# Looking for creative advice on Relay switches

Hi everyone

I am writing because I have a question about relay circuits. Is it possible to build a circuit that would:

1. use a small battery (~3 volts?) to power on a relay switch for 3 min. This would allow an independent circuit to become "closed" and turn on a light, for instance.
2. after the 3 min, the relay must be de-energyzed and turned off (the light in the independent circuit turns off because circuit becomes open)
3. when the relay is turned off, a timer of some sort (maybe within the relay itself, or another alternative?) waits 15 minutes to turn the relay switch back on, in order to repeat step #1

My question is... would something like this be possible to do? And if so, what alternative would work best so that it uses the least amount of power possible?

Thank you all so much! I look froward to hearing your creative ideas

Luc

would something like this be possible to do?

Yes

what alternative would work best so that it uses the least amount of power possible?

You have not said what voltage / current is going to be on the relay contacts, without that information it is impossible to recommend a specific relay.
A reed relay takes very little power like this one
http://www.pickeringrelays.com/productnews.html
You can drive them direct from an arduino pin, remember to put a reverse biased diode across the coil.

Hi Mike

thank you for your reply! Awesome news… sorry that I wasn’t more specific. So, what I was envisioning was to have the relay switch turn “on” and “off” a circuit that has 5 volts going through it (<1 amp).

For the circuit that would be controlling the relay switch, my idea was to have this outdoors, disconnected from the grid. Therefore, I was thinking of connecting a battery (~4v) that would power the relay switch, and an additional component (not sure what this/these would be) to turn the relay on for 3 min, and then turn it off for 15 min. Is there anything else that I could use, aside from an arduino to control this? Some kind of mechanical switch, or something like that?

Thanks, L.

sorry... meant "timer" instead of "mechanical switch"

turn the relay on for 3 min, and then turn it off for 15 min.

That is a long time for things like NE555 timers, they are difficult to get an accurate time over that period, you need some sort of micro. However there is no need for a full arduino, why not use one of the small ATtiny chips, they are arduino compatable.

or this

How about a latching relay? Relay - Wikipedia

These are relays with a cam inside. A momentary pulse of current changes the relay from on to off, and the cam holds the relay in that position until the next momentary pulse. This way, you don't have to supply a constant current to keep the relay in the state you want.

Search Digikey for "latching relay" - in the Signal Relays there are some available that can handle 2A but only require about 15mA coil current to actuate. Cost: less than \$4US.

Word of caution: Some latching relays are not mechanical and require a polarity reversal on the coil to "flip" states. Other non mechanical type have 2 separate coils. This makes them slightly more difficult to interface with than the mechanical type mentioned. It also means that you need to be sure what kind you are purchasing if you chose the latching relay approach.

Hey guys!

thank you SO much! These ideas & suggestions are awesome... It looks like the components that you mentioned would be just what I would need.

1. a low-power consumption controller (ATtiny85) to control the relay
2. a latch relay (without polarity reversal) that would use a very low amount of power to turn itself "on" and "off"

I think I'm ready to place an order to get these components and get started on the project. In digikey I found these two items, but which ones should I get? They have many different "flavors"... I am looking for a low power and low-consumption components that would runn on ~3-4v lipo (3.7v with 2700mah).

In DigiKey, for the ATtiny85s:

In DigiKey, for the latching relay (I would need a simple, non-polarity reversal switch [thus, something that you just send a bit of power to turn on, and then again to turn it off]):

Thus, which one of each should I get?

Thanks again for your awesome ideas & help as I navigate through these huge electronics catalogs

Luc

Sadly there is no relay there to fit your needs. The only one that is 3V can only be bought on a minimum quantity of 500.

Also all those relays need the polarity reversal because they are only single coil designs.

I don't see why polarity reversal is a deal breaker. Just use two digital pins connected to both ends of the coil. Set them both to INPUT most of the time. When it's time to flip, set them to OUTPUTs and make one HIGH and the other LOW for a few ms to make the relay flip the light on (or set the pins opposite for a few ms to flip the light off). Then return to INPUT mode on both pins.

