# A relay that behaves like a simple one but doesn't continuously use power?

I'd like to find a simple way to switch the audio output of my Amazon Echo between its internal speakers and an external audio system. Fortunately, Amazon was gracious enough leave a little cavity at its base.

I was thinking of installing a switched phone connector and a DPDT relay such that the switch inside the phone connector controls the supply of power to the relay: plugging the jack into the connector switches the audio to the external speakers; unplugging it switches to internal.

I could quit here but it bothers me that the relay will likely be energized, consuming 140mW, for days or even weeks at a time.

An impulse relay comes close to solving this, except it transitions on an impulse, not a step. I'd rather not have to deal with the headache of building a differentiator, experimenting with pulse durations, rectifying the output, and fitting it all in a small space...

It seems like a relay that had this behavior built-in should exist, and actually be quite common, but I can't seem find it. Is there a simple way to accomplish this objective?

• You can reduce the power consumption by reducing the voltage to just about the hold voltage. It requires more energy to energize the coil, but once energized, it takes less energy to maintain it. – efox29 Sep 4 '16 at 23:40
• The type of relay described in the question is called latching relay. These are not too uncommon. – Nick Alexeev Sep 4 '16 at 23:57
• I think you'll find that a latching/impulse relay doesn't have to be driven with an impulse. You can drive it with a step signal that goes away after some time when you remove power, as long as you're prepared to supply another/opposite polarity signal to put the relay back in the other position. I'm not sure how your proposed level-triggered latching relay could work mechanically; obviously you can make it using an impulse relay and some logic, but then you consume power for the logic, which seems contrary to your purpose. – Oleksandr R. Sep 4 '16 at 23:57
• The best option seems to be to have the DPDT switch physically actuated by the connector insertion. – Oleksandr R. Sep 5 '16 at 0:01
• @Nick Alexeev a latching relay will not function as described in the post. – tschmit Sep 5 '16 at 0:51

## 1 Answer

You're looking for an "Analog Switch". They are basically solid-state relays that can pass analog signals.

There are many types, made by many manufacturers. Here is a link to Maxim's MAX14689 to get you going. It is a solid-state DPDT multiplexor suited to audio frequencies. It can be switched (and powered!) by a low-voltage signal from a microcontroller, or from your phone connector (if wired judiciously).

• I didn't realize a solid state solution could handle the switched signal voltage extending outside the range of the power supply, but apparently it can. That seemed like a good solution, so I started searching for some kind of socket that would allow me to connect to that SMD, but had no luck. Then I realized that that chip it only about 1mm on a side (!). And the ones on a more human scale seem to have an unacceptably high On resistance – tschmit Sep 5 '16 at 1:00
• @tschmit Yeah, they're really small :) It looks like TI and Fairchild only make tiny ones, too. Try Analog Devices. Just a sec, let me find a search link for you... Hmm, the link is so long I can't embed it here. I'll post it right after this comment. I only filtered it for package size; you'll have to refine it further :) – bitsmack Sep 5 '16 at 1:19
• @tschmit Here's a search link with only larger ICs selected.. – bitsmack Sep 5 '16 at 1:19
• @tschmit By the way, how do you make those short goo.gl hyperlinks? – bitsmack Sep 5 '16 at 1:25
• @bitsmack Go to goo.gl and it prompts you to enter the full link, and then gives you the shortened hyperlink. – Steve G Sep 5 '16 at 8:09