Hello,
I want to build an attenuator using a couple Intact Audio Autoformers and control the attenuation level from an Arduino device over I²C. I am really a software guy but I didn't see anything quite like this on these forums, this is my first crack at PCB design. Does this circuit look ok? Am I missing anything? Is something wrong? Is there a better way that reduces the number of relays required?
Thanks in advance!
I want to build an attenuator using a couple Intact Audio Autoformers and control the attenuation level from an Arduino device over I²C. I am really a software guy but I didn't see anything quite like this on these forums, this is my first crack at PCB design. Does this circuit look ok? Am I missing anything? Is something wrong? Is there a better way that reduces the number of relays required?
Thanks in advance!
As no one else has commented here goes.
Without knowing the design intent my questions might be irrelevant.
Why are both terminals of the relay coils (COIL+ and COIL-) driven?
What are the integrated circuits driving the relay coils?
What is the reason for using so many relays? Wouldn't just a one relay contact per transformer tap be enough?
Without knowing the design intent my questions might be irrelevant.
Why are both terminals of the relay coils (COIL+ and COIL-) driven?
What are the integrated circuits driving the relay coils?
What is the reason for using so many relays? Wouldn't just a one relay contact per transformer tap be enough?
Thanks for replying! I am using DPDT latching relays (IM43TS) in the schematic so I have positive going to both set and reset the coil, or at least that is how I think this works?
For ICs, I have 2x MCP23017 driving 4x ULN2803.
The transformer has 14 taps -3db… -42db and I also wanted to define a mute and 100% volume position so there is 16 total positions. I could use 1 DPDT relay per tap to control the volume of the right and left channel in tandem, one position of the relay would be connected and the other disconnected but that would require 16 total relays vs the 15 I have here in a binary tree structure.
For ICs, I have 2x MCP23017 driving 4x ULN2803.
The transformer has 14 taps -3db… -42db and I also wanted to define a mute and 100% volume position so there is 16 total positions. I could use 1 DPDT relay per tap to control the volume of the right and left channel in tandem, one position of the relay would be connected and the other disconnected but that would require 16 total relays vs the 15 I have here in a binary tree structure.
I forgot to mention, R1-6 are 10kΩ resistors, R7, R8 are 4.7kΩ. C1, C2 are 820 µF aluminum electrolytic capacitors.
Thanks for the explanation that the relays are latching types.
The ULN2803 can only sink current by turning on a darlington transistor to connect the output to GND. The COM connection is only for the 'free wheeling' diodes to clamp any signals on the outputs above the COM pin voltage. I think that an alternative part is required that can both source and sink current to drive the relay coils.
Also it might be prudent to add some high frequency decoupling capacitors between the VSS and VDD pins (say 100 nF) to the I2C expanders. They may be sensitive to transients on the +5V power supply. Depending on PCB layout.
The ULN2803 can only sink current by turning on a darlington transistor to connect the output to GND. The COM connection is only for the 'free wheeling' diodes to clamp any signals on the outputs above the COM pin voltage. I think that an alternative part is required that can both source and sink current to drive the relay coils.
Also it might be prudent to add some high frequency decoupling capacitors between the VSS and VDD pins (say 100 nF) to the I2C expanders. They may be sensitive to transients on the +5V power supply. Depending on PCB layout.
You can get latching relays with a centre-tap on the winding meaning you can drive them either direction without an H-bridge. H-bridges are a drudge to work with, get relays that are easy to drive...
1) you have four relay contact in series (e.g. for -3dB) 😱
2) this kind of application requires make before brake, otherwise there will be clicks
3) think about normally open reed relays
2) this kind of application requires make before brake, otherwise there will be clicks
3) think about normally open reed relays