add project description

README.md

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-these are the hardware design files for **insert project name here**.
+the PD blocker removes Power Delivery (PD) capabilities from USB-C connections
 
 purpose
 =======
 
+some USB-C devices do not require the Power Delivery features available on the connector, mainly higher voltage and current.
+the device might not even be rated for higher voltages.
+mischievous eMarked cables (with PD-capable chips built in) or chargers might raise the voltage without the device requesting it.
+the higher voltage could then damage the device.
+
+the PD blocker protects your [expensive] USB-C device from such damages.
+
 usage
 =====
+
+insert the PD blocker in line between the power source (e.g. charger or host computer) and the cable connecting your device.
+this is the ideal position to also safeguard against mischievous eMarked cables, still being able to use them.
+else you can also insert the PD blocker between your device and the cable or power source.
+your device will still be protected against mischievous eMarked cables, but you won't be able to use them.
+
+all other features of USB-C are preserved (e.g. USB2, USB3, USB4, SBU, orientation detection)
+some alternate modes requiring PD communication will not work anymore though.
+
+mode of operation
+=================
+
+to prevent PD communication, capacitors are placed on the CC lines.
+they smooth out the 300 Kbps BMC signal used for PD communication.
+it still allows the identification of Rp, Rd, and Ra used for orientation and role detection.
+
+an additional circuit will cut the power when VBUS exceeds 5.5V (on either side of the PD blocker).
+
+warning
+=======
+
+the PD blocker can block up to 36V.
+PD rev 2.0 specifies voltages up to 20V.
+PD rev 3.0 specifies voltages up to 48V, but I haven't seen any power source using this new capability yet.