Yet another relay attenuator

jwb · 8th July 2003, 05:32 AM

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.

thylantyr · 8th July 2003, 09:19 PM

pretty slick..

Bjerre · 8th July 2003, 11:34 PM

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 ?

jwb · 9th July 2003, 12:47 AM

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.

jwb · 10th July 2003, 05:50 AM

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"

jwb · 10th July 2003, 05:57 AM

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

jleaman · 10th July 2003, 06:43 AM

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.?

janneman · 10th July 2003, 06:48 AM

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 &plusmn;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

jwb · 10th July 2003, 07:19 AM

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.

capslock · 10th July 2003, 04:47 PM

Nice work! I gather you are still working on the AVR software?

8th July 2003, 05:32 AM	#1
jwb diyAudio Member Join Date: Mar 2002 Location: San Francisco, USA	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.

10th July 2003, 06:43 AM	#7
jleaman diyAudio Member Join Date: Apr 2002 Location: Nanaimo	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.? __________________ Baby Danika, Born January 20th 1:54am.. http://itsparks.dyndns.orgMix a LittleUntangle&Opendns.org Perfect..

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8th July 2003, 09:19 PM	#2
thylantyr diyAudio Member Join Date: Feb 2001 Location: Mars	pretty slick..

8th July 2003, 11:34 PM	#3
Bjerre diyAudio Member Join Date: Mar 2003 Location: Aalborg, Denmark	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 ?

9th July 2003, 12:47 AM	#4
jwb diyAudio Member Join Date: Mar 2002 Location: San Francisco, USA	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.

10th July 2003, 05:57 AM	#6
jwb diyAudio Member Join Date: Mar 2002 Location: San Francisco, USA	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

10th July 2003, 04:47 PM	#10
capslock diyAudio Member Join Date: Feb 2002 Location: Germany	Nice work! I gather you are still working on the AVR software?

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