An emergency indicator light uses a 10 V, 2-W incandescent lamp. It is to be ON when a digital output is high (5 V). The digital circuit does not have sufficient power to turn on the lamp directly. However, as is common practice, a transistor driver is used as a digital switch. Select RB in the circuit of Figure P4–22 so to drive the transistor into saturation causing it to act as a short circuit between the lamp and ground when the digital output is high. The Thevenin equivalent for the digital circuit is also shown in the figure.

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+1 +10 V iLamp Lamp=10V @ 2W RD RB ww www =50 Vy=0.7 V Digital circuit VD=5 V RD = 500 2 FIGURE P4-22