In both circuits, the SCR (and thereby the lamp) can be latched on by momentarily closing SI, thereby fe
eding gate drive to the SC via Rl. In both circuits, the gate is tied to the cathode via R2 to improve circuit stability. Of course, after the SCR turns on, it can be turned off again only by momentarily reducing anode current below the device"s IH value. The SCR is turned off by momentarily opening S2, by using S2 to short the anode and cathode terminals of the SCR momentarily.