Buzzer Control Circuit
Purpose
This circuit provides high-side switching control for 5V loads such as buzzers, relays, LED lights, and solenoids. The design uses a P-channel MOSFET with NPN transistor gate drive to enable microcontroller control of inductive and resistive loads with flyback protection.
Control Signal
- Origin: ESP32 GPIO (or any 3.3V microcontroller)
- Voltage: 3.3V logic level
- Logic: Active HIGH (GPIO HIGH = Load ON, GPIO LOW = Load OFF)
Main Components
| Component |
Part Number |
Function |
| Q6 |
FDN340P |
P-channel MOSFET high-side switch |
| Q5 |
BC847 |
NPN transistor gate driver |
| R38 |
1kΩ 1% |
Base current limiting resistor |
| R65 |
6.8kΩ |
Gate pullup resistor |
| D1 |
SMAJ6.0A |
Flyback protection diode |
Circuit Connections
Power Connections
- Pin 1 (Source): Connected to 5V supply rail
- Pin 3 (Drain): Connected to load positive terminal
- Load negative terminal: Connected to GND
Control Connections
- BuzzerControl: Connected to ESP32 GPIO output
- Q5 Base: Connected to BuzzerControl through R38 (1kΩ)
- Q5 Emitter: Connected to GND
- Q5 Collector: Connected to Q6 Gate
- Q6 Gate: Pulled to 5V through R65 (6.8kΩ)
Protection
- SMAJ6.0A diode: Cathode to load positive (Pin 3), Anode to GND
- Function: Clamps inductive flyback voltage during load turn-off
Operating Voltages
- Supply Voltage: 5V from switching power supply
- Control Logic: 3.3V (GPIO high), 0V (GPIO low)
- Load Voltage: 5V (when switch ON)
Circuit Operation
Normal Conditions - Load OFF (GPIO = LOW)
- Q5 (BC847 NPN) base is LOW → Q5 is OFF
- Q5 collector is pulled HIGH through R65 (6.8kΩ)
- Q6 (FDN340P P-MOSFET) gate is at ~5V, same as source
- Q6 gate-source voltage = 0V → Q6 turns OFF
- No current flows through load → Load is OFF
Load Activation (GPIO = HIGH)
- Q5 base gets current through R38: (3.3V - 0.7V) / 1kΩ = 2.6mA
- Q5 (BC847 NPN) turns ON
- Q5 collector pulled LOW (to GND)
- Q6 gate is now LOW (0V), source is still at 5V → VGS = -5V
- Q6 turns ON → P-MOSFET conducts
- Current flows: 5V → Q6 → Load → Load turns ON
Current Capability Analysis
FDN340P Specifications
- Maximum continuous current: 2A (datasheet limit)
- Maximum pulsed current: 10A
- RDS(on): ~65mΩ at VGS = -2.5V
- Maximum power dissipation: 0.5W
Thermal and Power Analysis
| Load Current |
MOSFET Power Loss |
Voltage Drop |
Load Power |
Efficiency |
| 50 mA |
<1 mW |
3 mV |
0.25 W |
99.9% |
| 100 mA |
1 mW |
7 mV |
0.50 W |
99.9% |
| 200 mA |
3 mW |
13 mV |
1.00 W |
99.7% |
| 500 mA |
16 mW |
33 mV |
2.48 W |
99.4% |
| 1 A |
65 mW |
65 mV |
4.93 W |
98.7% |
Practical Current Limits
- MOSFET capability: 2A continuous
- 5V supply sharing: ~500mA practical maximum (50% of 1A 5V rail)
- Recommended maximum: 500mA for system reliability
Load Compatibility
Supported Load Types
| Load Type |
Typical Current |
Compatibility |
| Piezo buzzer |
5-15 mA |
✅ Excellent |
| Magnetic buzzer |
20-80 mA |
✅ Excellent |
| Small relay (5V) |
50-100 mA |
✅ Good |
| Automotive relay |
100-200 mA |
✅ Good |
| LED strips |
100-300 mA |
✅ Good (within limits) |
| Small solenoids |
200-500 mA |
✅ At practical limit |
| Large solenoids/relays |
>500 mA |
⚠️ Exceeds recommended limit |
Idle Power Consumption
Circuit Idle Current (Load OFF)
- GPIO LOW (load OFF): Only R65 conducts: 5V / 6.8kΩ = 0.74mA
- 5V rail idle current: 0.74mA
- 12V input equivalent: 0.74mA × (5V/12V) / 0.874 = 0.35mA
- Note: Assumes 87.4% overall power supply efficiency (12V→5V→load)
- Quiescent power: 4.2mW at 12V input
Active Power (Load ON)
- GPIO HIGH: Base current (2.6mA) + Load current + R65 current (0.74mA)
- Total 5V current: Load current + ~3.4mA overhead
- Minimal control overhead compared to load power consumption
Bill of Materials (BOM)
| Reference |
Part Number |
Description |
Package |
Quantity |
| Q6 |
FDN340P |
P-Channel MOSFET, -20V, -2A |
SOT-23 |
1 |
| Q5 |
BC847 |
NPN Transistor, 45V, 100mA |
SOT-23 |
1 |
| R38 |
ERJ-3EKF1001V |
Resistor, 1kΩ, 1%, 0603 |
0603 |
1 |
| R65 |
ERJ-3EKF6801V |
Resistor, 6.8kΩ, 1%, 0603 |
0603 |
1 |
| D1 |
SMAJ6.0A |
TVS Diode, 6V, 400W |
SMA |
1 |
Design Considerations
Flyback Protection
- SMAJ6.0A clamps inductive kickback to 6V above ground
- Essential for relay and solenoid loads
- Protects both MOSFET and supply circuit
Switching Speed
- Turn-on time: Limited by BC847 and gate drive
- Turn-off time: Limited by R65 pullup
- Suitable for DC switching, not high-frequency PWM
PCB Layout
- Keep gate drive traces short to minimize noise pickup
- Place flyback diode close to load connection
- Use adequate copper area for current-carrying traces
- High efficiency: >99% at typical buzzer loads
- Reliable operation: Wide safety margins on current handling
- Low idle power: Minimal impact when load is OFF
- Inductive load safe: Flyback protection for all load types
Notes
- Logic is active HIGH: GPIO HIGH turns load ON
- Can handle PWM control if switching frequency is kept low (<1kHz) (? not tested)