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Old 28th April 2015, 01:19 PM   #1
hajj is offline hajj  Lebanon
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Default Help needed designing signal switching circuit

I've been working on building my amplifier enclosure and need to switch between two inputs.
The problem is i can only use a dpdt switch to do so.
I tried using a simple relay circuit to do the switching, but noticed that when the relays are powered up they tend to inject some noise into the signal.
So i got my hands on some bistable relays to do the switching and need a circuit to send a 12v to switch the relay one way and a -12v pulse to switch it the other way.
There are lots of circuits out there that work with temporary switches, but alas these don't work in my case since i am forced to use a simple DPDT toggle switch.
Any ideas how to go about doing it?
Thanks!
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Old 28th April 2015, 06:36 PM   #2
Mooly is offline Mooly  United Kingdom
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Would a resistor and cap work. Cap charges to -12 volt and can pulse the relay. You would have to allow the cap to charge between switching's. Perhaps something like 1000uf and 10k for starters. You might be able to get the cap value much lower and it still work.
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Old 29th April 2015, 08:24 AM   #3
hajj is offline hajj  Lebanon
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Hello Molly,
A Cap and resistor was one of the first things i thought about, but couldn't figure out how to male ot work with a single dpdt switch, knowing that I need to feed the relay a +12V pulse to switch it one way and -a 12V pulse to switch it other way, all the while finding a way for caps to charge.
I also looked into 555 based circuits, but found nothing that could work with the challenging dpdt toggle switch
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Old 29th April 2015, 12:39 PM   #4
Mooly is offline Mooly  United Kingdom
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Like this. The cap not in uses charges fully. The high value of resistor keeps the current in the relay at a minimal value.
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File Type: png Relay.png (34.7 KB, 98 views)
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Old 29th April 2015, 02:28 PM   #5
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OK I know I must be missing something here, but what's the relay for?

A relay is simply a switch, so it seems overdone to have a switch switching the relay. It won't work to just switch the inputs with...the switch?

Unless, of course, you're talking about more than a DPDT relay.
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Old 29th April 2015, 02:49 PM   #6
hajj is offline hajj  Lebanon
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Thanks Molly!
I'll try it later today and see what resistor/cap combo works.
Bentsnake, the problem arises because i'm using a dpdt switch and i need to switch at least 3 signals (right pos, left pos and ground).
this would be a non issue if I had 3pdt switch
So i'm planning to use Molly's schematic to switch 4 relays in series actually (right pos, right ground, left pos, left ground)
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Old 29th April 2015, 02:55 PM   #7
DF96 is offline DF96  England
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Do you have to switch ground?
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Old 29th April 2015, 02:57 PM   #8
hajj is offline hajj  Lebanon
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Yes, dont know what sources will be plugged into this amp as time goes by, and any slight difference in ground potential would be problematic
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Old 29th April 2015, 03:17 PM   #9
DF96 is offline DF96  England
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You will need to switch signal and ground in the right order, unless you want clicks. Ground connections should be 'make first, break last'. Signal is 'make last, break first'.
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Old 29th April 2015, 05:27 PM   #10
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<< i need to switch at least 3 signals >>

Oops, I missed that part, got it now.

Just to mention it--now that you already have the bistable relays--eBay has "3PDT switch" (sans quotes) from Asian sellers. Also at taydaelectronics.com a very large Asian seller that ships worldwide. Of course I don't know how any of that might help things in Lebanon.
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Last edited by bentsnake; 29th April 2015 at 05:32 PM.
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