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Here the analog flip-flop circuit which built based on 2 transistors work as switch. This is very easy made and very cheap circuit. You can use this circuit as “the first project” for your students… 
and the result should be like this:
Replace the Resistor 10K with variable resistor 20K or replace the electrolytic capacitor 100uF with other value for change the frequency of LED’s flash.
Flip flop PCB layout:
