I have a project were I need to switch 13 relays. I read an article that mentioned a mentioned of using shift registers to cut down the number of pins required. As I really don't know anything about this thought I would get you smart people to check over my schematic. The circuit uses a ULN2003AD Darlington array and a 74HCT595D shift register. Am I doing this right? is there a better way to achieve this?
also do I need to use diodes across my relays I took these off as there are Diodes in the Darlington array.
The schema looks reasonable, though you should check whether you need protection diodes on the relays.
Also, why are you driving the 74hc595 individually and not chain them into one 16-bit shift register?
The diodes in the Darlington package are in-line and probably intended to protect the device from being incorrectly connected. The diodes for relays are across the coil and reverse polarity and intended to short the dangerous back EMF when the coil is switched off. You need the diodes on the relay in other words.....
ialpha:
How do I chain the shift registers, I can connect the clock and Latch together, do I do the same with the data or does this need to be separate?
Check on the link below the part where the green LED are added. This will answer all your questions.
I'm not really sure what the mute and 7.1 relays are doing but they do make 4 pole double throw relays, that might cut down on parts a little. But they cost around $50. Don't they make chips specifically for audio and video switching? Or are you switching the powered speaker wires?
I purchased a I2C relay board with 8 relays on it, pretty happy about it as I am not using any fancy approaches, just good old plain I2C. You can purchase two of these to have 16 relays array at no I/O cost.
Google for "RLY08 I2C" to check out the one I have
