Calculating Resistor Size for Time Delay in Alarm Circuit

[ehabmozart]

Homework Statement

A student is designing an alarm circuit. She wants a time interval between closing a switch and the circuit becoming active. She decides to charge a 2200 mu (x10 -6) F capacitor through a resistor from a 5V supply. The potential difference across the cap. must rise to 4.3 V for the alarm to become active. Calculate the size of the resistor that she should connect in the circuit to achieve a 1 minute delay

Homework Equations

V=V(initial)e^-t/RC

The Attempt at a Solution

t should be 60 seconds
ln(4.3/5)=-60/Rx2200x10-6
RC=719.5831403
R should be .. Oh .. That's too big.. I NEED HELP!

 

[Muphrid]

Notice that the problem says the voltage across the capacitor should rise to 4.3 volts, yet when we plug in t=0 into your formula, you get 5 volts. I would consult notes and make sure that this is the correct formula for the capacitor's voltage.