What kind of relay would I need?

[Rygar]

Hi folks. I like to think of myself as a tinker. I am by no means an educated person in the world of electronics. I can do simple, logical circuits and stuff, but that is about all I understand at this time.

I have a project that I am wanting to work on, and it calls for a specific type of switch. Only problem, I don't know what it would be called so that i can go buy one. Let me describe what I need it to do.

It will run on DC, probably a 9v or equal voltage in AA batteries. I need a switch that will be held open when the current is running to it. If that current is shut off, the switch will close and allow current from another source to flow. So, basically, the 9v will hold the switch open. if the 9v source is shut off, the switch will close and allow a buzzer to operate for a signal that the first source was shut off.

Could someone point me in the right direction as to what kind of switch that would be, if in fact they make something like that? The voltages are not set in stone, so if the requirements are different, I can probably still adjust at this planning stage. but it would still need to be DC powered, and preferably smaller instead of larger. Thanks!

 

[JMW]

What you are looking for is relay DC Coil they come in various voltages so choose accordingly. The contact configuration is what is unique. You want Normally Open and Normally Closed contacts. You will connect to the N. C. contacts

 

[KrisBlueNZ]

As JMW says, you want a relay, with a 9V DC coil. You use the "common" and "normally closed" ("NC") contacts.
Here's an example:
http://www.digikey.com/product-detail/en/G5V-1-DC9/Z2539-ND/369041
The coil of this relay draws 17 mA of current. When your control signal is active, it will need to be able to supply this much current, to keep the relay energised and hold the normally closed contact open.

 

[Rygar]

thanks for the help so far guys. I think I got the right part. Went to Radio shack and got the 9VDC mini relay. It only has 5 posts compared to the one you linked to above. The tech guy at the shack drew me a mini pictured as to how I need to wire it to get the effect I want, but something isn't working. I'm thinking I don't have enough juice to do the trick.

The specs on the relay says the coil resistance is 500 ohm. the nominal coil current is 18mA and the contact rating is 12A at 120VAC/24DC

So, is a 9v battery (the square ones) gonna have enough juice to activate this relay?
And, I'm not sure if I'm wiring this up right. The tech at the shack said to get the effect I want, I put power from the 9v to both the pins for the coil. the middle one between them is the common ground. then, power lead to device #1 on the top one, and the lead on the bottom to Device #2. (which would be my buzzer.) If the power to #1 is cut, then the switch would turn on to #2, powering my buzzer.

The image above (please forgive my kindergarten drawing) is a copy of the schematic on the package of the relay I bought, and the red lable is the wiring that the tech told me would work.

In testing with my 9V...If i put power to both the ones that are the 'coil' posts, and the ground on the COM GRD I get nothing on the other side for device #1 or 2. If I put power to the COM GRD, then Device #2 has power. I can't seem to make it do the switch like I want it to do.

so, my question is, where am I going wrong? I know I need two power sources in..If one goes dead , then the other should kick on activating my buzzer to let me know.


Edit to add: I think I figured it out. Not sure why this site didn't pop up before, or I didn't look at it. http://www.1728.org/project3.htm This is basically exactly what I want. Will try it out in the morning.

 

[donkey]

just quickly. a spdt would work.... but it all depends on how the power source would be "cut off" as you put it. anyway if you are absolutely sure you need secondary stuff I will draw for you.

so when power is applied to coil and load the circuit for the buzzer is open and power cannot move. when the load or battery connected to the coil is removed(or any break in the line) the relay switches activating the buzzer.

 

[JMW]

No, that will not work. You need a relay with DPDT contacts, yours is labeled SPDT (Double Pole Single Throw and Single Pole Double Throw respectively) one set of contacts to switch supplies the other to run the buzzer. Are you using batteries or power supplies? If batteries, you want to use a single relay to conserve power, if power supply and you have sufficient space, just get another relay. So, the negative supply terminals should be connected together, and connected at "Power 2". the pin marked Power one remains as it is, it should also be tied to the N.O. pin. Common should be the input of the device you want to power. The N.C. pin should be the output of Power Supply 2.
If using a single DPDT relay, common goes to the buzzer the other side of the buzzer goes to common. The N.C. contact gets tied to Power Supply 2.

 

[Rygar]

I used this diagram to wire it up, and it worked!

http://www.1728.org/alarm1.gif

Thanks for the help folks!! I have learned a ton in the last couple days!

 

[donkey]

It will run on DC, probably a 9v or equal voltage in AA batteries. I need a switch that will be held open when the current is running to it. If that current is shut off, the switch will close and allow current from another source to flow.

when you said another source I thought you meant another battery. hence my diagram with 2 batteries.
as for the pic you sent that is not badits almost identical to mine except the "C" on the relay is not right. besides that its perfect.
if you connect it as shown in your diagram the positive will not make a connection instead it will just shut off completely. so if you got it working I am assuming you hooked it up to common and not the plug shown in your second diagram.

JMW why would he need a dpdt? I cannot think of any reason at all