133 Pages

## Conceptual Design Edit

Voltage-controlled oscillator circuit, for the sound/light circuit, using a 555 timer: [1]

Sedra/Smith has an analysis of the standard 555 astable multivibrator circuit on page 1201. However, it completely ignores the control voltage input (pin 5). When used as an input to the circuit as in the above schematic, the frequency of the oscillation can be controlled. I've re-done their analysis with this input included.

Instead of $V_{TH} = \frac{2}{3}V_{CC}$ and $V_{TL} = \frac{1}{3}V_{CC}$, we now have $V_{TH} = V_{CONT}$ and $V_{TL} = \frac{1}{2}V_{CONT}$.

Eqn 13.41 becomes

$T_H = C ( R_A + R_B ) \ln \left( \frac{V_{CC} - \frac{1}{2}V_{CONT}}{V_{CC} - V_{CONT}} \right)$

showing that [the time the output remains high] increases with the control voltage. Eqn 13.43 remains unchanged as

$T_L = C R_B \ln ( 2 )$

showing that [the time the output remains low] remains constant. Because of this the duty cycle $\frac{T_H}{T_H + T_L}$ will also vary with the control input, and Eqn 13.39 becomes something very horrible.

## Components Needed Edit

• A good old 555 timer
• LED
• Piezo Buzzer
• Jaycar cat. no. AB3458 PC mount buzzer, 3.8kHz, 3-30V dc input voltage, 16mA max input current, 23.8mm wide, 16mm high. $3.95 • Jaycar cat. no. AB3459 mini PC mount buzzer, 2.3kHZ, 3-14V dc input voltage, 30mA input current, 9.5mm high, 12mm wide.$4.95
• Jaycar cat. no. AB3462 mini piezo buzzer, 3.3kHz, 3-16V dc input voltage, 15mA max input current, 22mm diameter, 12mm high. \$3.95