This backup light circuit guarantees the fact that if Lamp 1 burns out then Lamp 2 will light up. But remember to replace the Lamp 1 or else this circuit won’t work again when Lamp 2 is burned out.
How does the backup light circuit works
In series with La1 is the Tri2 triac. R3 and C2 form a network delay. As soon as the voltage on C2 is increasing over 30 V, the diac D2 will start to conduct and this will trigger Tri2 resulting in the lighting of La1. The command circuit of La2 is in parallel with the one for La1 but because R2-C1 has a double timing compared to R3-C2 the firing of Tri1 won’t happen when Tri2 conducts.
If La1 is burned out then there is voltage on both RC networks, through La2 and R1 and so the Tri2 will be triggered first but because its current is lower than the holding current it will stop conducting almost immediately. In this situation C1 will continue to charge and shortly Tri1 will trigger.
Because the time constant for La2 is a little higher than the one of La1, then the La2 light intensity will be a little lower. If you want that both light intensity to be the same then choose La1 with a higher nominal power.
Without heatsinks the triacs can outstand powers up to 100 W each, and with heatsinks up to 1000 W. It is not indicated to use light bulbs with power less than 25 W because it might flicker.