Infrared (IR) Sensor Alarm Circuit using TSOP1738

This tutorial show how to make a Infrared (IR) Sensor Alarm Circuit using TSOP1738.

It finds intruder using light which we cannot see.

This alarm uses infrared light so people call it IR alarm.

It make one invisible light beam with small chip named IC 555.

If someone walk in front of light, then the beam breaks and then alarm goes OFF.

This project is fun way to learn little bit of electronics.

What is a Infrared (IR) Sensor Alarm Circuit:

When some condition takes place then one electric system called Infrared (IR) Sensor Alarm Circuit is made to find infrared light and make alarm sound or give others an alert.

This system uses IR sensor like motion detector or IR receiver to see change in infrared light in small areas.

It is mostly used for security and this circuit find moving person or intruder in one place and then make an alarm to warn people or security guard.

Circuit Working:

Parts List:

This circuit uses IC 555 in monostable mode it stays stable until something turn it ON.

The time of output signal at pin 3 depend on value of resistor R and capacitor C which is explained below with formula.

One BC547 transistor work like switch and it sends signal to pin 2 by one capacitor and turn ON the IC 555.

The capacitor charges and discharges to make proper trigger.

Second BC547 transistor is also there which keeps first transistor OFF and then this is important to keep circuit in standby.

This makes IC 555 stay OFF until someone break the system.

Another IC 555 works in astable mode which makes 38 kHz signal again and again.

Capacitors C1, C2 and resistors R1, R2 control how fast the signal come.

This 38 kHz signal goes out through one infrared LED connected to astable IC 555.

TSOP1738 sensor catch this signal and keep second BC547 transistor ON.

If signal break then second transistor goes OFF and this makes IC 555 in monostable mode turning ON.

Then alarm turn ON using output relay from pin 3.

Formulas and Calculations:

Below are easy formulas for IR Sensor Alarm Circuit using TSOP1738:

1. Formula for Pulse Time:

Pulse Time = 1.1 * R * C

where:

• 1.1 is just a guess number and it helps because IC 555 is not always perfect and other parts in the circuit can also change it.
• R means resistor value in ohms Ω.
• C means capacitor value in farads F.

Limits:

This formula only work while using IC 555 in monostable mode.

It does not work with other kind IR circuits.

1.1 is only close guess and actual pulse time may change little bit.

Example:

If R = 10k and C = 1uF:

Pulse = 1.1 * 10,000 * 0.000001 = 0.011 seconds = 11 milliseconds

2. Formula for Time Constant:

Time Constant (τ) = R * C

where:

• τ tau is time constant and it is important in RC circuits.
• R is resistor in ohms Ω
• C is capacitor in farads F.

3. Formula for Frequency in Astable Mode:

Frequency = 1.44 / ((R1 + 2 * R2) * C1)

where:

• Frequency means how fast signal goes HIGH and LOW which is measured in Hertz Hz.
• R1 and R2 are resistors in ohms Ω.
• C1 is capacitor in farads F.
• 1.44 is constant from IC 555 chip.

How to Use:

If circuit gives R1, R2 and C1 values then use this formula to find frequency.

We can change R1, R2 or C1 to change the frequency.

Example:

R1 = 10k, R2 = 10k, C1 = 1uF

Total = 10k + 2 * 10k = 30k

Frequency = 1.44 / (30,000 * 0.000001) = 48 Hz

IC 555 IR Transmitter:

Parts List:

One more IC 555 is used in astable mode it makes 38 kHz signal all the time like a transmitter for IR light.

Capacitors C1 and resistors RV1 and R2 have fixed values and these control the square wave signal.

To get correct 38 kHz signal from pin 3 of IC 555 and IR LED then RV1 preset must be adjusted carefully.

The IR LED is connected to pin 3 of this astable IC 555 which then send out the square wave IR signal.

This part is very important because it ensures alarm system keep working by sending 38 kHz IR signal always.

How to Build:

Making Infrared (IR) Sensor Alarm Circuit using TSOP1738 following steps are required for Connections:

IC 555 Section:

• Connect pin 2 of IC1 555 to collector of first BC547 transistor with one capacitor C2.
• Connect pin 6 of IC1 to pin 2 using resistors R1 and R2.
• Pin 2 of IC1 also connects to base of first BC547 transistor.
• Connect pin 7 to pin 6 using resistor R3.
• Connect pin 7 and pin 6 to collector of second BC547 transistor.

IR Signal Section:

• Set second IC 555 in astable mode.
• Connect pins 6 and 2 to pin 7 using resistors R1 and R2.
• Also connect pin 6 to pin 2 using capacitors C1 and C2.
• Connect pin 3 of this IC 555 to anode (+) of IR LED.
• Connect cathode (–) of IR LED to power supplies positive side.

IR Signal Detection Section:

• Add TSOP1738 IR sensor in the circuit.
• Its output connects to base of second BC547 transistor.
• Collector of second BC547 connects to base of first BC547 transistor.

Testing:

• Give power to circuit.
• Be sure IR LED is sending 38 kHz signal.
• Block the IR light to test.
• When blocked alarm should turn ON.

Important Notes:

• To get correct timing and IR signal change resistor and capacitor values.
• Check every part are in placed right and polarity is correct.
• Before using for real test in safe place.

Safety Tip:

• When working with electronics always be careful.
• If not sure ask someone who knows electronics for help.

Conclusion:

This Infrared (IR) Sensor Alarm Circuit using TSOP1738 in monostable mode is turned ON using BC547 transistor.

The IR signal at 38 kHz from astable IC 555 keeps the system in standby.

When this signal is broken the alarm gets activated.

References:

How to use 2 TSOP1738 (infrared sensor) in one circuit?

Datasheet TSOP1738