The simple ultrasonic remote control consists of an ultrasonic transmitter and a receiver. Both units operate at 40KHz. The receiver controls a relay according to the received remote commands from the transmitter. The control range is up to about 15 meters. The circuits are quite simple and do not require any adjustment at all.
The ultrasonic transmitter
The ultrasonic remote control transmitter is very simple. Its circuit is shown in Figure 1. It consists of a single integrated circuit, a button, some passive components and the MA40S4S ultrasonic transmitting module.
U1 contains four NAND gates. The U1D NAND gate is used as an oscillator and produces a square waveform at 40KHz. The oscillator uses a 40 KHz crystal to ensure frequency accuracy and stability.
The oscillator signal is inverted and amplified from the U1B gate which acts as an inverting current amplifier. U1C and U1A, as a pair, acts as a non-inverting amplifier. The amplifiers drive the CTX ultrasonic transducer which produces ultrasound at 40KHz. This is a non-audible frequency.
The 40KHz ultrasound is produced every time the SW1 button is pressed. The current consumption of the circuit is around 4mA for as long as the button remains pressed. Due to this low current consumption, the ultrasound transmitter will be able to operate for a very long time with the use of a classic 9V battery.
The ultrasonic receiver
The receiver is a bit more complex than the transmitter. Its circuit is shown in Figure 2.
The signal received from the CRX receiving capsule is amplified by the Q1 FET. In the drain of the FET there is a tuning circuit for 40KHz which consists of the L1 inductor and the C4 capacitor. The signal is also amplified by a second transistor, the Q2. At Q2 there is another 40KHz tuning circuit consisting of the L2 and the C8 as well as resistor R4. R4 is used to reduce the quality factor (Q) of the tuning circuit. The reason we use tuning circuits is to prevent the receiver from activating from ultrasound frequencies other than 40KHz.
The received signal is rectified by diodes D1 and D2 and charges the C10 capacitor with a DC voltage. The DC voltage is applied to the next stage which is a voltage comparator based on the LM311 IC. DC voltage at C10 is only generated when a pure tone at 40KHz is received. Otherwise, the voltage across C10 remains approximately equal to 0V.
U2 is used as a comparator and its threshold voltage is set to about 0.6V by diode D3. This means that whenever the receiver receives a signal at 40KHz and the voltage at the ends of the C10 becomes above 0.6V, the output of the comparator switches to logic 1.
The comparator output is used to trigger the U1A flip-flop. The U1A acts as a T-type flip-flop and toggles at each pulse received form the comparator.
The output of U1A drives the Q3 transistor which turns a relay on and off. LED D5 lights up when the relay is activated. Each time the button is pressed on the transmitter, the relay is switched on or off, depending on its previous state.
The receiver is powered by 12V from J1 and the current consumption is around 20 to 80mA when the relay is off or on, respectively.
Assembling the simple ultrasonic remote control
You may assemble the simple ultrasonic remote control on the boards we provide below or in any breadboard if you just wish to experiment. The components should be placed to the transmitter and the receiver board according to the assembly guides of Figures 3 and 4.
All resistors we use in this project are of 1/4W type and of 5% tolerance or better.
The circuit boards are single-sided. All components should be placed on the components side and soldering should be done on the other side (copper lines side). The receiver circuit board also requires three jumper wires, as shown in Figure 4.
The ultrasonic modules should not be mounted directly on the boards but on the J2 terminals, as shown in Figures 3 and 4. The transmitter should be portable. Thus requires a battery. You should use a 9V battery. The battery and the board can be placed in a small plastic box. The receiver requires a 12V power supply unit. For this purpose you may use a small stabilized power supply unit.
The whole ultrasonic remote control design was made on Kicad. The Kicad files of the project are provided below for free. These files contain the schematics and the printed circuit boards artwork.
Transmitter details in compressed Kicad files
The details of the receiver in compressed Kicad files