Remote control using VHF modules

Description A few designs for remote control switches, using VG40T and VG40R remote control pair, are shown here. The miniature transmitter module shown in Fig. 1, which just measures 34 mm x 29 mm x 10 mm, can be used to operate all remote control receiver-cum-switch combinations described in this project. A compact 9-volt PP3 battery can be used with the transmitter. It can transmit signals up to 15 metres without any aerial. The operating frequency of the transmitter is 300 MHz. The following circuits, using VG40R remote control receiver module measuring 45 mm x 21 mm x 13 mm, can be used to: (a) activate a relay momentarily, (b) activate a relay for a preset period, (c) switch on and switch off a load. To activate a relay momentarily (see Fig. 2), the switch on the transmitter unit is pressed, and so a positive voltage is obtained at output pin of VG40R module. This voltage is given to bias the relay driver transistor. The relay gets activated by just pressing push-toon micro switch on the transmitter unit. The relay remains energised as long as the switch remains pressed. When the switch is released, the relay gets deactivated. Any electrical/electronic load can be connected via N/O contacts of the relay. To activate a relay for a preset period (refer Fig. 3), the switch on the transmitter unit is pressed momentarily. The transistor gets base bias from VG40R module. As a result the transistor conducts and applies a trigger pulse to IC 555, which is wired as a monostable multivibrator. The relay remains activated till the preset time is over. Time delay can be varied from a few seconds to a few minutes by adjusting timing components. To switch on and switch off a load (refer Fig. 4), a 555 IC and a decade counter 4017 IC are used. Here the 4017 IC is wired as a flip-flop for toggle action. This is achieved by connecting Q2 output to reset terminal while Q1 output is unused. Q0 output is used for energising the relay. The relay is activated and deactivated by pressing the transmitter switch alternately. So, to activate the load, just press the transmitter switch once, momentarily. The relay will remain activated. To switch off the relay, press the transmitter switch again. This process can be repeated. Time delay of monostable multivibrator is set for about one second. Note: Short length of shielded wire should be used between VG40R receiver module output and the rest of the circuit. The transmitter with 9V battery must be housed inside a nonmetallic (say, plastic) cabinet for maximum range of operation.

 Circuit Diagram

ULTRASONIC REMOTE CONTROL

Description Here is a low cost, wireless switch controller. It uses ultrasonic sound waves for remote control of a switch. As with any other remote control, the system basically comprises a transmitter and a receiver circuit. Frequencies up to 20kHz are audible. Frequencies above 20kHz are not audible. The transmitter circuit generates an ultrasonic frequency between 40-50kHz. The receiver senses the ultrasonic sound and switches on a relay. The transmitter uses a 555 astable multivibrator. It oscillates at a frequency of 40-50kHz. An ultrasonic transducer is used to transmit the frequency. The transmitter runs on a 9v battery. The ultrasonic receiver uses a receiver transducer to sense ultrasonic signals. It uses a twostage amplifier, a rectifier stage and an operational amplifier in inverting mode. Output of the operational amplifier is connected to a relay through a driver stage. A 9v adapter can be used to power the receiver circuit. When switch S1 is pressed, it generates ultrasonic sound. The receiver amplifies the received signal via transistors Q3 and Q4. The amplified signal are then rectified and filtered. The filtered DC voltage is given to the inverting pin of operational amplifier 1C2. The non-inverting pin of 1C2 is connected to a DC voltage through VR2 that determines the threshold value of the signal received, for operation of relay RL1. The inverted output of 1C2 is used to bias transistor Q5. When transistor Q5 conducts, it supplies base bias to transistor Q6. When transistor Q6 conducts, it energises the relay RL1 . The relay can be used to control any electrical or electronic appliance. Frequency of the circuit can be varied by adjusting VR1. Adjust it for maximum performance. Ultrasonic sounds are highly directional. So when you are using the transmitter, the receiver should face towards the transmitter. The receiver is always kept on

Circuit Diagram

FM TRANSMITTERS

THE SIMPLEST CIRCUIT The following circuit is the simplest FM circuit you can get. It has no microphone but the coil is so MICROPHONIC that it will pick up noises in the room via vibrations on a table. The circuit does not have any section that determines the frequency. In the next circuit and all those that follow, the section that determines the frequency of operation is called the TUNED CIRCUIT or TANK CIRCUIT and consists of a coil and capacitor. The transistor and components surrounding the tuned circuit simply keep the tuned circuit operating at its RESONANT FREQUENCY. This circuit does not have this feature. The transistor turns on via the 47k and this puts a pulse through the 15 turn winding. The magnetic flux from this winding passes through the 6 turn winding and into the base of the transistor via the 22n capacitor. This pulse is amplified by the transistor and the circuit is kept active.