C Program crashing when character is entered into menu option - c

Question

Hi this program keeps crashing when I enter a character into the menu. I have figured out it is because scanf is looking for an integer and cannot find one so it goes into an infinite loop. I have yet to find how to correct this anywhere online and am looking for some much needed help. I cant seem to create a function to do input validation and that is where I need help as our professor does not seem to help us with these. Thank you.
#include <stdio.h>
void resistor (void);
float calc_freq(float ra, float rb, float c) // function to calculate Frequency
{
return 1.44/((ra+(2*rb))*c);
}
float calc_duty_cycle(float ra, float rb, float c) // function to calculate duty cycle
{
return (rb)/(ra+(2*ra));
}
int main (void)
{
int choice; // choice to run program from start menu or quit
while (1)
{
// Menu for entering resistor and capacitor values
printf("Welcome to the 555 Timer Frequency and Duty Cycle Calculator\n");
printf("Please enter two resistor values in between ");
printf("1 kOhms and 100 kOhms and a capacitor value\n\n");
printf("Menu\n\n");
printf("1. Continue\n");
printf("2. Exit\n");
scanf("%d", choice);
switch (choice)
{
case 1: resistor();
break;
case 2 : printf("Goodbye.");
break;
default: printf("Sorry I do not understand\n\n");
main();
}
break;
}
return 0;
}
void resistor (void)
{
float ra, rb, c; // Float variables for two resistor values and capacitor
while(1) // While loop to gather resistor A value
{
printf("Enter a value for Resistor Ra (kOhms): "); // First Resistor Value
scanf("%f", &ra);
if(ra < 1 || ra > 100) // Will repeat loop until value between 1 and 100 is entered
printf("Invalid selection, choose again.\n");
else
break; // breaks loop when valid data is entered
}
while(1)
{
printf("Enter a value for Resistor Rb (kOhms): "); // Second Resistor Value
scanf("%f", &rb);
if(rb < 1 || rb > 100) // Will repeat loop until value between 1 and 100 is entered
printf("Invalid selection, choose again.\n");
else
break; // breaks loop when valid data is entered
}
while(1)
{
printf("\nCapacitor (uF) : "); // Capacitor Value
scanf("%f", &c);
if(c <= 0)
printf("Invalid selection, choose again.\n");
else
{
float freq = calc_freq(ra, rb, c); // call function to calculate frequency
float duty_cycle = calc_duty_cycle(ra, rb, c); // cal function to calculate duty cycle
printf("\nFrequency is: %.2f kHz\n", freq); // print frequency
printf("Duty cycle is: %.2f percent\n", duty_cycle*100); // print duty cycle
break; // break while loop
}
}
}