Basic Electronics, Chapter 3: Challenge 3 solution (light sensor and pull-up)

Circuit: 5 V, LDR on top (R1), fixed resistor R2 to ground, Vout from the middle.
  Vout = 5 x R2 / (R_LDR + R2)

(a) R2 = 10 k
  Dark,   R_LDR = 500 k:  Vout = 5 x 10 / (500 + 10) = 50 / 510 = 0.098 V
  Room,   R_LDR = 15 k:   Vout = 5 x 10 / (15 + 10)  = 50 / 25  = 2.000 V
  Bright, R_LDR = 2 k:    Vout = 5 x 10 / (2 + 10)   = 50 / 12  = 4.167 V
  Swing: 0.098 V to 4.167 V, with room light at 2.0 V (a little under half supply).

(b) Make room light give 2.5 V (half supply)
  Half supply needs R2 = R_LDR at that light level, so R2 = 15 k (an E12/E24 value).
  Dark:   Vout = 5 x 15 / (500 + 15) = 75 / 515 = 0.146 V
  Room:   Vout = 5 x 15 / (15 + 15)  = 75 / 30  = 2.500 V
  Bright: Vout = 5 x 15 / (2 + 15)   = 75 / 17  = 4.412 V
  Swing: 0.146 V to 4.412 V. Room light is centred, and the bright end has more
  room before reaching 5 V. The dark end is a little higher because 15 k is
  larger, but it is still a clear "dark" level.

(c) Pull-up for a push button on 3.3 V
  Pressed current limit:  R >= 3.3 V / 0.2 mA = 16.5 k ohm
  Leakage limit (1 uA, 0.99 V allowed drop):  R <= 990 k  (not a constraint)
  Pick the next standard value above 16.5 k: 22 k (E12). (Anything from 22 k to
  47 k is reasonable; 10 k also works if 0.33 mA is acceptable.)

  Pressed current:  I = 3.3 / 22 k = 0.15 mA   (under 0.2 mA: passes)
  Power in resistor (pressed):  P = V^2 / R = 3.3^2 / 22,000 = 10.89 / 22,000
                                  = 0.000495 W = 0.495 mW  (trivial for a 1/4 W part)
  Rise time constant: tau = R x C = 22,000 x 20e-12 = 4.4e-7 s = 0.44 us
  (the line reaches 95 % of the rail after about 3 tau = 1.3 us: much faster
  than any button or a microcontroller reading it)
  Open-button level: 1 uA x 22 k = 22 mV below the rail, so a solid logic 1.
  Pressed: the pin is held at 0 V by the button, so a logic 0.

Build check (expected, using your own measured LDR resistances):
  Use the formula in (a) with the resistance your meter shows in each light
  condition. Vout / 5 V must equal 10 / (R_LDR + 10) with R in k ohm.

Checklist
 [ ] LDR on top: output rises with light
 [ ] R2 equal to the LDR resistance at the light level that matters gives half supply there
 [ ] pull-up: lower limit from pressed current, upper from leakage and speed
 [ ] check the pressed current, power and rise time with the real value chosen