A point-type switch with automatic power-off function

1. Circuit Working Principle

The circuit is shown in the figure. In the figure, S1 is a click-type reset switch. When S1 is clicked, the battery BT1 quickly charges C1 through R1. When the voltage charged on C1 is higher than the conduction threshold voltage of the field effect tube T1 control gate (G pole), T1 is turned on, the light-emitting diode D1 lights up, and BT1 supplies power to the load RL. At this time, as long as the voltage on C1 is higher than the conduction threshold voltage of the field effect tube, T1 remains on. As C1 discharges to R2, when the voltage on C1 is lower than the conduction threshold voltage of T1, T1 is turned off. If S1 is clicked again before T1 is turned off, T1 can be extended for a conduction time period. The length of the circuit conduction period is determined by three factors: one is the value of the discharge resistor R2 of the C1 discharge circuit; the second is the capacitance of C1; and the third is the high or low battery voltage.

2. Key points of circuit production:

If the circuit is made according to the component values ​​selected in the figure, under the condition of 5V battery power supply, the conduction time of T1 is about 10 minutes. If you need to extend or shorten the switch conduction time, you can increase or decrease the capacitance of C1 and the resistance value of R2 accordingly, but it should be noted that when increasing the capacitance of C1, the resistance value of the current limiting resistor R1 should be appropriately increased to avoid extending the charging time of C1 and causing long-term overload impact damage to the battery.

T1 is selected according to the size of the load RL. For small current loads, BS170 field effect tubes can be selected, and its maximum operating current is 100mA; for large current loads, IRL540 field effect tubes can be selected, and its maximum operating current is 1A. When these two types of field effect tubes are in the on state, the voltage drop between the S and D electrodes is very small and can be basically ignored.

The performance of this circuit is stable and reliable for controlling analog circuits or resistive and inductive loads. However, for electronic equipment composed of controlling digital circuits, the cut-off and conduction transition process of the switching circuit may cause a brief delay and hysteresis when the equipment enters the working state.