V Batt RD a RD RA Figure 7: Zombie Alarm Circuit b RD RD
Night has fallen and you’ve taken shelter in a room with 4 doors. You’ll need a good sleep so that you
move quickly the next day to keep moving away from the zombie horde. The zombies have not been
reported in your part of the city yet, but they move quickly at night and could show up while you are
resting. You find some wire and AA batteries and set up the alarm system shown schematically in Figure
7.
In Figure 7, RD represents the resistance of the doorknob on each of the 4 doors in the room. ?? = 500 ?.
RA represents the alarm. It has a resistance of 1.5 kΩ and will sound an alarm when the voltage across it
reaches a magnitude above 1 V (i.e. Vab > 1 V or Vba > 1 V). The idea is that if a zombie touches the
doorknob of one of the doors, it will act as a resistor in parallel with the resistance of the doorknob. You
estimate that a zombie represents a resistance of 300 Ω.
1. Answer the following questions. You can use Circuit JS to determine your answers.
a) AA batteries have a voltage of 1.5 V. What is the minimum number of AA batteries you need to
connect in series to create VBatt such that a zombie touching one of the doorknobs sets off the
alarm?
b) If the zombies are clever, how could they open the doors without setting off the alarm and waking
up their warm meal (you)?
2. Include a screenshot of the simulated circuit that you used to determine the number of batteries use
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