user16647: What is the cheapest and best method to convert 12v(14)<->5V(3.3)? Need to connect Arduino to a car.
I need logical shifter not buck converter to connect Arduino to car 12v circuit. I would like to control electrical equipment of car by Arduino. For example, I want to connect input pin to heater, when heater is on from 12v circuit I get HIGH 12v signal which processes by Arduino, but like everyone knows, Arduino can handle only 5/3.3V. Then Arduino going to change state of equipment by connecting optocoupler/relay/transistor to  button(of elect. equip.) and will act like physical press. 
P.P.S I know that more clever to hack and control by CAN bus. But not all eq. are connected to CAN bus and it`s a little bit risky but I'm working on it as well. 
I'm looking for best low cost solution. I have found it on ebay . Will it work like I want? Offer DIY solutions as well.

user4143: Your safest option is to use an opto-isolator. Connect the LED side in parallel with the load with a suitable resistor, then the transistor side becomes, effectively, a button connected to the Arduino.
As far as the car is concerned you have just connected an LED. As far as the Arduino is concerned you have a button connecting to ground. 
It provides complete isolation of the car's electrics from your Arduino.

user10214: Ok, so... I can see your question is a bit obscure, so I'll try to cover a generic approach.

Power If you need to power the arduino, you can get a cheap car USB adapter. If you want a "solderable" solution, I'd avoid linear regulators (such as a 7805)and go with a cheap switching module. For low power applications I usually buy some modules called mini360 (for instance here is a link from aliexpress) which are cheap and ok for even some hundreds of milliamps. You FIRST turn the potentiometer to set the output voltage (checking with a multimeter) and THEN connect the arduino to it.
Get status If you need to get the status of something (e.g. whether the lamp is on or off) you have to analyse the circuit. If you have to detect if either 12V or GND are applied, you can solve the problem with a voltage divider. But if the lamp is always connected to 12V and then the GND side is he controlled one then you will always force a (small) current inside it. If this is the case, maybe an optocoupler is the best solution. If the signal is a PWM maybe you need some filtering, if it is a digital wire maybe it is already at the correct voltage, .... You have to figure out what is the best solution for you
Set status If you need to control something (e.g. turn on a light) your best bet is to use a 5V relay. There are plenty of them, even modules with the transistor and diode necessary to control them with an arduino.

Here are some ideas and rough schematics for the three points


simulate this circuit – Schematic created using CircuitLab
Just one very big WARNING: you have to carefully double check that the modification does not blow something up (for instance, you can make a short circuit if the body computer tryes to force a wire to 12V and you close the relay to ground). So... Be very careful, since what you do can break a lot of things, and if you are very lucky the bill can be only some couple of thousand dollars (I'll let you figure out what happens if you are not lucky enough)

16647: <p>What is the cheapest and best method to convert 12v(14)&lt;->5V(3.3)? Need to connect Arduino to a car.</p>

<p>I need logical shifter not buck converter to connect Arduino to car 12v circuit. I would like to control electrical equipment of car by Arduino. For example, I want to connect input pin to heater, when heater is on from 12v circuit I get HIGH 12v signal which processes by Arduino, but like everyone knows, Arduino can handle only 5/3.3V. Then Arduino going to change state of equipment by connecting optocoupler/relay/transistor to  button(of elect. equip.) and will act like physical press. 
P.P.S I know that more clever to hack and control by CAN bus. But not all eq. are connected to CAN bus and it`s a little bit risky but I'm working on it as well. 
I'm looking for best low cost solution. I have found it on <a href="http://www.ebay.com/itm/Level-Shifter-Logic-Level-Converter-12-Channel-5V-to-12V-or-12V-to-5V-/251604416861?hash=item3a94cab95d:g:DUYAAOSwxH1T2d~7" rel="nofollow">ebay</a> . Will it work like I want? Offer DIY solutions as well.</p>
 4143: <p>Your safest option is to use an opto-isolator. Connect the LED side in parallel with the load with a suitable resistor, then the transistor side becomes, effectively, a button connected to the Arduino.</p>

<p>As far as the car is concerned you have just connected an LED. As far as the Arduino is concerned you have a button connecting to ground. </p>

<p>It provides complete isolation of the car's electrics from your Arduino.</p>
 10214: <p>Ok, so... I can see your question is a bit obscure, so I'll try to cover a generic approach.</p>

<ol>
<li><p><strong>Power</strong> If you need to power the arduino, you can get a cheap car USB adapter. If you want a "solderable" solution, I'd avoid linear regulators (such as a 7805)and go with a cheap switching module. For low power applications I usually buy some modules called mini360 (for instance <a href="https://www.aliexpress.com/item/1pcs-Mini360-DC-DC-Buck-Converter-Step-Down-Module-4-75V-23V-to-1V-17V-17x11x3/32835208399.html?spm=2114.search0104.3.9.372623d0lCPKb6&amp;ws_ab_test=searchweb0_0,searchweb201602_1_10152_10151_10065_10344_10068_10342_10343_10340_10341_10696_10084_10083_10618_10304_10307_10302_5711211_10313_10059_10184_10534_100031_10103_10627_10626_10624_10623_10622_5722411_10621_10620_5711311,searchweb201603_12,ppcSwitch_5&amp;algo_expid=3bf6a53d-cc3c-479b-b30a-5a108f557776-1&amp;algo_pvid=3bf6a53d-cc3c-479b-b30a-5a108f557776&amp;priceBeautifyAB=0" rel="nofollow noreferrer">here</a> is a link from aliexpress) which are cheap and ok for even some hundreds of milliamps. You FIRST turn the potentiometer to set the output voltage (checking with a multimeter) and THEN connect the arduino to it.</p></li>
<li><p><strong>Get status</strong> If you need to get the status of something (e.g. whether the lamp is on or off) you have to analyse the circuit. If you have to detect if either 12V or GND are applied, you can solve the problem with a voltage divider. But if the lamp is always connected to 12V and then the GND side is he controlled one then you will always force a (small) current inside it. If this is the case, maybe an optocoupler is the best solution. If the signal is a PWM maybe you need some filtering, if it is a digital wire maybe it is already at the correct voltage, .... You have to figure out what is the best solution for you</p></li>
<li><p><strong>Set status</strong> If you need to control something (e.g. turn on a light) your best bet is to use a 5V relay. There are plenty of them, even modules with the transistor and diode necessary to control them with an arduino.</p></li>
</ol>

<p>Here are some ideas and rough schematics for the three points
</p>

<p><img src="https://i.stack.imgur.com/d8fmw.png" alt="schematic"></p>



<p><sup><a href="/plugins/schematics?image=http%3a%2f%2fi.stack.imgur.com%2fd8fmw.png">simulate this circuit</a> &ndash; Schematic created using <a href="https://www.circuitlab.com/" rel="nofollow">CircuitLab</a></sup></p>

<p>Just one <strong>very big</strong> WARNING: you have to carefully double check that the modification does not blow something up (for instance, you can make a short circuit if the body computer tryes to force a wire to 12V and you close the relay to ground). So... Be very careful, since what you do can break a lot of things, and if you are very lucky the bill can be <em>only</em> some couple of thousand dollars (I'll let you figure out what happens if you are not lucky enough)</p>
