Dead simple script to find optimal resistor and capacitor values for a 555 timer circuit

555.py

#!/usr/bin/env python

import sys
import itertools

TIME_HIGH_GOAL = int(sys.argv[1])
TIME_LOW_GOAL = int(sys.argv[2])

print 'Seeking optimal R1, R2 and C1 values for a %i pulse every %i milliseconds...' % (TIME_HIGH_GOAL, TIME_LOW_GOAL)

# Common resistor values
RESISTORS = [0, 1.5, 4.7, 10, 47, 100, 220, 330, 470, 680, 1000, 2200, 3300, 4700, 10000, 22000, 47000, 100000, 330000, 1000000]

# Common capacitor values
CAPACITORS_MICRO = [1000, 470, 100, 47, 22, 10, 4.7, 2.2, 1]
CAPACITORS_NANO = [470, 200, 100, 22, 10, 4.7, 2.2, 1]
CAPACITORS_PICO = [47, 22, 10, 470, 220, 100]

CAPACITORS = [float(c) / 1000 for c in CAPACITORS_MICRO]
CAPACITORS.extend([float(c) / 1000000 for c in CAPACITORS_NANO])
CAPACITORS.extend([float(c) / 1000000000 for c in CAPACITORS_PICO])

guesses = itertools.product(RESISTORS, RESISTORS, CAPACITORS)
best_guess = (0, 0, 0)
best_off_by = 999999999

for r_one, r_two, c_one in guesses:
    # R1 minimum is 1K ohms
    if r_one  < 1000:
        continue

    # C1 inimum is 0.0005 microfarads
    if c_one < (0.0005 / 1000):
        continue

    time_high = 0.693 * (r_one + r_two) * c_one 
    time_low = 0.693 * r_two * c_one

    off_by = abs(TIME_HIGH_GOAL - time_high) + abs(TIME_LOW_GOAL - time_low)
    
    if off_by < best_off_by:
        best_guess = (r_one, r_two, c_one)
        best_off_by = off_by

r_one, r_two, c_one = best_guess

print 'Best guess:'
print 'R1:', r_one
print 'R2:', r_two
print 'C1:', c_one
print 'Time high:', 0.693 * (r_one + r_two) * c_one
print 'Time low:', 0.693 * r_two * c_one