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#### Rikard Bosnjakovic

- Jan 1, 1970

- 0

Using these formulas:

T1 = 0.693 * (R1 + R2) * C [ ms ]

T2 = 0.693 * R2 * C [ ms ]

F = 1.44 / [ (R1 + 2*R2) * C ] [ kHz ]

for my R1, R2, C, will yield these results:

T1 = 8.62785 (ms)

T2 = 0.10395 (ms)

F = 0.1142857 (kHz)

So far so good.

The problem is now that I need to lower C to 1uF and still try to retain

T1, T2 and F as much as possible. For 100% accuracy, I simply reevaluate

the equation and extract.

First R2:

R2 = T2 / (0.693*C)

R2 = 150k

then R1:

T1 / (0.693*C) = R1 + R2

R1 = T1 / (0.693*C) - R2

R1 = 12.3M

Verifying with the equation for F:

F = 1.44 / [ (12.3M + 2*150k) * 1u)

F = 1.44 / (12.6M * 1u)

F = 1.44 / 12.6

F = 0.1142857

This means that the new values for R1 and R2 are correct, since all of the

variables T1, T2 and F are correct.

However, a resistor of 12.3 megaohms sounds pretty silly, so going for

100% accuracy on T1 and T2 is probably not a good idea. What I'm doing is

not rocket science, but a small pulse generator. The problem is that I

don't have any 1.5uF-caps at home, and buying one of them will cost me

$12. That's not an option, and that's why I have to rescale the resistors

to fit my needs.

As a last resort, I can ofcourse use 1uF in parallell with two 1uF in

series to get 1.5 uF total, but I would prefer - if possible - just

changing the resistor values.

Unfortunately, this type of equation system is too difficult to me, so I

would appreciate a hand.