# How to drop a 12v signal to 5v

jimxms
Member - Posts: 2
Joined: June 16, 2017
Posted: June 16, 2017 at 3:39 AM / IP Logged
Hi all, first post here from a complete electronics novice.
I have a 0-12v signal that I would like to monitor in my car using a datalogger that I have installed. However the datalogger inputs are only 0-5v.
So I think what I need to do is find a resistor that will drop the voltage like so: 12.0v would be 5.0v, 10v would be something like 4.1v, 6v would be something like 2.4v.....etc
stuc
Member - Posts: 9
Joined: September 12, 2008
Location: Massachusetts, United States
Posted: June 16, 2017 at 12:16 PM / IP Logged
A resistor won't do the trick. You want to use a voltage regulator
https://electronics.stackexchange.com/questions/109039/reducing-12v-to-5v
If you have a newer car you can probably pull it off the USB port. If you want to be lazy you can use the electronics in a car phone charger to get the 5V
jimxms
Member - Posts: 2
Joined: June 16, 2017
Posted: June 16, 2017 at 12:22 PM / IP Logged
The top answer on that link refers to using a LM7805 voltage regulator. However the specs sheet states: "7V To 25V In, 5V Out". This sounds like no matter what boltage between 7-25v you put in, you will always get 5v out? If so, this is not what I want.
stuc
Member - Posts: 9
Joined: September 12, 2008
Location: Massachusetts, United States
Posted: June 16, 2017 at 2:29 PM / IP Logged
Assuming signal integrity isn't critical you can use a voltage divider (two resistors). You'll want to throw a opamp/voltage follower on there so you don't have to mess with tuning the resistors based on the dataloggers properties
Theoretically you'd choose resistors with values R1 = (7/5)*R2 but you may want to round down a bit so you don't put the signal in the datalogger over 5V
You could go with a 3.3k and a 2.2k
http://tinyurl.com/y794e5ea
(the bottom resistor is where you'd put the input to the datalogger)
You can do it without the op amp but you'll want to measure the datalogger channel resistance and account for that
i am an idiot
Platinum - Posts: 13,334
Joined: September 21, 2006
Location: Louisiana, United States
Posted: June 16, 2017 at 9:01 PM / IP Logged
A regulator will not work for what you are trying to do. A regulator will put out 5 volts any time the input voltage is over 6 volts or so. You need to build a voltage divider. 1.2 K resistor and a 1K resistor. Connect the 1.2 K to to 12 volt wire. Connect the 1K to ground. Connect the 2 resistors to each other. Connect your device to the junction where you connected the 2 resistors together. I did not do the math on this, but with 12 volts across the 2 resistors there should be around 5 on the junction. 6 volts across the resistors should yield around 2,5 at the junction.
No idea what the input resistance of your device, you may have to adjust values if the voltage drops when you connect the device.
I did not read the above post till after I made this post. I too so not think you will need an op amp. Build either of the dividers, check voltages after you connect your device. Let us know what the junction voltage is and which resistors you used and we can guide you on necessary adjustments.

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