This is a very interesting application. We shall vary the speed of DC motor from a remote place without any wire connection. I am using PWM (pulse width modulation) method to vary the speed of DC Motor. To make it wireless I am using IR transmitter and IR sensor. There are two sections in the circuit 1) PWM generator with IR modulator and 2) IR receiver and motor driver. Transmitter generates PWM wave of 50 Hz (20 ms) and modulates it over 38 KHz frequency. The IR sensor on receiver side will demodulate the PWM wave and drives the DC motor.
Let us first understand the block diagram of the system
PWM Generator: - Its astable multi-vibrator which generates frequency of 50 Hz (that means time period is of 20 ms). We may vary the pulse width from almost 2 ms to up to 18 ms.
IR Modulator: - Its also an astable multi-vibrator that generates exact frequency of 38 KHz. It generates IR light beam of 38 KHz
IR sensor: - It detects IR light beam of 38 KHz and triggers monostable multi-vibrator
Monostable multi-vibrator: - It simply reproduces same PWM that is generated by PWM generator at the transmitter side
DCM driver: - It drives DC motor using PWM (also we may change the direction of DC motor if needed)
DC Motor: - Its standard 12 V DC motor with max RPM of 1000
The complete circuit is shown in Circuit Diagram 1 tab and is divided in different sections
1) PWM Generator: - This is an astable multi-vibrator made up of IC555. The output is fed back to threshold and trigger inputs through two diodes D1 & D2. The frequency is given by
Freq = 1.44 / (R2 + R4)*C2
= 1.44 / 11000*2*10-6
= 65 Hz
Here when output is high, C2 charges via D1, part of R4 (say R4') and R2. When it charges more then threshold limit, output goes low. Now C2 starts discharging through R2 another part of R4 (R4") and D2. Again when it discharges to the trigger limit output again goes high and this cycle repeats. If we turn R4 clockwise, charging time is more so output pulse width is more and vice versa. Thus as we rotate R4 in either direction the output pulse width changes and we shall get PWM wave.
2) IR Modulator:- This is again an astable multi-vibrator. Depending upon the given values of R5, R6 and C3 if we calculate the value of frequency then
Freq = 1.44 / (R5 +2*R4)*C2
= 1.44 / [470+2*1690]*10-7
= 37.4 KHz
As shown in figure the PWM output is connect to the biasing pin (Vcc) of this astable multi-vibrator. So when PWM output is high this will generate burst of 38 KHz wave. This means the low frequency 50 Hz signal is modulated using 38 KHz signal. As this output is directly given to IR LED, it will generate IR light beam of 38 KHz.
3) IR sensor: - This sensor will detect the IR light beam of 38 KHz and produces low output. Means when IR light falls on it its output is low and vice versa
4) Monostable multi-vibrator: - The output of IR sensor is connected to trigger input of IC555 which is configured in monostable multi-vibrator. Its time period is again kept very low (1.1 ms) as compared to transmitted pulse time period (20 ms). So its output will be nearly same as the PWM generated at transmitted side.
5) Motor driver: - Finally the output of IC555 is given to the base of Darlington pair TIP122 which is used to drive DC motor. As the width of pulse is more, more the speed of motor and vice versa
Thus as anyone changes the width of pulse on transmitter side the motor speed changes on receiver side. So we can tirelessly change the speed of DC motor. If you want to change the direction of motor then it requires slight modification in the circuit. Instead of single TIP122 we require four TIP122 to construct H-Bridge circuit as shown in Circuit Diagarm Tab 2
The output of IC555 is given to two H-Bridge inputs through SPDT switch. When switch is on position 'A', Q1 and Q4 are ON and Q2 and Q3 are OFF. So motor rotates in one direction. As the switch is moved to position 'B' Q2 and Q3 becomes ON and other two are OFF. So motor changes its direction of rotation.