited to IC supported byhttp://wiringpi.com/as I their are very well documented, tested and widely used in RPi community.Electrical characteristics (Canon EOS 400D)Voltage (no load) - 3.3 VThreshold voltage - 1.8 VShort circuit current - 68 µACurrent at threshold voltage - 30 µAI think values are within < 100 µA & < 5V for most DSLR.

author: nsayer
author_rank: Regular Contributor
timestamp: June 08, 2014, 02:29:08 am
content: Quote from: jlmoon on June 04, 2014, 02:34:40 pmI made the bold assumption that he was using an optoisolater because, well, he wanted to isolate the damn thing.

author: nsayer
author_rank: Regular Contributor
timestamp: June 08, 2014, 02:40:16 am
content: Quote from: gcardinal on June 05, 2014, 10:25:25 amOh, well, that should easily be able to be handled by the opto.