Who-is-first

  
Using two SCRs, this control circuit is designed to lock out the other SCR when one has been triggered, so only one lamp will light. Indicator lamp Il is controlled by SCRl. The operator simply presses switch Sl. Lamp I2 is similarly controlled by S2 and SCR2. With both switches open, neither lamp is lit. The result is insufficient gate current to trigger SCR2 into conduction, so lamp I2 does not light.
Who-is-first - schematic

If S2 is pressed first, the reverse situation occurs. Once one of the SCRs is activated, it is necessary to open S3 to tum the light off.




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