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Discussion Starter #1 (Edited)
I have a pair of power fold mirrors to play with. I shan't be using the auto dimming feature, but I thought it could be useful to share their properties for anybody experimenting or considering their own control of dimming, even simply manual dimming with a knob.

Auto dimming mirrors use an electrochromic technology consisting of layers separated by a conductive electrolyte. They behave like a very large capacitor. When a small voltage is applied to the mirror, the layers darken. When the voltage is removed and falls to near zero, the mirror lightens. Changes in brightness up or down are fairly slow (8 seconds plus) as the capacitor layers build up or release charge. They remind me of those self darkening spectacles. Great in sunlight until you drive into a tunnel and can't see. Their response can be quick when new, but slows down as they age. However, V.W fit these with the rear view mirror to keep us safe.

I bought a super mini d.c-d.c step down switching supply for mirror tests. I was being ultra cautious not wanting to damage my mirror. These supply boards are adjustable down to 0.7 Volts and deliver a maximum current of 3A. They are the size of a thumb!

After I started testing, I found the mirror voltage needed to be <0.7V to completely lighten. In fact, if you buy these used, hold them up to reflect a strong light on the surface and you can just make out the natural mirror around the edges and a very slight darkening inside this boundary. If this arrives dark with no voltage, it's a bad mirror and won't match the standard mirror on the other door.

I adjusted the power supply to 3V and followed it with a 220 ohm variable resistor and two series connected diodes. These ensure the voltage applied to the mirror is in the range 0-1.65 Volts.

I aimed a white led lamp 45 degrees to the mirror surface and setup an electronic LUX meter to measure reflected incident light at 45 degrees to the mirror surface. I no longer have access to lab grade photometry instruments and a cheap LUX meter is all I have. But you could also use a camera and count the stops. The switched supply board is very efficient and most of the current input from a bench power supply goes to the mirror. This is the current I measured for these tests.





The red curve shows how the light reflected is reduced from 0V applied to the mirror. Look at how the mirror current only increases slowly and there is a crossover at 1100mV when the mirror is darkest at 30 LUX (relative to 126 LUX). As the mirror voltage and current increases, there is no further large change in mirror darkness. High mirror current should be avoided.

Therefore, the mirror will operate over its full range of dimming with voltage 0-1100mV and current <30mA. The mirror doesn't therefore need a complex high current power supply to work, although a semiconductor driven power source could help reduce the lag and improve response times?

Revisiting my test circuit to make a manual mirror dimmer, the dc-dc power supply board I used could be replaced by a simple resistor connected to the vehicle supply, a 220 ohm variable resistor and 2 diodes. The series resistor could be about 100 ohms 2 watts. A low driver current of 30mA doesn't require a powerful power source and I can see now why this can come from the interior mirror. IMHO, I don't like the slow response times, but each to their own.

Life is Good - Vox.
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