Introduction |
The infra-red (IR) toggle switch project described here is aimed to
provide control mechanism for electrical appliances that do not
have remote operation features. The goal is to construct a black
box where you can plug-in your 120V AC appliance and control
ON and OFF operations with any modern IR remote control devices.
Modern IR remote controls generate modulated pulse train of 38KHz frequency
when any key on the remote is pressed. With the use of capacitive filtering we
will convert the stream of pulses into one pulse regardless of the key entered.
This way, we will be able to toggle a relay switch with any key pressed on the
remote. This project has been tested with varieties of IR remote control devices
like that for TV, DVD, digital camera, etc., and it worked well.
Circuit Diagram |
The TSOP 1738 IR receiver module detects the 38KHz input pulses
received from the IR remote control device. Under stand-by condition,
the output pin of the IR module is at logic High, and when it detects
the train of pulses, they appear at its output. The output from IR
receiver is fed to a PNP transistor (BC557) with a series base resistor
of 4.7K. At the collector of the NPN transistor, the train of pulses
will be inverted. There is a 10uF capacitor and 100K resistor connected
from the collector to ground. The function of capacitor is to convert the
train of pulses into a single pulse, and the resistor is to provide the
discharge path for the capacitor.
So lets see what happens when a key on the remote is pressed.
During standby, the output of IR receiver module is High, so BC557
is cut off. The capacitor is fully discharged, and the collector of BC557
is at ground. When a key is pressed on the remote, the train of pulses
arrived at the base of BC557 turns it ON and OFF very fast. When it is
ON, the capacitor gets charged through the collector current of BC557,
and when it is OFF, the capacitor starts to discharge through 100K
resistor. But the train of pulses is so fast (38000 pulses per second)
that the capacitor doesn't get chance to discharge. So, the bottom
line is, every time a key is pressed from the IR remote, a positive
going clock pulse is generated at the collector of BC557 transistor.
Next comes CD4017, a decade counter. It counts low-to-high going
pulses up to 10 that are arrived at its CLK pin (14) and pulls the
corresponding output (Q0-Q9) High. When it is just turned on, Q0
goes High, and when it gets a first low-to-high pulse (when a key
is pressed from the IR remote) at CLK i/p, Q0 goes Low and Q1
goes High. Q1 output is connected to a LED through a current
limiting resistor to indicate the ON/OFF status. The Q1 output
is also used to drive a relay switch through a NPN transistor (BC547).
I used 5V DC relay that requires about 70mA current from 5V source
to turn ON. This current is provided by BC547.
Complete circuit before putting inside the box
Packing inside a box for safety as it deals with 120V AC.
Now, lets see what happens when a key is pressed again. The counter
advances by 1, Q1 goes Low (relay is OFF), and Q2 goes High. If we
connect Q2 to Reset input of CD4017, the counter is going back to the
initial condition (Q0 High, Q1 and all others Low), and is ready to get
another key press signal to turn the relay ON. This way the switch is
toggled every time a key is pressed from the remote.
Power Supply |
The power supply for the circuit is provided through a 5V regulator IC
LM7805. A 12V step-down transformer with a bridge rectifier and a
capacitor filter provides an unregulated DC input to LM7805. The
regulator IC provides a constant 5V supply for the circuit.
Be careful on the output side of the relay that connects to 120V AC line. A
reversed biased diode parallel to the relay input is for back emf protection.
Switch in Operation |
Download |
Download assembly code (.asm)
