Today we analyze a classic microcontroller power supply circuit, the schematic diagram of the circuit is shown below:
▲ Simplified switch circuit
Before powering the circuit, the switch “TEST” is open, and the microcontroller is not powered through VCC. At this point, the base of T1 is grounded through R9 (100k), putting it in the cutoff state. The base resistor R7 connected to T3’s base, as well as Test, keeps T1 in the cutoff state, so T3 is also in the cutoff state.
The power supply +9V is isolated by T3 and is not loaded onto the voltage regulator IC2, keeping the output VCC of IC2 at a low level.
▲ Circuit in off state
Pressing the button “TEST” starts the circuit, the base of T3 is grounded through R7, Test, and the b-e of T2, causing T3 to conduct. At this point, +9V is applied to the voltage regulator IC2 through T3. The output VCC of IC2 is applied to the microcontroller.
After the microcontroller is powered on, it outputs a high voltage through IO2, causing T1 to conduct via R8. Even if Test is released at this point, the base of T3 can still be grounded through R7, LED1, and T1, achieving power self-locking.
▲ Press TEST to start the circuit
▲ After the circuit starts, the MCU provides the base voltage for T1, thus keeping T3 conducting
Afterward, the microcontroller software can make the IO2 port return to a low level, causing T1 to cut off, and consequently T3 to cut off.
The state of T2’s switch can be read based on the IO1 port, thus determining whether the user has pressed the function key. After determining that the user has pressed Test, wait until the user releases Test before setting IO2 to a low level.
Software functions can also implement automatic power-off delays, thereby reducing power consumption.
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