Make cheap and easy Panic Alarm Button Circuit using IC 555.
IC 555 work like bistable circuit with two states: alarm ON and alarm OFF.
Press panic button, timer start and stay ON until reset button is pressed.
Timer give high output, turns ON transistor and then LED light and buzzer makes sound.
Circuit Working:
Parts List:
| Category | Item | Quantity |
|---|---|---|
| Resistors | 10k 1/4 watt | 2 |
| 1k 1/4 watt | 2 | |
| Capacitors | 0.01μF | 1 |
| Semiconductors | IC 555 | 1 |
| Transistor BC547 | 1 | |
| Tactile switches | 2 | |
| LED red 5mm 20mA | 1 | |
| Buzzer | 1 | |
| 9V Battery | 1 |
When circuit starts it keep oscillating and give continuous output.
Parts R1, R2 and C1 control how fast it oscillate and when alarm goes ON.
How it works:
When power is ON, IC 555 output pin 3 is LOW.
Buzzer and LED are OFF and transistor BC547 is also OFF with no signal to base.
Press SET button which is normally open and it connect pin 2 to ground for short time.
This make IC 555 output pin 3 go HIGH and starts the timer.
High signal turn ON the transistor BC547.
Current goes through buzzer and LED and they turn ON and make alarm.
RESET pin 4 is always HIGH through normally closed button.
This keep IC 555 ready and alarm OFF by default.
When RESET button is pressed pin 4 disconnect from HIGH.
This reset IC 555 make output LOW.
Transistor goes OFF, buzzer and LED stop and alarm goes OFF.
Formulas:
These are main things and formulas for making set-reset (SR) type IC 555 panic alarm circuit:
1. Trigger Resistor (Rtrigger):
Rtrigger help IC 555 trigger smooth and stop button bounce problem.
Normal value are 1k to 10k ohm
Value depend on button and how sensitive we want it
2. Timing Resistors R1 and R2:
R1 and R2 set how long alarm stays ON or OFF after pressing panic button.
Time formula:
T = 0.693 × (R1 + R2) × C
where:
- R1, R2 are the resistance in ohms Ω
- C is the capacitor in farads F
Note:
This circuit use IC 555 in set-reset mode.
Change R and C values to set right timing for alarm ON and OFF.
How to Build:
To build a Panic Alarm Button Circuit using IC 555 follow the below mentioned steps for connections:
- Join all parts as shown in circuit diagram.
- Pin 1 of IC 555 connects to ground.
- Pin 2 connects between resistor R1 and SET push button.
- Pin 3 connects to base of transistor Q1 using resistor R4.
- Pin 4 connects between resistor R2 and RESET push button.
- Pin 5 connects to ground with capacitor C1.
- Pin 6 connects to ground.
- Pin 8 connects to +9V battery.
Transistor Q1 Connections:
- Collector connects to +9V through resistor R3 and LED1.
- Base connects to pin 3 of IC 555.
- Emitter connects to ground.
- Buzzer connects to one end of +9V and other end between LED1 and Q1 collector.
Safety Tips:
- Check all wires and parts again to avoid short circuit or damage.
- Check all resistors, transistors, capacitors are rated for used voltage.
- Place LED and capacitor in correct direction.
- Use heat shrink tube or solder wires properly.
- Keep circuit safe from water, dust or touch and use plastic box if needed.
Conclusion:
In this Panic Alarm Button Circuit using IC 555 when panic button is pressed then IC 555 sends oscillating signal.
This turns ON transistor, buzzer and LED start alarm.
Press RESET button to stop alarm and reset timer.