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relay to turn on led


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nguyenphananh 
Member - Posts: 19
Member spacespace
Joined: May 04, 2005
Posted: March 19, 2014 at 8:50 PM / IP Logged  
Hi, Im new, and don't know much about electronic, but like to mess out cars. Please let me know what I should do in this situation:
I want to install a LED strip in my car using the power source from the cigarette lighter (12vdc). I want the LED strip to turn on/off with the dash's lights. The dash's lights output is 5vdc. Are there anyway to use the relay to turn on/off the 12vdc source using the dash's lights 5vdc?? how do I do it? what parts do I need?? Right now I hooked up the LED strip directly to the dash's lights (5vdc), but it was too dark.
Thanks in advance
Ween 
Platinum - Posts: 1,364
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Joined: August 01, 2004
Location: Illinois, United States
Posted: March 19, 2014 at 10:30 PM / IP Logged  
Since the dash lights usually come on with the parking lights, it would be easier to locate a parking light wire in the car. You can use that wire to activate your relay. If the LED strip doesn't consume that much power, it could be connected directly to the parking light circuit. A fuse inline would be advisable, in case there is an issue with the strip. What is the year, make and model of your car?
nguyenphananh 
Member - Posts: 19
Member spacespace
Joined: May 04, 2005
Posted: March 19, 2014 at 10:48 PM / IP Logged  
Ween wrote:
Since the dash lights usually come on with the parking lights, it would be easier to locate a parking light wire in the car. You can use that wire to activate your relay. If the LED strip doesn't consume that much power, it could be connected directly to the parking light circuit. A fuse inline would be advisable, in case there is an issue with the strip. What is the year, make and model of your car?
it is a 2006 Mercedes slk280. To tap into the parking light, it would be very complicated on this car (as many already tried), due to all the insulation, seal etc... I already tapped into the dash lights, hope I can use this to switch on the relay....
but thank you for this ideal.... it will be my last option.
Ween 
Platinum - Posts: 1,364
Platinum spacespace
Joined: August 01, 2004
Location: Illinois, United States
Posted: March 19, 2014 at 11:24 PM / IP Logged  
An easy solution would be to obtain a PAC-AUDIO TR4 universal trigger module. Has power and ground, a trigger input and +12volt output.
How many LED's are in the strip? The TR4 may be able to power them directly.
oldspark 
Gold - Posts: 4,913
Gold spacespace
Joined: November 03, 2008
Location: Australia
Posted: March 20, 2014 at 1:30 AM / IP Logged  
Why not use a 5V relay?
Or the +5V could turn on a transistor or MOSFET that turns on the LEDs. That's especially easy of you can GND the LEDs strips to turn them on (ie, a single NPN transistor or N-ch 'logic' MOSFET).
What I don't like about relays for LEDs is that usually the relays consume more power than the LEDs themselves, tho this is a case or voltage conversion - or rather, using a relay to interface equipment on different power supplies.
BTW - if they are "12V" LED strips as opposed to "automotive 12V", ensure you add an appropriate resistor or other voltage clamping method.
nguyenphananh 
Member - Posts: 19
Member spacespace
Joined: May 04, 2005
Posted: March 20, 2014 at 3:00 AM / IP Logged  
Ween wrote:
An easy solution would be to obtain a PAC-AUDIO TR4 universal trigger module. Has power and ground, a trigger input and +12volt output.
How many LED's are in the strip? The TR4 may be able to power them directly.
Thank you so much... that will work for me, can you tell me exactly what to buy (link will be perfect). I have no knowledge of electronic.. just like to play around with my car, and common sense will work for me
nguyenphananh 
Member - Posts: 19
Member spacespace
Joined: May 04, 2005
Posted: March 20, 2014 at 3:03 AM / IP Logged  
oldspark wrote:
Why not use a 5V relay?
Or the +5V could turn on a transistor or MOSFET that turns on the LEDs. That's especially easy of you can GND the LEDs strips to turn them on (ie, a single NPN transistor or N-ch 'logic' MOSFET).
What I don't like about relays for LEDs is that usually the relays consume more power than the LEDs themselves, tho this is a case or voltage conversion - or rather, using a relay to interface equipment on different power supplies.
BTW - if they are "12V" LED strips as opposed to "automotive 12V", ensure you add an appropriate resistor or other voltage clamping method.
Thank you very much for the option. Can you please tell me exactly what to buy? (link will be perfect). Im not familiar with these stuff. I used the 12v automotive LED strip bought from pepboy.... I can follow the instruction well, just don't know exactly what to buy..
nguyenphananh 
Member - Posts: 19
Member spacespace
Joined: May 04, 2005
Posted: March 20, 2014 at 3:17 AM / IP Logged  
Ween wrote:
An easy solution would be to obtain a PAC-AUDIO TR4 universal trigger module. Has power and ground, a trigger input and +12volt output.
How many LED's are in the strip? The TR4 may be able to power them directly.
Right now, the LED strip works with 5vdc, but it is very dim. It is much brighter with 12vdc. I saw a PAC tr4 on ebay for around $15 (to turn on car's amplify).... is that the correct part?? thank you
oldspark 
Gold - Posts: 4,913
Gold spacespace
Joined: November 03, 2008
Location: Australia
Posted: March 20, 2014 at 4:14 AM / IP Logged  
Google '5V relay'. They are available from ~80c upwards.
However, if daunted by the stuff below, maybe the PAC relay is a better option.
You only need an SPST type though it doesn't matter of it's SPDT (or even DPDT).
And it probably only needs to be rated for 12V DC and 1A for LEDs though 5A & 10A are fine and probably more common, and cheaper.
Note that they are 'contact ratings' - ie, what the relay can switch - as opposed to its 'coil' rating (ie, 5V @ 10mA or 100mA etc).
EG - maybe OMRON 5V 10A SPDT Relay. (Yes, it can be confusing - the coil is 5V but the 10A refers to the contact rating which in this case is AC but you want its DC rating which is 5A... but you need to see its datasheet for that.)   
And you'd probably have to solder its terminals. Though 12V relays often have spade connectors, 5V relays are usually solder-type terminals to suit 5V (computer etc) applications.   
You could refer to the Relay info on this site, but in brief, one side of the coil (86) to your dashlight's +5V & the other side (85) to GND with the NO = Normally Open contacts/terminals (30 & 87) between your +12V and the LED's +12V.
If using an SPDT relay I'd suggest the +12V supply to 87 with the LED +12V off the 30 aka Common aka Comm aka C terminal.
The numbers 85, 86, 30 & 87 (and 87a for SPDT) refer to a common DIN numbering format, but your relay might not have the same numbering. But during the several decades that I used relays BEFORE joining this forum I only ever used the Legend on the relay case and not those DIN numbers...
As to fuses, a fuse will not protect the LEDs, BUT...
A 1A or 2A or 5A fuse could be used in your +12V supply in case of shorts in your wiring...
And a small fuse could be used in your dashlight +5V coil connection in case you have wired it wrong. That's only to hopefully prevent damage to your dashlight supply but should not be required (and would not be used) if your wiring is correct. The fuse needs to be rated higher than your relay coil current - say twice as high. (EG - that OMRON relay has a 72mA coil current at 5V, hence a 100mA or 200mA or 250mA fuse.)
However a small bulb might be used instead of a fuse to test - eg, a 2W or 3W light bulb.
Of course it is assumed that the dashlight +5V can handle the extra load of the relay coil. (But IMO it should.)
And it assumes there is no dash light dimming (control).

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