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31.8.2 AMPLIFIER INPUT STAGE
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The circuit in Fig. 31-10 is a basic amplifier for an electret condenser microphone (refer to parts list in Table 31-7). The circuit is designed around the common LM741 op amp, which is wired to operate from a single-ended power supply. Potentiometer R1 lets you adjust the gain of the op amp, and hence the sensitivity of the circuit to sound. After experimenting
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FIGURE 31-10 Sound detector amplifier. Adjust R1 to increase or decrease the sensitivity, or replace the potentiometer with the circuit that appears in Fig. 31-10.
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31.8 SOUND INPUT SENSORS
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TABLE 31-7 IC1 Q1 R1 R2,R3 R4,R5 C1,C2 MIC1
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Parts List for Sound Detector LM741 op amp IC 2N2222 transistor 500K potentiometer 6.8K resistor 1K resistor 0.47 F ceramic capacitor Electret condenser microphone
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with the circuit and adjusting R1 for best sensitivity, you can substitute the potentiometer for a fixed-value resistor. Remove R1 from the circuit and check its resistance with a volt-ohm meter. Use the closest standard value of resistor. By adding the optional circuit in Fig. 31-11, you can choose up to four gain levels via computer control. The resistors, R1 and R2 (you decide on their value based on the gain you wish), are connected to the inverting input of the op amp and the inputs of a 4066 CMOS 1-of-4 analog switch. Select the resistor value by placing a HIGH bit on the switch you want to activate. The manufacturer s specification sheets for this chip recommend that only one switch be closed at a time.
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31.8.3 TONE DECODING DETECTION
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The 741 op amp is sensitive to sound frequencies in a very wide band and can pick up everything that the microphone has to send it. You may wish to listen for sounds that occur only in a specific frequency range. You can easily add a 567 tone decoder IC to the amplifier input stage to look for these specific sounds. The 567 is almost like a 555 in reverse. You select a resistor and capacitor to establish an operating frequency, called the center frequency. Additional components are used to establish a bandwidth or the percentage variance that the decoder will accept as a desired frequency (the variance can be as high as 14 percent). Fig. 31-12 shows how to connect a 567 to listen to and trigger on about a 1 kHz tone (refer to the parts list in Table 31-8). Before you get too excited about the 567 tone decoder, you should know about a few minor faults. The 567 has a tendency to trigger on harmonics of the desired frequency. You can limit this effect, if you need to, by adjusting the sensitivity of the input amplifier and decreasing the bandwidth of the chip. Another minor problem is that the 567 requires at least eight wave fronts of the desired sound frequency before it triggers on it. This reduces false alarms, but it also makes detection of very low frequency sounds impractical. Though the 567 has a lower threshold of about 1 hz, it is impractical for most uses at frequencies that low.
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SOUND INPUT AND OUTPUT
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+5 vdc
Sensitivity A On 1 Off 0 14 13 12 11 10 9 8 Note: Tie unused inputs to ground.
To Pin 2 of Op-Amp
To Pin 6 of Op-Amp
On 1 Off 0
Sensitivity B
FIGURE 31-11 Remote control of the sensitivity of the sound amplifier circuit. Under computer control, the robot can select the best sensitivity for a given task.
Being an older part, the 567 has been obsoleted by several manufacturers that used to make it. For the time being, however, you can still purchase 567 chips from most new and surplus retail and mail-order electronics companies. Another option is to use a telephone Touch-Tone (known as DTMF) decoder. There are several chips available, such as the MT8880 (available from Parallax) that can decode the dual tones of a Touch-Tone phone and output a digital value. An obvious advantage of using a DTMF decoder over the 567 is the greater number of outputs, allowing you to implement multiple controls for your robot. The remote control sound source for this chip could be an old telephone handset that is powered by external batteries and the tones are amplified, or a microcontroller, like the BS2, which has a DTMFOUT command.
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