Ultrasonic Transmitter and Receiver

THE Ultrasonic Transmitter circuit shown in Fig. generates ultrasonic sound waves at a  frequency of 40kHz. The key component is a 555 timer, IC1, which is wired as an astable to produce an output frequency of 40kHz. This stream of electrical signals drives the ultrasonic
transmitter transducer TX1.

Potentiometer VR1, used as a variable resistor, enables you to make fine adjustment to the frequency when setting up the control system to ensure that TX1 is resonating at its optimum natural frequency, thereby ensuring maximum range. This adjustment is described in the ‘synchronising’ section at the end of the receiver section.

Ultrasonic Receiver:
The circuit shown in Fig. is designed to receive and process the 40kHz pulse of sound waves generated when the transmitter’s pushswitch is pressed. On the first push the relay is energised, and de-energised on the next push, enabling an electrical device to be switched on and off via the relay contacts. The Receiver circuit comprises the following building blocks:

Integrated circuits IC1 and IC2, which together amplify the ultrasonic sound waves received by sensor RX1 from the transmitter. Just like the Bat Detector project (Part 6), two coupled
amplifiers are used to boost the voltage gain and ensure remote control over a useful range of around 25 metres.

The amplified 40kHz pulses are fed to the base (B) of NPN transistor TR1 via capacitor C3. When a burst of ultrasonic waves is received, the voltage at the collector (C) terminal of TR1 suddenly falls as current flows through resistor R2.

The sharp fall in voltage is transferred to the trigger pin 2 of IC3, which is a 555 timer  connected as a monostable circuit. Once triggered it produces a short pulse from its output
terminal (pin 3) of about one second duration. This pulse triggers flip-flop IC4 via its clock input pin 13, each successive pulse ‘toggling’ the output (pin 15) between logic high and logic low.

The final stage is the relay driver using NPN transistor TR2. When a logic high is produced at pin 15 of IC4, the transistor switches on, energising relay RLA1 and turning on the  optional LED1. The next burst of 40kHz sound waves sets the output of IC4 to logic low, switching off the transistor and denergising the relay and LED1.

Do not omit capacitor C5, as it helps to stabilise the power supply voltage to the circuit.

Synchronising the system
The transmitter needs adjusting so that it generates ultrasound at a frequency of 40kHz. This is easy if you have an oscilloscope or frequency counter handy. If you do not have one,
proceed as follows.

Make sure the transmitter is connected to the 9V battery and its sensor is pointing towards the receiver placed about a metre away and also connected to its 9V battery. LED1 should be on or off depending on whether the output of IC4 is high (on) or low (off) when powering up the circuit. Now it’s a matter of trial and error!

Press and release pushswitch S1 on the transmitter while watching LED1 on the receiver. Make small adjustments to preset VR1 on the transmitter until repeatedly pressing and releasing S1 causes LED1 to go off and on. Gradually increase the distance between the
transmitter and receiver to 20 metres and continue to make fine adjustments to VR1. If the receiver is to be used to switch on and off an electrical device, a relay needs to be connected in parallel with LED1 and R5, as shown, and its switching contacts used to control the device in a separate circuit.