How do I build a circuit that is normally getting power but switches something else on when power is shut off?

Question

I'm not an electronics expert but I am trying to build a low voltage battery operated circuit where voltage is normally present but does not do anything (ie doesn't light an LED) and when that voltage goes out, I need something else to turn on (ie and LED). So basically I guess I need some kind of switch or relay that doesn't let voltage through until there is no power through one of the three wires.

Also, battery life is important so something like a relay that keeps power from flowing when voltage is applied would probably use too much battery. Thanks

Everyone thank you. This is a low voltage DC Battery application! 1.5-9 volts

Answer 1

Need more detail, anyone who attempts to answer with this little information could get you KILLED!!!!!!!

Answer 2

A relay would do the job, and since you're talking about a low power application, you should be able to get a small one that doesn't require a lot of power to hold it open. You'd need a "normally closed" (NC) relay, where applying power opens the contacts. That way, when you lose power, the contacts close and turn on your other circuit. Of course, you'd also have to make sure you bought a relay that uses the voltage that you have available.

Another option (if all you want to do is run an LED) is to use a digital logic integrated circuit (IC) called an "inverter" (not to be confused with a "power inverter"). You'd need to provide a 5VDC source to power the IC at all times and would connect the power that you want to monitor to the input of the inverter. If 5VDC is applied to the input, the output is 0VDC. When the input goes to 0VDC, the output goes to +5VDC. (That's why you have to keep power applied to the IC at all times... so it's always sensing the power line that you want to monitor.) The beauty of an IC is it draws VERY little power, but it can only supply a limited amount of power (~10mA). Fortunately for you, an inverter can easily light an LED!

Using an IC requires a little knowledge, but I bet you'd be able to figure out how to hook it up with the instructions on the back of the IC package. (You can buy most of the common ICs at Radio Shack for a couple of bucks. Ask for any part number from the family "LS04".)

By the way, AA batteries and 9V transistor batteries are about as safe as you can get for experiments like the one you're talking about. Good luck, and have fun!

Answer 3

So you want the same battery that powers the LED to power the detector? What happens when that battery goes dead -- no LED?

I suppose that the closest thing to what you want is a set of CMOS comparators biased so when the input drops below a certain voltage, the output turns on. But you'll need more than 1.5V if you want something like that to work.

Why not just light LED's with the three voltages and when one drops out, you'll know there's a problem? Most people can make the adjustment that when an LED goes out, there's a problem, as opposed to when an LED goes on, there's a problem. Doesn't require near the circuitry expertise that the comparator design does, either. You just have to set the LED's to just barely operate at the required voltage (using resistors) so that when the voltage lowers, the LED goes out.

Answer 4

I cringe when I see questions like this because a novice shouldn't be monkeying around with AC voltages. One careless solder joint and nasty electrical burns can result, if not worse.

But you're talking about an Uninterruptible Power Supply, or UPS for short. Essentially, in its simplest form, the AC input holds a relay open until power has been lost. Then the relay closes which connects a battery to the circuit to continue powering it. When power is restored, the relay is opened again, and the battery isn't used until the next power interruption.

Commercial UPS's are more complex as rechargeable batteries are used, and so battery recharge takes place during non-use. Additionally, over-charging could result, so more sophisticated design is necessary to stop the charging process when complete.

Since there is no single simple design, you'll probably be better directed to use a search engine for UPS SCHEMATIC and choose from the several that will come up.

Answer 5

The simplest way to do that is with a double throw relay. You would hook it up so that it was energized when voltage is present. When the voltage is removed the relay would de-energize. You would connect the circuit you want working when the power is shut off to the normally closed contacts of the relay.

I see you now say power consumption is a limitation so I would qualifly my answer a bit by saying - use a bistable relay. You would have to make a circuit to pulse the relay when the power goes off though. Things get much more complicated when power limitations are an issue.

Answer 6

A relay would typically draw more current than necessary. One route would be to use IC's and an inverter. If you only have 1 input, a simple inverter would work. As long as the input voltage is present there is no output. If the input goes away the output goes high. Usually an IC could provide enough current to drive and LED. If you need more than 1 input look into the various types of logic gates, AND, NAND, OR, and NOR.

Answer 7

sounds like you need a double pole double throw switch I use this on my radio set up I run a power to one side of the switch to power the radio but when the main power goes out I flip the switch and the other two termails are connected to my car battrey and the radio runs off the battrey till the power comes back on this is a manual switch but there are more compalcatied relays out there. actually I do have an extra switch get ahold od me if youd be intrested in using it but this other one I have has a center off postion thats why I didn't use it.