Lab 1 - Pager Motor Circuit

For the second part of the lab, the team was asked to swap out the LED for a pager motor. The circuit schematic is shown below.

The resistor values for this circuit were justified as follows:

Current Limiting Resistor (Motor/Speaker):

In order to limit the current through the pager motor, the team inserted the resistor R4 shown in the schematic diagram above. The resistor was sized to limit i4 to 65mA using the following equation, where the input voltage Vcc=5V, the resistance of the motor RL=51.7 ohms, and the collector-emitter voltage VCE=0.3V:

The above equation yields R4=20.6 ohms. For the circuit shown above, the team used a 20-ohm resistor.