We did a remoter IR heart rate monitor for horses on our Horse Treadmill... I designed the 555 circuit that produced the carrier that the heart rate modulated. I was dutifully working out the resistor value, and my boss said, "don't put so much into it, it'll have to be changed anyway (ie, you won't get it right until testing)"... so I put extra effort into picking the correct value. But one thing I almost overlooked, the RC time constant is for 1/2 of the period (charge time, discharge time). So I had to correct to make sure I got the correct 'frequency', not half period. It didn't have to be changed!!!!