Add Youtube thumbnail image into README.MD file.

README.md

@@ -4,15 +4,18 @@ In this project, we will upgrade a traditional shortwave [superheterodyne receiv
 
 In this upgrade, we enhanced the local oscillator with [Si5351 clock generator module](https://s.click.aliexpress.com/e/_9iD0bV) and Arduino control circuit. Compared to the [old design](http://jayakody2000lk.blogspot.com/2015/03/two-transistor-superheterodyne.html), this new receiver uses an improved version of an intermediate frequency amplifier with 3 I.F transformers.
 
+[![Arduino Superheterodyne Receiver](https://raw.githubusercontent.com/dilshan/arduino-superhet/main/resources/arduino-superhet-youtube.jpg)](https://github.com/dilshan/arduino-superhet)
+
 In this new design, we divide this receiver into several blocks, which include, mixer with a detector, a local oscillator, and an I.F amplifier. The I.F amplifier builds into one PCB. The filter stage, mixer, and detector stages place in another PCB.
 
 In this prototype build, the *Si5351* clock generator drives using an *Arduino Uno* board. With the given sketch, the user can tune and switch the shortwave meter bands using a rotary encoder. The supplied sketch support clock generation from 5205kHz (tuner frequency : 4750kHz) to 16285kHz (tuner frequency : 15830kHz). If required, this range can further extend or reduce by modifying the Arduino source code. In the given source code, we use [*Etherkit's Si5351 library*](https://github.com/etherkit/Si5351Arduino) to control the clock generator module.
 
+![Prototype version of the filter, mixer and detector PCB.](https://raw.githubusercontent.com/dilshan/arduino-superhet/main/resources/arduino-superhet-mixer.jpg)
 This receiver uses a conventional 455kHz I.F transformer kit in an intermediate frequency amplifier. We use a [2SC724](https://www.web-bcs.com/transistor/tc/2sc/2SC724.php) transistor for each filter stage. The value of this transistor is not critical and can be replaced with any suitable mid-frequency NPN transistor, for example, [2SC930](https://s.click.aliexpress.com/e/_9vi1D5), [BF494](https://s.click.aliexpress.com/e/_Ap8gur), [BC337](https://s.click.aliexpress.com/e/_9gRclD), or [2SC829](https://s.click.aliexpress.com/e/_ACedU7), etc.
 
+![Prototype version of I.F amplifier.](https://raw.githubusercontent.com/dilshan/arduino-superhet/main/resources/arduino-superhet-if-amp.jpg)
 The 2nd PCB consists of a filter, mixer, and detector circuit. This mixer stage also uses the NPN low power transistor and can replace with any of the above transistors. The crucial component of the detector phase is the [OA90](https://s.click.aliexpress.com/e/_AknfCT) or [1N34](https://s.click.aliexpress.com/e/_AdOUUn) Germanium diode. It can also change with any suitable Germanium signal diode. 
 
 The front filter stage constructs using a pair of readymade [1µH inductors](https://s.click.aliexpress.com/e/_ApVWb9). The suggested supply voltage to this tuner is 6V to 9V. 
 
 All the design files of this project were released under the [Creative Commons Attribution 4.0 International](https://creativecommons.org/licenses/by/4.0/) license. The Arduino sketch is released under the [MIT License](https://github.com/dilshan/arduino-superhet/blob/main/LICENSE). 
-