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Old 8th July 2003, 04:32 AM   #1
jwb is offline jwb  United States
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Default Yet another relay attenuator

Inspired by the ftorres' nice balanced preamp, and slightly turned off by the design decisions made by the Dip Chip folks, I decided to make my own relay attenuator. Using the ftorres' schematic as a starting point, I created a balanced stereo attenuator with 20K input impedance and 64 levels, in 1.25dB steps. The attenuator is controlled by an AVR micro and a 32-step-per-revolution rotary encoder.

Schematic
Layout
Bill of Materials

I hope the drawings make sense. Total cost is about $150 for parts and varying PCB price.
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Old 8th July 2003, 08:19 PM   #2
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pretty slick..

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Old 8th July 2003, 10:34 PM   #3
Bjerre is offline Bjerre  Denmark
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Assuming that the latches are ordinary TTL (I haven't been able to figure out what the D at the end of 74LS595D stands for), then please verify that they can actually supply the current needed for the relay coils... Normal TTL are inheretly poor at delivering current compared to what they can sink. And I suspect even the sinking capabilities of the latches might not be enough ?

Are you sure there is any need for the zeners btw ? The anti-parallel diodes should be enough, shouldn't they ?
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Old 8th July 2003, 11:47 PM   #4
jwb is offline jwb  United States
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The datasheet from Fairchild for MM74HC595 (which is the part used, nevermind what it says on the schematic) claims current drive of ±35mA per pin. The Omron G6K-2P-Y-5V relay requires 21mA drive current when on. I think it will work.

As for the zeners, I simply did not want to blow up the outputs of the '595. Perhaps I was too conservative, but whomever builds this could optionally use a diode rather than a zener, if desired.
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Old 10th July 2003, 04:50 AM   #5
jwb is offline jwb  United States
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I'm only just coming to realize that this attenuator is tiny enough to use in my planned 6-channel amp. If you view this on a 96-dpi computer monitor (most of you), this image is the actual size of 5.4" x 3.9"
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File Type: png atten-actual.png (93.7 KB, 3520 views)
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Old 10th July 2003, 04:57 AM   #6
jwb is offline jwb  United States
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Here's the manufacturing data, suitable for sending to a PCB shop. Gerber files are included for silkscreen and solder mask, although they can be omitted to save cost. The drill sizes are chosen to coincide with the ones offered by pcbexpress.com.

atten-manuf.zip
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Old 10th July 2003, 05:43 AM   #7
jleaman is offline jleaman  Belgium
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that does look awesome.. What kinda control is used to use this pcb and Schematic.. How much it is from start to finnish.. ? is this balanced or just Line.. This looks something that i want to tackle. How much for a pcb ? Will you charge.? do you need 2 boards one for left and one for right.?
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Old 10th July 2003, 05:48 AM   #8
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Quote:
Originally posted by jwb
The datasheet from Fairchild for MM74HC595 (which is the part used, nevermind what it says on the schematic) claims current drive of ±35mA per pin. The Omron G6K-2P-Y-5V relay requires 21mA drive current when on. I think it will work.

As for the zeners, I simply did not want to blow up the outputs of the '595. Perhaps I was too conservative, but whomever builds this could optionally use a diode rather than a zener, if desired.
What you should check is when you pull these 21mA, what the output voltage of the mux is, certainly no longer 5V. It should be enough to pull in the relay.

I used them in a similar project and needed to go to Pickering low-current relays for this reason.

Jan Didden
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Old 10th July 2003, 06:19 AM   #9
jwb is offline jwb  United States
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Quote:
Originally posted by JasonL
that does look awesome.. What kinda control is used to use this pcb and Schematic.. How much it is from start to finnish.. ? is this balanced or just Line.. This looks something that i want to tackle. How much for a pcb ? Will you charge.? do you need 2 boards one for left and one for right.?
Hey, one question at a time! Not sure what you mean about the controls. It uses a rotary encoder for input. Anything that runs on +5V and has a quadrature output would work. The software runs on an Atmel AVR. The design is two balanced channels per board, so only one board needed. Depending on how many boards i get made, it could be as much as $150 per board or much less. I was planning to build two and sell one of them. Later if I build 6 channels I might build 10 and sell the surplus.

Jan, thanks for the tip I'll see how the output holds up under load. I've tested with Coto relays and 74HC595 with no troubles. Note that only one output on each 595 is powered at once.
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Old 10th July 2003, 03:47 PM   #10
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Nice work! I gather you are still working on the AVR software?
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