You must select it according to it's Ic that must be greater than I L and it's current gain h FE. It must be at least 5 times the load current IL divided by the maximum output current from the Input to the base of the transistor The values for Rb and Qs vary accordingly. The protection diode Dp is used to protect the transistor from the reverse current generated from the coil of the relay during the switch off time. The relay will be actuated when the input of the circuit goes high. It is used to switch high loads or loads that needs AC current to operate. This circuit will drive a relay coil from a low power output, usually from an IC like 555 or a TTL/CMOS.
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