An emergency indicator light uses a bright \(15 \mathrm{~V} @ 0.5 \mathrm{~W}\) LED indicator. 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 LED directly. However, as is common practice, a transistor
driver is used as a digital switch. Select \(R_{\mathrm{B}}\) in the circuit of Figure \(\mathrm{P}_{4}-18\) so as to drive the transistor into saturation causing it to act as a short circuit between the LED and ground when the digital output is high. The Thévenin equivalent for the digital circuit is also shown in the figure.

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VD (+1) RD ww Digital circuit VD=5 V RD = 500 52 LED RB www +15 V LED 15 V @ 0.5 W B = 50 Vy=0.7 V