This tutorial show how to make a Infrared (IR) Sensor Alarm Circuit using TSOP1738, which finds intruder using light which we cannot see.
Also, this alarm circuit uses infrared light so people call it IR alarm, which make one invisible light beam with small chip named IC 555.
Moreover, if someone walk in front of light, then the beam breaks and then alarm goes OFF and hence, this project is fun way to learn little bit of electronics.
What is a Infrared (IR) Sensor Alarm Circuit:
When a specific condition occurs an electronic system called an Infrared (IR) Sensor Alarm Circuit detects infrared light and triggers an alarm or sends an alert.
Also, this system uses IR sensor like motion detector or IR receiver to see change in infrared light in small areas.
Furthermore, this circuit is mostly used for security; it detects a moving person or intruder in an area and then triggers an alarm to warn people or security guards.
Circuit Working:
Parts List:
| Components | Values | Quantity |
|---|---|---|
| Resistors | 10k 1/4 W CFR | 4 |
| 1k 1/4 W CFR | 3 | |
| 100Ω 1/4 W CFR | 1 | |
| 2.2k 1/4 W CFR | 1 | |
| 100k 1/4 W CFR | 1 | |
| Capacitors | Ceramic 0.01µF | 1 |
| Electrolytic 100µF 25V | 1 | |
| Electrolytic 1µF 25V | 1 | |
| Semiconductors | Transistor BC547 | 2 |
| Transistor BC557 | 1 | |
| IC 555 | 1 | |
| IC 7805 | 1 | |
| IC TSOP1738 | 1 | |
| LED Red 20mA 5mm | 1 | |
| Buzzer 12V | 1 |
To begin with, this circuit uses IC 555 in monostable mode and it stays stable until something turn it ON.
Then the values of resistor R and capacitor C show the output signal timing at pin 3, as explained below using the formula.
Also, 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.
After that, second BC547 transistor is also there which keeps first transistor OFF and then this is important to keep circuit in standby.
As a result, this makes IC 555 stay OFF until someone break the system and also this IC 555 works in astable mode which makes 38 kHz signal again and again.
Here, capacitors C1, C2 and resistors R1, R2 control how fast the signal come and also 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 and if signal break then second transistor goes OFF and this makes IC 555 in monostable mode turning ON.
Hence, the alarm turn ON using output relay from pin 3.
Formulas and Calculations:
Below are easy formulas for IR Sensor Alarm Circuit using TSOP1738:
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 and 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 describes how fast a signal switches between HIGH and LOW states and we can measure it 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 and 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:
| Components | Values | Quantity |
|---|---|---|
| Resistors | 470Ω 1/4 W CFR | 2 |
| 2.2k 1/4 W CFR | 1 | |
| Preset 10k 1/4 W CFR | 1 | |
| Capacitors | Ceramic 47nF | 1 |
| Electrolytic 10µF 25V | 1 | |
| Semiconductors | IC 555 | 1 |
| IR Emitter LED | 1 |
Another IC 555 in astable mode continuously generates a 38 kHz signal, acting as a transmitter for the IR light.
Also, capacitors C1 and resistors RV1 and R2 have fixed values and these control the square wave signal.
Adjust the RV1 preset carefully to obtain the correct 38 kHz signal from pin 3 of IC 555 and the IR LED.
After that, connect the IR LED to pin 3 of the astable IC 555, which then sends out the square wave IR signal.
Hence, this part is very important because it ensures alarm system keep working by sending 38 kHz IR signal always.
How to Build:
To make the Infrared (IR) Sensor Alarm Circuit using TSOP1738 follow the below connection steps:
IC 555 Section:
- First, connect pin 2 of IC1 555 to collector of first BC547 transistor with one capacitor C2.
- After that, connect pin 6 of IC1 to pin 2 using resistors R1 and R2.
- Join pin 2 of IC1 and also connects to base of first BC547 transistor.
- Then connect pin 7 to pin 6 using resistor R3.
- Now also connect pin 7 and pin 6 to collector of second BC547 transistor.
IR Signal Section:
- Also, set second IC 555 in astable mode.
- Then connect pins 6 and 2 to pin 7 using resistors R1 and R2.
- After that, connect pin 6 to pin 2 using capacitors C1 and C2.
- Now connect pin 3 of this IC 555 to anode (+) of IR LED and connect cathode (–) of IR LED to power supplies positive side.
IR Signal Detection Section:
- After that, add TSOP1738 IR sensor in the circuit and its output connects to base of second BC547 transistor.
- Then collector of second BC547 connects to base of first BC547 transistor.
- Give power to circuit, be sure IR LED is sending 38 kHz signal and block the IR light to test.
- Then 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 and if not sure ask someone who knows electronics for help.
Conclusion:
To conclude, the BC547 transistor turns ON this Infrared (IR) Sensor Alarm Circuit using TSOP1738 in monostable mode.
Also, The astable IC 555 generates a 38 kHz IR signal that keeps the system in standby and the alarm activates when this signal breaks.