When S2 is ON ,the circuit here operates as an astable multi vibrator and the LED is lit for about o.1sec,flasing every 1.5 seconds.Since the human reaction time is more than this,you cannot catch it once it is seen o,by pressing S1.If your sense of time interval is good,and you press S1 with in that 0.1 sec,the discharging of C1 stops and then the lamp stays lit.you may change the ON and OFF periods by changing R1 and R2 or C1 to suit your convenience.
Circuit diagram with Parts list.
- Switch S1 can be a pushbutton switch.
- Switch S2 can be a SPST switch.
- The circuit can be powered from a 9V battery or 9V DC power supply.