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I was wondering if there is someway to have relay-like behavior, but without constant power to the relay. For example, if my understanding is correct, if I wanted to control power from an outlet using a relay, it would either be normally on or off, but when power was applied it would switch to off or on respectively. Is there a way to achieve the same type of behavior but without constant power? For instance, a pulse of power would switch the "relay" to on, and another pulse would switch it to off?

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    \$\begingroup\$ Google Latching relay \$\endgroup\$ – sstobbe May 20 '17 at 15:17
  • \$\begingroup\$ There are two types of relays keeping their position without power. Bistable relays use a permanent magnet to hold the position. Latching relays use a mechanic lock to hold the on position. See en.wikipedia.org/wiki/Relay#Latching_relay and \$\endgroup\$ – Uwe May 20 '17 at 15:26
  • \$\begingroup\$ @Uwe Wow, this looks like exactly what I am looking for haha! Would it be possible to wire up something to control outlet power (120V) with a 5V control? Sparkfun has a good tutorial on this for normal relays, but I can't find any documentation on latching relays. \$\endgroup\$ – G K May 20 '17 at 15:27
  • \$\begingroup\$ When you Googled on "latching relay circuit", what did you get? How many links did you look at? \$\endgroup\$ – WhatRoughBeast May 20 '17 at 17:20
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Digik-Key has lots of power latching relays. Go here and under Coil Type select single or dual coil, and Coil Voltage 5VDC. Here's a dual coil one for about $4. Note the hefty coil current though (150 ma), so you'll need to drive each coils with a transistor.

Although it will take two output pins (and two transistors), dual coil is probably a better way for you to go, since I assume the single coil ones work by operating the coil in one direction, and then reversing the current to disengage -- which would require an H-bridge.

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Aside from the everyday latching relays which have either one coil (reverse the polarity and pulse to change the state) or two coils (pulse one for 'on' and the other for 'off') there exist alternating latching relays. Each pulse reverses the state of the switch. The Tyco (née Potter and Brumfield) ones I show below have 120VAC or 240VAC coils available (as well as DC) and operate microswitches to switch substantial (15-20A) AC current. As you can see, the mechanism is completely open and somewhat resembles that of a ball-point pen or alternating switch.

enter image description here

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A single coil relay requires a normally off momentary contact SPST push button or toggle switch.

A double coil relay requires a center off momentary contact DPDT toggle switch. Switched one way to turn the relay on and switched the other way to turn the relay off.

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