Circuit Description

An ultrasonic transducer mike (TD1) picks up high – frequency sounds and converts them to electrical sig­nals via the piezoelectric effect (see Figure 26-2). Inductor LI tunes the inherent capacity of the trans­ducer to a window frequency centering around 25 kHz. This parallel, equivalent resonant circuit pro­duces a high-impedance signal source that is coupled to field-effect transistor (FET) Q1 amplifier through capacitor C2. Resistor R1 and capacitor Cl decouple the bias voltage to the drain. Layout and input lead shielding is important, as this section is prone to noise, feedback, and extraneous signal sensitivity.

The output of Ql is taken across the drain resistor R2 and is capacitance coupled to amplifier II A. Gain is set to X50 by the ratio of resistors (R6/R4).

The output of 11A is AC coupled to the combina­tion mixer/amplifier 11B through capacitor C4.The output of oscillator 11C is coupled into the circuit by a “gimmick” capacitor, CM. This is a short lead from pin 8 of IC1 and is twisted with a similar lead from pin 2 of 11B. (It is suggested to check performance without this gimmick.) The oscillator now generates a frequency that is mixed with the picked-up signals. The resultant is two signals, one being the sum and the other the difference.

Capacitor C7 and resistor RI7 form a filter net­work attenuating the higher-frequency component of the mixed frequencies while allowing the lower fre­quency to pass by a factor of 20 decibels (db).The lower-frequency results are the difference between the oscillator frequency and the actual signal fre­quency. This is similar to a superheterodyne effect. The high frequency is obviously bypassed by filter C7 and R17. The filtered signal, being the composite dif-

Circuit Description

"Gimmick"- See text as may

Proper routing of input power leads will improve noise figure.

Leads to J1 must be short and direcl as possible

Leads lo power must be routed direct to underside of mounting plate.

Rd is chosen to dampen transducer response Suggested value is around 3SK.

Twist all lead pairs wherever possible

Figure 26-2 Ultrasonic microphone schematic

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Circuit Description

ference, is rectified by diode Dl and integrated with capacitor CB. This signal is in the audio frequency range and is what you actually hear. It is tuned by control pot R12 in the oscillator section and enables the selective tuning of certain target frequencies within an acceptance window of the transducer TD1. These resulting audio frequencies are coupled to vol­ume control R19 through DC blocking capacitor CIO. Capacitor Cl2 bypasses any higher frequencies that may leak through. The arm of R14 teeds the audio signal into headphone amplifier 12. The output is H ohms and is capacitance-coupled to the headset jack J1. You may use a small speaker in a quiet loca­tion for group listening. Network R21 /С4 further attenuates any further higher frequencies.

Power to 12 is decoupled through resistor R20 and capacitor С15. This provides circuit stability, prevent­ing feedback oscillations and other undesirable effects.

The operating points of 11 A, 11B. and 11С are set at the supply voltage midpoint selected by divider resistors R7 and R11. Resistors R5, RIO, and R15 compensate for offset currents.

Construction Steps

To begin constructing the device, follow these steps:

1. Identify all the parts and pieces and verify them with the bill of materials.

2. Insert the components, starling from one end of the perforated circuit board, and follow the locations shown in Figure 26-3, using the indi­vidual holes as guides. The board is cut 2.25 x 2.25 X.1. A printed circuit board (PCB) is also available from www. amazingl. com.

Use the leads of the actual components as the connection runs, which are indicated by the dashed lines. It is a good idea to trial-fit the larger parts before actually starting to solder.

Always avoid bare wire bridges, messy solder joints, and potential solder shorts. Also check for cold or loose solder joints.

Pay attention to the polarity of the capacitors wilh polarity signs and to all the semiconduc­tors. The positioning of the control pots must

Circuit Description

allow physical alignment with the mounting holes in RPl.

3. Cut, strip, and tin the wire leads tor connect­ing to J1 and solder them. These leads should be 2 inches long and twisted.

