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@@ -54,17 +54,51 @@ I recommend soldering the bare minimum to power up the processor, and to flash i
 
 ### The proximity sensor
 
+Penny has two eyes, each one is composed of an infrared LED and a corresponding phototransistor.
+The LED emits infrared light; this light propagates through the air and once it hits an object it is reflected back towards the phototransistor.
+If the object is close, the reflected light will be stronger than if the object is further away.
+Note that while the infrared light is not visible by a human eye, some cameras may see it and show it on the recordings,
+it can be a handy tool for debugging Penny:
+
 ![](https://raw.githubusercontent.com/ssloy/penny/master/doc/ir-leds-on-camera.gif)
 
+The schematics is very simple:
+
 ![](https://raw.githubusercontent.com/ssloy/penny/master/doc/pcb-proximity-sensor.png)
 
+We power up two LEDs, when the phototransistors are not lit, the collector pins of Q3 and Q4 are tied to Vcc, and if the phototransistors
+sense the light, the voltage on the collector pins will go down.
+Here is the circuit test before I have installed it in the Penny's eyesockets:
+
 ![](https://raw.githubusercontent.com/ssloy/penny/master/doc/proximity-sensor.gif)
 
+Note that you may need to adjust the R6 resistor value.
+47 Ohms provides 55mA to the LEDs; some leds might need more or less current.
+For example, I have scraped a couple of LEDs from a broken toy, and they work perfectly on 3mA (910 Ohms)!
+
+I recommend to assemble first the sensing unit on a breadboard without the LEDs.
+Then take a CR2032 or a similar battery, put the LED pins on the battery directly and put it against the phototransistor.
+Verify that the voltage drops as expected.
+Once the sensing unit is okay, try to find a good resistor value for the LEDs to obtain the behaivour you see on the above video.
+Note that it is important to put a heatshrink around both the LEDs and the phototransistors to avoid parasitic lights.
+Moreover, with heatshrink it fits neatly into the eyesockets.
+
+If you fail to assemble the proximity sensor, or simply dislike it, there are plenty of options:
+
+* You can use isf471 instead of the phototransistors and all the 2n3904 circuitry.
+* You can buy Sharp GP2Y0A21YK0F distance measuring units:
+
+<img src="https://raw.githubusercontent.com/ssloy/penny/master/doc/GP2Y0A21YK0F.jpg" width="320"/>
+* Or a basic binary proximity sensor based on a LM393 differential comparator:
+
+<img src="https://raw.githubusercontent.com/ssloy/penny/master/doc/lm393.jpg" width="320"/>
+
 # Wishlist
 If you are a good soul willing to create a V2 of the motherboard, you are very welcome to do so. Here are the things that I'd like to be fixed/added/modified in the motherboard:
 * The main thing is the on/off switch to cut off servo motors during flashing;
 * Remove the crystal, internal RC should be just enough;
 * Replace through-hole components by their SMD analogs;
+* Replace R6 with a potentiometer to easily adjust the proximity sensor
 * Propose good (small and foolproof) connectors instead of pin headers and optimize their placement;
 * IR LEDs pads are very hard to reach under the center servo. The only viable option with the V1 motherboard is to solder the wires;
 * Move a little bit the big capacitor. I had to incline it, otherwise the screw in center legs would destroy it;