Disclaimer: I haven't tried this myself, so I could be spouting nonsense.

If your load is 5 volts DC, they you can bypass the power drain of a relay coil. Use a mosfet rather than a relay.
Virtually no drain to hold a mosfet on.

5v 1amp.

A mosfet is probably a better option or even a power transistor (eh tip41)

I see... thanks for the tip, guys. The MOSFET option sounds like it could work well.

Which mosfet would you recommend for using with the 3.7v lipo battery? Does digikey carry it?

L.

Lol... i said it's probably better

I would opt for an npn over a fet because of the low voltage you use

A tip 31 for example will happily switch on at 3v, 4v to saturate it, a mosfet may require a voltage high than 3v to switch it on.

Sorry coding1227 but cjdelphi often pops up and gives very poor advice. If you look at some of the threads he starts you will see he is always arguing about something he dosn't understand. He even contends that the world would be a better place if Hitler had wone the war!

Now in your first post you talked about an indipendant circuit. In electronic terms that means an isolated one. Using a transistor or a FET requires the switching circuit to be connected to the ground of the circuit you want to switch, so it would not be an isolated or indipendant circuit. If cjedelphi were any good he would have asked about that first.

If you don't mind the circuits being connected then using a FET would require some sort of driver to boost the output voltage to 5V, a transistor could be used without such a driver. But the point is that to drive a circuit like this you need to know exactly what you are driving.

Maybe you should think about the usage of a Digispark. Digispark USB Development Board - Digistump

That's a small Board with a Attiny85 on it that can be programmed with the Arduino IDE. For the usage with a battery you can add a boost kit that transform 2 - 4.5 V up to required 5 V. Additionally there is a MOSFET as well as a relay shield. Simply stack all together and ready to program

Maybe you should consider going up to the 5volt arduinos, such as a small nano. Such as a nano clone:
http://dx.com/p/nano-v3-0-avr-atmega328-p-20au-module-board-usb-cable-for-arduino-844118037
with usb cable, for less than \$15USD.
Lots of devices like 5volts.

Hi everyone for the very valuable ideas!

As a clarification, there would be a total of two circuits: one being a 3.7v battery powering a ATtiny85 chip that controls the latching relay or mosfet; the other being an arduino connected to a separate 3.7v lipo battery that powers an LED, and who's "open" or "closed" status is controlled by the relay/mosfet mentioned earlier).

Therefore... what would you recommend? Should I use a mosfet, or a latching relay? I guess the main question would be: "which of the two options (latching relay or mosfet) would allow for the ATtiny85 to make the "arduino circuit" to turn on & off for the longest time?

Hi, just a question, what is the second arduino doing that will not allow it to do the job of the ATtiny85.

So far all that you want to do can be done with one arduino.

We will need to have an explanation of the complete project to advise on getting the most from your hardware.

Thanks Tom...

Hi Tom et al.

sorry... I should have been a bit more detailed in regard to the project that I had in mind. I guess I made a "simplified version" to make things easier

What I would like to do is basically run an arduino on a 3.7v battery to collect air temperature data in my backyard, and record that to an SD card. Since I would like to extend the duration of the battery power, I want this circuit to be powered "on" every 15 minutes for a duration of 3 minutes, and then power off until the next cycle (15 min). The "on and off" cycles would be controlled by a separate circuit, in this case a programmed ATtiny85 connected to a 3.7v battery and a switch (latching relay? mosfet?).

I specifically don't wish to use the "sleeping" mode in the Arduino because this would continue to make it use some battery power (while it is "sleeping"). Therefore, I just want to turn it off completely. Hence, my interest in using the ATtiny85 circuit... Thus, the idea is to have the Arduino work as long as possible by controlling the circuit's "open" or "closed" status with the use of the separate ATtiny85 circuit.

Sorry... is this description of more help? Hopefully it can help figure out what alternative would work best... a mosfet, or a latching coil (one or two coils)

Thanks