Electronic components

I know this likely isn't the place to ask this, but I couldn't find anywhere else...

Does anyone know of an electronic component that works kind of like a transistor, only it turns on when the electricity stops?

I have a pocket radio in my truck because the FM radio is broken, and I'm trying to build something that will switch between power from a cigarett lighter adapter and two AAA batteries.  I need it to switch to the cigarett lighter adapter when the power starts comming from that, and for it to switch to the batteries when the truck is turned off and the cigarett lighter adapter stops providing power.
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Crash2100Asked:
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drelectroConnect With a Mentor Commented:
A relay is the answer, but probably not a reed relay, reed relays tend to be single pole single throw i.e either on or off you need a changeover type.

Look for a single pole double throw (or changeover) relay, connect the commonn to the +ve battery  terminal of the radio.
Connect the normally closed relay contact to the battery +ve.
Connect the normally open relay contact to the adapter +ve.

Connect 1 side of the relay coil to the adapter +ve.

Connect the other side of the relay coil, the battery -ve, the adapter -ve together to the radio -ve.

The parallell charging idea is good so long as you take care not to overcharge the batteries, if you can't be sure you're not over charging them then don't, the last thing you need is an exploding battery in a moving vehicle.
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parkerigCommented:
I suggest just use rechargeable batteries and have the cigarette lighter charging them.
When truck power on batteries charge / radio plays
When truck power off radio still plays.

Cheers
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Crash2100Author Commented:
I am using rechargable NIMH batteries.  But won't connecting the batteries and the adapter together double the voltage when the batteries are charged?
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parkerigCommented:
Your will need to trickle charge them via a proper NIMH battery charger. You will also have to make that the charger is connected in such a way it is correct for the AAA batteries being in series. Without pulling my charger apart I can't help there sorry.
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parkerigCommented:
Howdy again,
Just checked my spotlight - where I am doing same as you for radio - now I think it works for the spotlight as it only has ONE battery. I don't know if my advice works for TWO or more batteries.
Apologies
Ian
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CallandorCommented:
If the charger is set up to charge the batteries, running the radio at the same time from the batteries means the radio is set up in parallel to the charger, not series.  This will work for you.
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rogerperkinsCommented:
Back to the orginal question, a transisor also can turn off when power is applied if it is a Junction Field Effect Transistor, I suggest using a high power JFET.  Some radios have a hole for you to plug in an adapter such as a cigarette lighter adapter, when you plug that in, the radio should automatically use that power instead of the battery power, but when the adapter power is turned off you may need to pull the adapter for the radio to revert back to the batteries.
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rogerperkinsCommented:
A Reed relay can also be used.
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Crash2100Author Commented:
Unfortunately, there is no hole for an adapter, I have to hook it directly to the battery clips.
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rogerperkinsCommented:
My grandpa did that one time, years ago, and blew out the radio, I think he reversed the polarity, actually voltage goes up in series, so if you hook up the adapter in parallel with the batteries it shouldn't increase the voltage, just be sure you are using rechargeable batteries, because you will get the added benefit of charging the batteries at the same time.
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Crash2100Author Commented:
do you know where I can get a relay like that, do you think radio shack would sell them?
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publicCommented:
Just use a dual common cathode diode. Connect one anode to the battery, the other to the lighter. The lighter has a higher voltage when the engine runs, so it will supply the current.
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drelectroCommented:
Re Relays - Radio shack should sell them.
How many batteries does your radio take ?
Make sure you get one with a coil voltage to match the adapter output voltage.

Re diodes you could aslo use 2 regular diodes with the cathodes joined together, BUT you need to make sure that the adapter voltage is more than about 0.6V higher than the battery pack, this may not be the case if the adapter is regulated.
Of course you can't make it too high much higher or you  may fry the radio.


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Crash2100Author Commented:
two AAA's, it's 3V
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drelectroCommented:
Then you'll need a relay with a 3V coil.

Digikey (www.digikey.com) 255-1391-ND should do the trick.
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Crash2100Author Commented:
Can I use a relay with a higher rated coil, like 5V?

Because I found this one at radio shack:

http://www.radioshack.com/product.asp?catalog%5Fname=CTLG&category%5Fname=CTLG%5F011%5F002%5F013%5F000&product%5Fid=275%2D232&hp=search
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publicCommented:
If you really want a relay then use 12V coil with C contacts type, and connect the NC to the battery, the NO to the adapter, and the common to the radio.
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Crash2100Author Commented:
I got that relay and I have it working.  But I've got one other question.

How could I use a transistor to switch the radio off when it goes to the battery?  I just need it to switch for a split second.  I figured out how to make it turn the radio on using the transistor, but how can I turn it off?
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drelectroCommented:
Sorry I for one don't follow.

Is the transistor in addition to the relay ? or instead Of ?

How it the transistor connected at the moment ?
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Crash2100Author Commented:
I'm going to try to connect a transistor to the radio's power button so that when the car's turned off, it turnes the radio off.
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rogerperkinsCommented:
Just take out the batteries.
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Crash2100Author Commented:
Sorry for the delay, I haven't had much time to mess with this.

I got this relay from radio shack and I have it working in a circuit.
http://www.radioshack.com/product.asp?catalog%5Fname=CTLG&category%5Fname=CTLG%5F011%5F002%5F013%5F000&product%5Fid=275%2D240

I haven't been able to get anything working with the switch, but at least the power works.

Thanks for the help!
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