Wireless On/Off switch

Introduction:

Imagine the convenience of selecting TV channels using your remote and then pointing the same remote to your switchboard to switch on/off the fan or the tube light. This is a simple circuit to remotely switch on/off any electrical device through a relay using the normal TV/ VCR/VCP/VCD remote control unit.

The circuit is built around a 3-pin IR IC receiver that can detect 38 kHz burst frequency generated by an I.R source (e.g. T.V remote).

Components:

1. IC LM7805:

            Gives a regulated 5V output voltage.

Features:

§  5V regulated output for input up to 35V.

§  Internal thermal overload protection.

§  No external components required.

§  Internal short circuit current limit.

2. IC TK1838:

      Detects burst frequency generated by an IR transmitter.

Features:

§  High immunity against ambient light.

§  Low power consumption.

§  High sensitivity.

§  Continuous transmission possible.

3. IC NE555:

       The 555 monolithic timing circuit is a highly stable controller capable of      producing accurate time delays or oscillations.                                                             

Features:

§  Turn off time less than 2μs

§  Max operating frequency greater than 500 kHz.

§  Output can source and sink currents up to 200 mA.

§  Timing from microseconds to hours.   

4. IC CD4027:

             It is a dual J-K Master/Slave flip flop with set and reset.

Features:

§  Wide supply voltage range

§  High noise immunity

§  Low power

5. Relay:

     A relay is an electrically operated switch. Current flowing through the coil of the relay creates a magnetic field which attracts a lever and changes the switch contacts.

Features:

§  Relays can switch AC and DC.

§  Relays can switch high voltages.

§  Relays are a better choice for switching large currents (> 5A).

§  Relays can switch many contacts at once.

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Circuit Description:

            The IC’s NE555, CD4027 and the IR sensor TK1838 get the supply voltage from IC 7805 which in turn gets the supply from the 9V battery. The capacitor C1 which is connected between the output terminal and common terminal (i.e. gnd) of the IC LM7805 is used to improve the transient response and the output impedance.

             

The output of IR sensor TK1838 is normally high. It responds to a frequency of 38khz.When it detects IR light of appropriate frequency the output goes low. The capacitor C5 is connected to avoid noise and false triggering.

            The timer IC NE555 is used in monostable mode of operation. The timing period is triggered (started) when the trigger input (555 pin 2) is less than 1/3 V_CC, this makes the output high (+V_CC) and the capacitor C2 starts to charge through resistor R2. Once the time period has started further trigger pulses are ignored. The threshold input (555 pin 6) monitors the voltage across C2 and when this reaches 2/3 V_CC the time period is over and the output becomes low. At the same time discharge (555 pin 7) is connected to 0V, discharging the capacitor ready for the next trigger. The reset input (555 pin 4) overrides all other inputs this instantly makes the output low and discharges the capacitor. The reset function is not required, hence the reset pin is connected to +V_CC.

            Output of the IR sensor acts as a trigger for the timer IC .The resistance R2 (100k) and capacitance C2 (10μF) determine the ON time of the IC which is given by:-

T_ON=1.1RC

Therefore the ON time is 1.1 second .The capacitor C4 is connected to ground noise pickup while the combination of R3, C3 is used to avoid false triggering of the monostable NE555.

           

The IC CD4027 is a Dual J-K Master/Slave flip flop. Each flip flop has independent J, K, Set, Reset and clock inputs. These flip-flops are edge sensitive to the clock input and change state on the positive-going transition of the clock pulse. Set and reset are independent of the clock (grounded in this case).  The J-K flip-flop is used in the toggle mode. Both the J and K terminals are connected to V_CC.

            Relays allow one circuit to switch a second circuit which can be completely separate from the first. There is no electrical connection inside the relay between the two circuits, the link is magnetic and mechanical. The relay RL1 is 9V, 100Ω SPDT (i.e. Single Pole Double Throw) switch. The current requirement for the relay is given by (Voltage rating)/ (Coil Resistance). In this case the current requirement is 90mA.

            Back-EMF diode 1N4007 is connected across the relay for the protection of transistors and IC’s from the brief high voltage spike produced when the relay is switched off. Diode is connected 'backwards' so that it will normally not conduct. Conduction only occurs when the relay coil is switched off, at this moment current tries to continue flowing through the coil and it is harmlessly diverted through the diode. Without the diode no current could flow and the coil would produce a damaging high voltage 'spike' in its attempt to keep the current flowing.

           

SL 100 is an npn transistor enclosed in a metal casing (better heat dissipation) and is used in a open collector mode. The output current of the J-K flip-flop is low (0.88 mA at 25˚ C) which is much less than that required by the relay, SL 100 is used to amplify the current.

                LED2, LED3, and LED4 are used to display the status of each output stage during circuit operation.

Working:

      Ø  The IR sensor detects the IR light from the transmitter and its output goes low.

Ø  This acts as a trigger for the timer IC555 which is used in the monostable mode of operation. The output of the timer IC goes high.

Ø  This toggles the J-K flip flop, whose Q output drives the relay through SL100 npn transistor

Advantages:

ü    Can be controlled by any ordinary TV/VCR/VCD remote.

ü    Can be used to switch device’s which require high voltage and high current.

ü    Can be used to switch any other 9V logic device by using the output across the relay coil terminal

Limitations:

ü    The operating distance is limited by the transmitting power of the IR source.

ü    Two units (not needed to operated simultaneously) cannot be juxtaposed.

ü    Requires external DC power supply.

ü    Due to the relay the circuit becomes bulky and requires high input power

Applications:

ü    Can be used to operate number of electrical appliances at home as well as in industries.

ü    Can be used by a bed-ridden person to control the fans and lights.

ü    Can be used to operate any 9V DC appliance in tandem to the relay.