How to bypass a circuit when no power

All frequent questions on switching: true or not true bypass, transistor-based or mechanical.
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boidek
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Post by boidek »

Hi,

I designed a circuit for a buffer that sounds really good!
BUT, when it doesn't get the 9DCV, well it obviously sounds bad since it' an active circuit.

How can I bypass the circuit when it has no power getting to it? (without a toggle)

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Lucifer
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Post by Lucifer »

You could fit a footswitch - or is that included in "toggle" ?

One way to have an automatic bypass is to use a relay that operates when 9v DC is present. In the operated state, it then passes your signal through the buffer circuit, but when power is removed, the relay can bypass the buffer.
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boidek
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Post by boidek »

I don't want to use toggles nor foot switches.

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Post by boidek »

Lucifer, thanks :thumbsup

Any schematics of such a relay?

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Lucifer
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Post by Lucifer »

You don't really need a schematic.

You need a relay that will operate at 9v (a 12v relay should operate OK at 9v) - and it needs to have two sets of change-over contacts (just like a DPDT toggle/foot switch).

Just wire the coil contacts across your 9v supply, and wire the switching contacts as if they were a toggle switch or a footswitch (input and output jacks to the wipers, the two normally-closed contacts wired together, and the normally-open contacts to your booster) . . . and that's it.

There are lots of different relay types, so I can't tell you what actual contacts to use - but the coil contacts are normally separated from the others slightly, so you can identify them. Most relays don't mind which coil contact goes to positive and which to negative - but if your relay has + and - marked, then that's the way you must connect them.

Personally, I like the miniature Post Office type relays - they have pin spacings that allow them to be mounted on strip board, so everything is nice and neat - just remember to cut the tracks between the two coil terminals, and between each set of switching contacts (apart from the normally closed ones, which need to be linked anyway).

You should also wire a 1N4148 type diode 'backwards' across the coil, as relays can generate hundreds of volts in 'back-EMF' when they de-energise - and the diode will prevent this high voltage from blowing up your booster (and anything else connected in the chain).

Good luck.
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electrosonic
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Post by electrosonic »

I am a fan of mictester's relay scheme.

https://www.freestompboxes.org/viewtopic ... 13&t=13295


from the original post (emphasis added by me)
The circuit gives six big advantages:

1. Hermetically sealed gold contacts - clean and quiet switching.
2. Additional relays can be put in parallel with the first one. You might have to increase the 100µF to 220 µF, but the switching will be entirely reliable.
3. Draws minute current - the whole of the battery power will be used to power your effect and the indicator LED!
4. Passes the "disconnected battery" "True Bypass" test.
5. Costs less than a good quality DPDT footswitch.
6. Allows simple connection of an indicator LED.
Andrew

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Post by Lucifer »

So am I, Andrew - but BOIDEK didn't want to use a footswitch.
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Post by mictester »

Lucifer wrote:So am I, Andrew - but BOIDEK didn't want to use a footswitch.
So don't! Leave the whole little circuit across the supply - with power, the audio gets routed through your buffer, and when the power is removed the buffer is powered down and the relay will revert to bypass mode.
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Post by Lucifer »

Hi MicTester,

It's good to know that your circuit (of which I am a great fan, as stated above) can also be used for the same ends here.

However, the main advantage your circuit offers for 'footswitching' (use of momentary, silent switch) is lost in this particular application.

The two alternatives do the same thing and would, therefore, appear to be on level pegging, but the simple non-latching relay (IMHO) just steals ahead slightly because they are more easy to find than latching types (I had to import my latching relays from Singapore), and require no additional components apart from the 'backlash' diode.

But your circuit has the benefit of drawing minimal power, which won't really matter if a power supply is used, but for a battery-powered unit this would be a major advantage.

I'm sure Boidek will be thrilled that he has a choice of options.

I wonder if there is a minimalist 'solid-state' solution - maybe a self-start 555 latch driving a couple of SS switches ? Nah - too complicated already.
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