diff --git a/Electrical/Board_003/simulation/README.md b/Electrical/Board_003/simulation/README.md
new file mode 100644
index 00000000..1a199dca
--- /dev/null
+++ b/Electrical/Board_003/simulation/README.md
@@ -0,0 +1,11 @@
+
+This directory contains various simulation of the M-Bus part of [MBus_USB.pdf](../doc/MBus_USB.pdf?raw=true).
+The simulattions are done with [qucs-s](https://ra3xdh.github.io/) using [ngspice](http://ngspice.sourceforge.net)
+as simulation backend.
+
+The first simulation is as close to the original as possible and gradulally changed to
+remove receiving parts and using a zener diode to have a constant 12V voltage drop
+regardless of the power supply. The final version works with any voltage in M-Bus range.
+
+![](mbus_master_004.png)
+