Board_003: Add readme for board

Electrical/Board_003/README.md

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+
+# Overview
+
+This board can be used to simulate a M-Bus master unit instead of
+connecting to the real AMS unit, i.e. usable for test and development.
+It takes a 5 V TTL level input (serial) signal and converts it
+to a M-bus level signal. It only supports sending. Based on
+[MBus_USB.pdf](https://github.com/rscada/libmbus/blob/master/hardware/MBus_USB.pdf)<sup>1</sup>.
+It depends on an external power source capable of delivering 25-40 Volt.
+Most DC-DC boost/step-up converter modules<sup>2</sup> should be fine for this.
+
+![](mbus_master.png)
+
+## BOM
+
+* D1 13V zener
+* Q1-Q3 NPN transistor
+* Q4 PNP transistor
+* R1 1k
+* R2 6.8k
+* R3 1 (optional, can be shorted instead)
+* R4 150
+* R5 22k
+* R6 220k
+
+Note for the zener. This board depends on the zener to operate below 1mA,
+e.g. like 1N5350 whereas a BZX55 will probably not operate properly.
+
+![](zener_ok.png?raw=true) ![](zener_not_ok.png?raw=true)
+
+This could be accomplished by reducing the 22k resistor so that it
+draws enough current.
+
+
+
+-------------
+
+<sup>1</sup>
+See also https://electronics.stackexchange.com/questions/99388/designing-a-m-bus-master-up-to-10-slaves/ and https://electronics.stackexchange.com/a/214477/568.
+
+<sup>2</sup>
+Like for instance [this one](http://hobbycomponents.com/power/698-xl60009-dc-dc-step-up-boost-converter) for £3.