Circuit Function Breakdown

flux

I can give a high-level breakdown from the project context, but I can’t inspect the live schematic further right now. Based on the project name and listed parts, this looks like a digital thermometer with a latching power switch built around an Arduino Nano.
Circuit block breakdown 1. Battery / power source
The multi-cell battery provides the main input power for the circuit. It likely powers the Arduino Nano either directly through its VIN/input path or through a switched power path.
2. Latching power switch
The push button, S8050 NPN transistor, resistors, and possibly the PC817 optocoupler appear to form a latch-style power switch.
The idea is usually:
  • Press the button briefly.
  • That momentarily powers or enables the circuit.
  • The Arduino then drives a control signal to “hold” the power on.
  • When the Arduino wants to shut down, it releases that signal and the circuit turns itself off.
This lets the thermometer use very little battery power when off.
3. Arduino Nano
The Arduino Nano is the main controller. It likely:
  • Reads the temperature sensor input.
  • Controls the display or output indicators.
  • Manages the latch power signal.
  • Turns the circuit off after a timeout or after measurement is complete.
The Nano contains the microcontroller, voltage regulation, USB interface, and GPIO pins.
4. Temperature sensing section
The project name suggests there should be a thermometer input, but from the listed components I don’t see a clearly named temperature sensor. It may be one of the generic parts, missing from the current component summary, or not yet added.
A temperature sensor block would usually be one of:
  • NTC thermistor with a resistor divider
  • Digital sensor such as DS18B20
  • I2C sensor such as TMP102, SHT31, BME280, etc.
  • Analog sensor such as LM35/TMP36
This is the main block I would verify next.
5. LED indicator
The LED with a series resistor is likely used as a status indicator. It may show:
  • Power on
  • Measurement active
  • Battery/status indication
  • Latch state
The resistor limits LED current so the Arduino pin or transistor is not overloaded.
6. Resistors
The fixed and generic resistors likely serve several jobs:
  • LED current limiting
  • Base current limiting for the S8050 transistor
  • Pull-up or pull-down resistors for the latch circuit
  • Possibly a sensor divider if using an analog thermistor
  • Biasing or isolation around the optocoupler
7. S8050 transistor
The S8050 is an NPN bipolar transistor. In this design it is probably used as a switch or signal inverter in the latching power section.
It may be controlling:
  • A power-enable node
  • The base/gate of another switching element
  • The latch feedback signal
8. PC817 optocoupler
The PC817 is an optocoupler. It electrically isolates one side of the circuit from another using light internally.
In this project it may be used to:
  • Allow the Arduino to control the latch while isolating signals
  • Interface between the battery/power switch section and logic section
  • Provide a safer control path if different voltages are involved
I would want to inspect its exact wiring before saying definitively why it is included.
9. Capacitor
The TCJA106M025R0150 is a 10 µF tantalum/polymer capacitor rated for 25 V. It is likely used for power smoothing or bulk decoupling.
It helps reduce voltage dips when the Arduino powers up or when the latch circuit changes state.
Current likely system behavior
  1. User presses the push button.
  2. Power is applied to the Arduino Nano.
  3. The Arduino starts up.
  4. The Arduino asserts a latch/hold signal.
  5. The circuit remains powered after the button is released.
  6. Arduino reads the temperature sensor.
  7. Arduino drives output/status indication.
  8. Arduino eventually releases the latch signal to turn itself off.
What remains to verify
The most important thing to confirm next is the exact wiring of:
  • The latching power switch
  • The Arduino power input
  • The temperature sensor block
  • The optocoupler purpose
  • Whether every IC/module has proper decoupling and safe current-limiting resistors
At a high level, the circuit is a battery-powered Arduino thermometer with a pushbutton-controlled self-latching power system.
Run ERC/DRC
Locate temperature sensor
Trace latch circuit
Check Nano power
Document breakdown
R8
Resistance
100 Ω
D1
R8
Resistance
100kΩ
BT1
R4
Resistance
100 Ω
R6
Resistance
100kΩ
R7
Resistance
500 Ω
R10
Resistance
100 Ω
Q4
U1
SW1
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Digital Thermometer with latch power switch

Digital Thermometer with latch power switch thumbnail
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