diff --git a/docs/about.md b/docs/about.md
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@@ -11,4 +11,8 @@ Various approaches considered as possible solutions for O-ESD are given along wi
 
 ## 3. Trade-offs and cost-benefit analysis of considered solutions
 The cost-benefit analysis of considered solutions is done by taking into the account the availability of electronic components on website of online distributors of electronic components. The understood trade-offs for considered solutions are given.<br>
-[Trade-offs and cost-benefit analysis of considered solutions](https://o-esd.etf.bg.ac.rs/forgejo/dragan.olcan/O-ESD/raw/branch/main/Documents/O-ESD-03.pdf) 
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+[Trade-offs and cost-benefit analysis of considered solutions](https://o-esd.etf.bg.ac.rs/forgejo/dragan.olcan/O-ESD/raw/branch/main/Documents/O-ESD-03.pdf) 
+
+## 4. Design and prototype printed-circuit boards for all candidate solutions
+All considered solutions are prototyped. Since some candidate solutions share some components or parts, PCBs/protypes are made for functional parts that can be mixed and matched between O-ESD solutions. For all considered parts that have PCB, gerber and drill files (needed for the production) are given. The photos of prototypes, functional parts and considered components are provided, too.
+[Snapshot of current hardware developement](https://o-esd.etf.bg.ac.rs/forgejo/dragan.olcan/O-ESD/src/branch/main/Hardware_developement) 
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