4. Fabricate the CHASl chassis, the RPl front panel section, the EN1 enclosure, and the HAND1 handle, as shown in Figure 26-4.

5. Prepare the SHI shielded cable at both ends as follows. If the optional reflector is used, you will need a length of 18 inches; if not, you will need 6 inches. This is shown in Figure 26-5.

a. Carefully remove 3A of an inch of the outer insulation, being careful not to nick the shielded braid.

b. Shred the shielding braid using a pointed object, such as a pin, and twist it into a lead. Carefully tin only the ends to hold the wires together.

c. Carefully strip 4a of an inch of the insul­ation from the center conducior and tin.

d. Check the finished cable for shorts or leakage with a high-resistance meter.

6. Solder the inductor LI and the damping resis­tor Rd, as shown in Figure 26-5. Solder one end of the SHI cable, being careful not to overheat the transducer pins or the insulation of the center conductor. Overheating these pins will ruin this part. If in doubt, you must perform a simple test of measuring a short cir­cuit to the metal case of the part to the shorter pin. If this resistance is above 1 ohm, you have trashed this part and need a replace­ment. The inset in Figure 26-5 suggests using mechanical connections such as crimpling, wire nuts, and so on.

7. Assemble as shown. Figure 26-6 shows an assembly using the parabolic reflector, whereas Figure 26-7 shows one without.

Note that hole in ENt for handle is best cut with a 1 g" circle saw. Fit must be tight to properly secure handle in place. The handle serves as the housing for the single 9-volt orS AA cells

From Figure 26-3

Cut out center of 3.5” plastic cap by placing on the enclosure tube and cutting out the center section with an x-acto knife, using the inner wall as a guide

Front Panel Fab

Dashed lines indicate t/2" mounting lip. Note clearance holes for SW1 mating to holes in RPt rear panel

section.

HAND1

RP1 PANEL

Circuit Description

Panel is cut from a3|x 3|’ piece of 045 aluminum or 03 gal. Cut corners to approach a circular shape.

Circuit Description

W.375 V^375

OFF/ON/VOL TUNING

Note that holes must be accuralely positioned for proper alignment to R12, R19, and J1 on the assembly or printed circuit board.

Figure 26-4 Final blowup and fabrication

Electrical Pretest

To run a test on the system, follow these steps:

1. Turn the controls to “off,” plug in the HS30 headset, and insert a 9-volt battery. Connect a meter set to read 100 milliamperes across the switch pins of R19 and quickly read a current of around 20 milliamperes. Remove the meter and turn control pot R19 to the midway point.

Note a smooth, rushing sound in the headsets. Then turn on a computer or TV monitor, and adjust the R12 control until you hear a clear tone. Turn off the sound source and gently rub two fingers together, noting a distinct sound. Check the range of the controls for unwanted feedback or spurious signals.

The unit is now ready for final assembly. Note the test points and wave shapes shown in Fig­ure 26-2.

її cannot be stressed the importance of proper heat sinking of the pins 10 the transducer TD1 before attempting soldering. If you are in doubt, use a vwre nut or "slip on" pin connections Note the shorter pin is internally connected to the transducer case and connects to the ground side of the circuit You should measure a short circuii from this pm to the aluminum shell or ihe transducer is no good!

CHAS1

Circuit Description

You may use a 9-volt battery or 12 volts using an 8 AA pack that will fit into the HA1 handle. The 12 volts allow a small 8-ohm speaker to be used as it produces more volume.

RP1

Short pin is connected to shield e note in Fig. 26-4 before connecting to ihe pins on TD1.

Circuit Description

Figure 26-5 Chassis assembly with board connections

Circuit Description

R12 Tuning

Circuit Description

R19/S1 Volume

Power Note that J1 is wired for mono operation

2. Complete the final assembly, adding parabolic reflector PARA 12 for a greatly enhanced, per­formance, as shown in Figure 26-6.

Note that your unit may pick up strong magnetic fields, as it is not shielded for such. Performing the Doppler shift test, as noted earlier in these plans, eas­ily differentiates these fields.

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