Update README.md

README.md

@@ -6,3 +6,18 @@ This is a DIY reader for 8-bit (9-hole) vintage paper tapes.
 
 For a short demonstration see my video here:
 https://www.youtube.com/watch?v=uZ2VWKNlPQU
+
+## Building the reader head
+
+When I started with this project, constructing the reader head was
+the most uncertain aspect. I found some other DIY attempts on the web
+but none seemed ideal. I wanted to use miniature photo-transistors that
+could fit on a 0.1" grid (to match the .1" spacing of the paper tape holes).
+I found the [Kingbright AM2520P3BT03](https://www.digikey.com/product-detail/en/kingbright/AM2520P3BT03/754-2338-1-ND/9647017)
+IR phototransistors and wavelength-matched 
+[ON Semiconductor QEB363ZR](https://www.digikey.com/product-detail/en/on-semiconductor/QEB363ZR/QEB363ZRCT-ND/3479521)
+IR LEDs. Initial tests with these mounted on a perfboard were very promising,
+so I made two matched PCBs ([emitter](schematics/readerhead-emitter-pcb.pdf) 
+and [sensor](schematics/readerhead-sensor-pcb.pdf)) that can be sandwiched together
+to form the reader head.
+