10. Things to Try

These exercises will solidify the chapter much better than re-reading.

1. RC differentiator. Build with $R = 10$ kΩ, $C = 100$ nF. Drive with a square wave from a function generator at 100 Hz, 1 kHz, 10 kHz. Watch the output on a scope. The spike behavior depends entirely on the relationship between input period and $RC$. Verify the math.

2. Compensated divider experiment. Build a 10:1 voltage divider with two resistors. Drive it with a 1 kHz square wave. Note the rounded corners on the output. Now add a 1 nF cap across the top resistor. Tune the cap (or add another in parallel) until the output corners are sharp. You've just compensated a probe.

3. Schmitt trigger. Use a comparator chip (LM393) plus a positive-feedback resistor pair to build a Schmitt. Feed it a slow triangle wave from a function generator. Watch the input and output simultaneously: the output transitions at clearly different input levels going up vs. down. Measure the hysteresis width.

4. 555 astable. Build with $R_A = 10$ kΩ, $R_B = 22$ kΩ, $C = 10$ nF. Compute the frequency by hand: $f = 1.443/((10k + 44k) \cdot 10nF) = 2.67$ kHz. Verify with a scope. Vary $R_B$ with a potentiometer and watch the frequency change.

5. 555 monostable. Use the trigger pulse from a button (with a 100 nF cap to differentiate the edge into a pulse). Output drives an LED through a 330 Ω resistor. With $R = 100$ kΩ and $C = 10$ µF, the LED should blink for 1.1 seconds after each press, regardless of how long you hold the button.

6. Scope-probe compensation. If you have a real scope, hook your x10 probe to its compensation output. Adjust the trim cap until the displayed square wave is perfect. Now check it with the probe attached to a known fast signal.

7. LTspice simulations. Build any of the above in LTspice. Sweep input frequencies or component values. Watch how the response changes. Compare predicted to simulated behavior.

8. 555 SPICE example. Here's a basic Schmitt-trigger and monostable simulation in LTspice syntax to play with:

* 555 Astable Oscillator (using built-in LTspice 555 model)
.SUBCKT NE555 GND TRG OUT RST CTL THR DIS VCC
* (typical model definition; download a free 555 SPICE model online
*  or use Falstad's web simulator instead for quick experiments)
.ENDS
 
* Astable circuit
V1 VCC 0 5
RA VCC PIN7 10k
RB PIN7 PIN6 22k
C1 PIN6 0 10n
C2 CTL 0 10n
RST_R RST VCC 1
X1 0 PIN6 OUT RST CTL PIN6 PIN7 VCC NE555
 
.tran 0 5m
.end

* Op-amp Schmitt trigger
V1 VCC 0 12
V2 -VCC 0 -12
Vin Vin 0 SIN(0 5 1k)
R1 OUT VFB 10k
R2 VFB 0 1k
* Op-amp model: input pins +, -, output, +V, -V
X1 VFB Vin OUT VCC -VCC LM741
.tran 0 5m
.end

When the above are intuitive (when you can predict roughly what an RC, Schmitt, or 555 will do before running the simulator), you have the chapter under your belt.