For a hobbyist, it has always been fun to activate or automate through a clap or a whistle. A whistle sometimes can act like a voice recognition actuator as the frequency varies from person to person. However, for this use the tuning is to be done around 1700 Hz, a pitch that is somewhere near to the middle C octave of the piano chords. A condenser microphone acts as the audio sensor and produces a few milli - volts of current to activate the electrical actuator like a relay. For this it has to be supported by a transistor or FET.
Driving such a switch with the resultant milli-Volt from the condenser microphone is not possible. Hence an overall gain 4000 times larger is needed to be divided into two amplifier sections having gain values of 65 each.
Resistors denoted by the values R1, R2 and R3 are responsible for the individual gain of the amplifiers and are 1.1K, 1.2K and 15K respectively. Thus we arrive at the formula given below
· R1 = Q/(G*C*2*Pi*F) = 8/(65*.01^-6*6.28*1700) = 1152 or 1.1K
· R2 = Q / ((2*Q^2)-G)*C*2*Pi*F) = 8/((128-65)*.01^-6*6.28*1700)= 1189 or 1.2K
· R3 = (2*Q)/(C*2*Pi*F) = 16/(.01^6*6.28*1700) = 150K
While choosing the resistor values for the proposed whistle remote control circuit, the quality factor (Q) has to be kept in mind. Q has to be greater than the square root of gain divided into two parts. It is a ratio of the central frequency and the band width. For this particular case, Q should have a value around 8.
It is to be noted that for a gain around 65 (the gain of individual amplifiers has a minimum value of 5.7), the quality factor has to be more than 5.7. Both capacitors employed in the amplifier need not have the same value, but for ease of calculation it can be kept at 0.01 microfarad. These particular values of capacitance are usable at audio frequencies and are readily available.
The op-Amps are biased in such a way that they are kept at nearly 50% of input supply (12 Volt DC). This is achieved by putting 10K resistors as potential dividers.
Output stage two rectifiers are fed into a capacitor (1 microfarad) which is connected to the base of a NPN transistor commonly denoted by 2N3904 or equivalent). Two resistors (2.7K and 3.3K) are used to bias the emitted voltage at 6.6 V. This enables the transistor to conduct a trigger for the flip-flop circuit when the peak value of the signal through the filter overtakes the combination voltage of the emitter 6.6V plus the emitter base voltage drop 0.7 V and the drop across a diode (0.7 V). If the clock circuits need to be triggered at a lower voltage than what it has set in the particular circuit, a 2.7K and a 3.3K resistor combination has to be altered. This will in turn determine the 6.6V DC input. Minor adjustments are possible like replacing the 3.3K resistor with a 5K. For the flip flop circuit IC number CD4013 is employed. This is basically a dual D type flip flop with recommended operating voltage between +3V DC to +15V DC.
Circuit diagram of the proposed whistle remote
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