The following figure shows the prototype block diagram:

 

Figure 1: Prototype Block diagram

 

555D was used as an oscillator with frequency of 25 kHz and a duty cycle of 50%

The output is high for t1 seconds:
t1(secs)=K*Ra*C/1000
K is worked out by the server and is calculated from Vcc and Vdd.
The output is low for t2 seconds:
t2(secs)=0.693*Rb*C/1000
Where R is in K Ohms and C is in uF (micro Farads).

V_cc = 9V

C = 220 pF

R_a = 110 kW

R_b = 131 kW

 

 

Comparator:

            A_v = 100 V/V

            R₁ = 1 kW

            R₂ = 100 kW

            V_o = -(R₂/R₁) (v_i1 – v_i2)

            v_i1 = Output of envelope detector.

            v_i2 = Output of band pass filter.

Butterworth high Q band pass filter:

            K = gain = 1

            f₀ = (673+134)/2 = 404 Hz

            w₀ = 2pf₀ = 2538.41 rads/sec

            b = 2p(673) - 2p(134) = 3386.64 rads/sec

            Q = w₀/b = 0.75

            R₁’ = Q/K = 0.75 W

            R₂’ = Q/(2Q^² –K) = 6W

            R₃’ = 2Q = 1.5 W

            C = 0.1 uF

            K_f = 2pf_o = 2538.41 rads/sec

            R₁ = K_m R₁’ = (10⁷ / 2538.41) * 0.75 = 3kW

            R₂ = K_m R₂’ = (10⁷ / 2538.41) * 6 = 23.7kW

            R₃ = K_m R₃’ = (10⁷ / 2538.41) * 1.5 = 6kW

Low Pass Filter:

            Low pass filter with a cut off frequency = 1.5 kHz and gain = 100V/V

            When R = 1 W and w = 1 rad/sec

            (2)^^0.5 = 1/C₁                           C₁ = 1.4142 F

            C₂ = 1/C₁ = 0.7071 F

            When f = 1.5 kHz:

            W_c = 9424.7 rad/s

            k_f = 9424.7                              k_m = 1000 = R’/R

            C₁’ = 1/(h_m h_f) C₁ = 0.15 uF

            C₂’ = 1/(h_m h_f) C2 = 0.08 uF

The Transistor Circuit that controls the Buzzer:

            b = 140

            I_c = (9 - 2)/1.2 = 5.8 mA

            a = b / (b + 1) = 0.993

I_E = I_c / a = 5.875 mA

I_B = I_E – I_C = 0.042 mA

R_B = (1.5 – 1.2) / 0.042 = 7.14 kW

R_E = 0.5 / I_E = 85 W

R_s = 1kW