The circuit presented here is used to transmit and receive low frequency pulses using IR modulator and demodulator. IR modulator takes low frequency pulses as input. It modulates them on 38 KHz carrier and transmits them through IR LED as 38 KHz IR light. On the demodulator side IR sensor detects (receives) 38 KHz IR light. It demodulates this 38 KHz pulses and recovers transmitted low frequency pulses. One pulse generator is built using IC555 to generate low frequency pulses. IR modulator is also built using IC555 and TSOP1738 IR sensor is used as IR demodulator.
Fig. 1: Prototype of 555 IC and TSOP1738 based Low Frequency IR Modulator and Demodulator
The circuit is divided in 3 sections.
1. Pulse generator
2. IR modulator
3. IR demodulator
It generates low frequency pulses that are to be modulated and transmitted. The circuit is made up around IC555. One 10 K pot R1 is connected between Vcc and pin no. 6. Its sliding terminal is connected to pin no. 7 as shown. A 100 µF capacitor C1 is connected between pin no. 6 and ground. R1 and C1 are timing components. Pin no. 2 can be connected to pin no 6 or it can be connected to push button through sliding switch as shown. Output pin 3 is connected to enable pin 4 of modulator IC555 and it also drives LED through current limiting resistor R2. Pin 8 is connected to Vcc and pin 1 is grounded to provide bias. Pin 4 is connected to Vcc to enable circuit operation. Pin 5 is grounded through 100 nF capacitor C2.
IC 555 is configured in astable mode with R4, R5-R6 and C2 as timing components. The values of these components are selected to produce 38 KHz pulses in the output. The output of this IC drives IR LED through current limiting resistor R7. The equation for output frequency is
F = 1.44 / (R4 + 2×Rp)×C2
Where Rp = R5×R6 / (R5+R6)
So Rp = 3300 / 2
And F = 1.44×10-9 / (470 + 3300)×10
IR sensor TSOP1738 works as IR demodulator. It is given bias on its pins 1 and 2 respectively. Its output pin 3 drives LED through NPN transistor Q1 connected in switch configuration. Final output is taken through collector of Q1.
Here are the snaps of circuits built on two different bread boards.
Fig. 2: Image showing IR receiver and transmitters on Low Frequency IR Modulator and Demodulator Circuits
· Pulse generator generates low frequency pulses of less than 1 Hz. In manual mode IC555 is configured in monostable mode and it generates single pulse when push button is pressed. In auto mode IC555 is configured in astable mode and it generates continuous pulses. Its frequency can be varied by varying potentiometer
· These low frequency output pulses are given to modulator IC555 on pin 4. It generates 38 KHz burst (of pulses) when pin no 4 is high. This is shown in figure given below with wave forms. These 38 KHz pulses are transmitted by IR LED. So low frequency pulses are modulated by 38 KHz pulses and they are transmitted as IR light
· IR sensor detects 38 KHz IR light. Its output goes low when it receives 38 KHz IR light and vice versa. So at the output of sensor we get exact inverted output of transmitted pulses. Finally when these inverted pulses are given to transistor input, it again inverts this input and generates same pulses in the output (similar to transmitted pulses).
Fig. 3: Waveforms for Low Frequency Pulse at Modulator and Demodulator Circuits