SOLIDSTATE REMOTE CONTROL SWITCH

C I R C U I T I D E A S

SEPTEMBER 2004 ELECTRONICS FOR YOU

Fig. 2: Receiver circuit

SANI THEO

SOLIDSTATE REMOTE CONTROL SWITCH

SEEMANT SINGH

Here is a solidstate remote control

switch which uses readily available

electronic components. The control

circuit comprises the transmitter and receiver

sections. The range of the transmitter

is around seven metres.

The transmitter circuit (shown in Fig.

1) is built around a timer IC (555) wired

as an astable multivibrator. It works off a

9V battery. When remote control switch

S1 is pressed, the astable multivibrator

built around IC1 starts oscillating at a frequency

of about 38 kHz. The signal frequency

at output pin 3 of IC1 is transmitted

through two infrared diodes (IR LED1

and IR LED2). A green LED (LED1) connected

to pin 3 glows whenever S1 is

pressed, indicating the presence of a signal

for transmission at the output of the

multivibrator.

The output frequency F at pin 3 of IC1

depends on the timing components, viz,

resistors R1 and R2 and capacitor C2. It is

given by the following relationship:

F = 1.443/(R1+2R2)C2

This frequency is fed to npn transistors

T1 and T2 (each BC547) through resistor

R4 (470-ohm) to drive the IR LEDs.

Resistor R5 limits the current flowing

through the IR LEDs.

The receiver circuit (shown in Fig. 2)

Fig. 1: Transmitter circuit

consists of regulator IC 7806 (IC4), IR receiver

module (TSOP1738), timer 555

(IC2) and decade counter CD4017 (IC3).

Timer 555 (IC2) is wired as a monostable

multivibrator.

The 9V DC power supply for the receiver

circuit is regulated by regulator IC

7806. The presence of power in the circuit is

indicated by glowing of the red LED (LED2).

The IR receiver module (TSOP1738),

which gets 5.1V power supply through zener

diode ZD1, receives the transmitted

signal of about 38 kHz. The signal is amplified

by transistor BC558 (T3) and given

to triggering pin 2 of IC2 through coupling

capacitor C6.

Initially, when no signal is received

from the transmitter, the

output of the IR receiver

module is high (approx.

5V).

When the transmitter

is pointed at the receiver

and switch S1 is momentarily

pressed, the transmitted

IR rays are sensed

by the receiver module

and its output pulses low

to trigger the monostable

(IC2). The output of IC2

goes high for about five

seconds. Thus, even if

you press the remote

switch more than one time by mistake,

there won't be any change in the output

of the receiver within this period and hence

no undesired switching of the appliance.

The signal reception is indicated by glowing

of the green LED (LED3).

The output of IC2 is given to the clock

input (pin 14) of IC3. Here, IC3 is wired as

a bistable circuit. For every clock input,

pins 2 and 3 of IC3 alternately go high.

Initially, when the power to the receiver

circuit is switched on, pin 3 of IC3

is high and therefore the yellow LED

(LED4) connected to it glows. The glowing

of LED4 indicates that the appliance is

in 'off' condition.

When a clock pulse is received at pin 14