Auto dimming rearview mirror

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Rearview mirror will automatically dim if it detects driving condition where driver can be blinded by a car driving behind.


There are two light sensors (photoresistors) required for the system to work properly. One checks for ambient light level, second one checks amount of light from following car. Based on that information electronics decides if dimming is required.

Ambient sensor

One of the sensors is usually facing forward to determinate ambient light level. It's installed on the rearview mirror but in some applications cars electronic can use data from ambient light sensor integrated into rain sensor or insolation sensor installed on the dashboard.

Blinding sensor

Second sensor is facing backwards and it detects light from following car. Usually it's located at the top of the rear view mirror assembly, sometimes there's a hole in the mirror itself, sometimes the sensor is at the bottom of the rearview mirror.


Reverse gear

In some cars dimming will automatically be turned off as long as the reverse gear is engaged.

Manual off

In some cars there a button the mirror to manually turn off dimming function.

Communication with side mirrors

In some cars (BMW, Audi) also side mirrors will dim. Rear view mirror will work as the master device and both side mirrors are slave devices which are controlled by signal send from the master rear view mirror through LIN-BUS, CAN-BUS or direct control lines.


To test if the mirror is working correctly:

  1. Turn the ignition on.
  2. Make sure the the dimming function is on if your have a button on the rear view mirror.
  3. Cover the front-facing sensor with your finger to trick the electronic intro thinking that there's no daylight.
  4. Use a flashlight and shine on the blinding sensor.
  5. Wait couple seconds for the mirror to dim.


It's possible to retrofit auto dimming mirror basically in any car. When you find a mirror that you only need to feed with 12V, wire it to the car using the Terminal 15, so the mirror is only working if the ignition is on. You can also use a relay on the reverse gear sensor or reverse lights line to turn the auto dimming mirror off when reverse gear is engaged.