ULTRASONIC TRANSMITTER AND RECEIVER

Most ultrasonic transmitters and receivers are built around timer IC 555 or complementary metal-oxide semiconductor (CMOS) devices. These devices are preset-controlled variable oscillators. The preset value of the working frequency is likely to drift due to mechanical vibrations or variations in temperature. This drift in frequency affects the range of transmission from the ultrasonic transducer.

The ultrasonic transmitter and receiver circuits described here use CD4017 decade counter ICs.

The transmitter circuit (Fig.1) is built around two CD4017 decade counter ICs (IC1 and IC2), D-type flip-flop IC CD4013 (IC3) and a few discrete components. The arrangement generates stable 40kHz signals, which are transmitted by transducer TX.

The crystal-controlled radio-frequency (RF) oscillator built around transistor T 1 (BC549) generates an 8MHz signal, which serves as input to the first decade counter built around IC1. The decade counter divides the oscillator frequency to 800 kHz. The output of IC1 is fed to the second CD4017 decade counter (IC2), which further divides the frequency to 80 kHz.

The flip-flop (IC3) divides 80kHz signal by 2 to give 40kHz signal, which is transmitted by ultrasonic transducer TX.

Coil L is made with 36SWG enamelled copper wire that is wound 15 times around an 8mm-diameter plastic former as used for radio oscillators, which has a ferrite bead.

The transmitter circuit works off 9-12V DC.

The receiver circuit (Fig.2) is built around a single decade counter CD4017 (IC4) and a few discrete components. To check the working of the transmitter, it is necessary to down-convert the 40kHz signal into 4kHz to bring it in the audible range. By using the receiver, the 40kHz ultrasonic transmitter can be tested quickly. The receiver’s transducer unit (RX) is kept near the ultrasonic transmitter under test. It detects the transmitted 40kHz signal, which is amplified by the amplifier built around transistor BC549 (T2). The amplified signal is fed to decade counter IC4, which divides the frequency to 4 kHz. Transistor T3 (SL100) amplifies the 4kHz signal to drive the speaker.

Use a 9V PP3 battery to power the receiver circuit.

House the transmitter and receiver circuits in separate small cabinets. If the 40kHz transducer under test is working, the receiver circuit produces audible whistling sound.

7MHz CW/AM QRP TRANSMITTER

The circuit of a 7MHz CW/AM QRP transmitter described here can be used to transmit either CW or audio frequency modulated signal over a 7MHz carrier.

The carrier frequency oscillator is crystal controlled using 7MHz crystal in its fundamental mode. The tank circuit comprises a shortwave oscillator coil which can be tuned to 7MHz frequency with the help of ½J gang capacitor VC1.

              Transistor T2 (with identical tank circuit connected at its collector as in case of transistor T1) serves  as a power amplifer. The RF output from oscillator stage is inductively coupled to the power amplifer stage. The output from power amplifier is routed via capacitor C3 and inductor L3 to a half-wave dipole using a 75-ohm coaxial cable. ½J gang capacitor VC3 along with inductor L3 forms an antenna tuning and matching network between the output of power amplifier stage and coaxial transmis-sion line for maximum power transfer. Suitable heatsink should be used for transistor T2.

Tuning adjustments may be accomplished using a 6-volt torch bulb. Connect the bulb to the collector of transistor T1 frst through a coupling capacitor and tune ½J gang VC1 for maximum brilliance. (Note: the bulb would light according to intensity of RF energy.) Same procedure may be repeated for power amplifier stage and antenna tuning network for ensuring maximum power transfer. For CW operation, switch S1 is to be kept on for bypassing the audio driver transformer and Morse key is used for on/off-type modulation. CW would be generated during key depressions. For AF modulation, Morse key points should be closed and switch S1 should be fipped to ‘off’ position. Any suitable mic. amplifer may be used to feed audio input to the audio driver transformer X1. (For  transformer X1 you may use the transistor-radio type AF driver transformer.)