Radio power automatic shut-off circuit

For battery-powered equipment, if you forget to turn off the power supply, the battery power will be exhausted unknowingly; and dry batteries often leak and corrode circuit boards after running out of power.

The circuit in Figure 1 can automatically shut down the power supply of the device after a certain period of time. The circuit uses the CMOS type time base integrated circuit ICM7555 for one-time action. Take radio power supply control as an example. First press the start switch SW1. When the voltage of pin ② of the time base Ic reaches less than 1/3 of the power supply voltage, the internal trigger of the time base Ic flips and the output pin ③ changes to high level "H "The drain-source terminal of MOSFET tube Tr1 is connected, turning on the power of the radio.

At the same time, the ⑦ pin of the time base Ic becomes high impedance, the capacitor c1 is charged through the resistor R1, and the voltage Vx across it slowly rises until it reaches 2/3 of the power supply voltage VCC. This period of time t can be expressed by the following formula:

Vx=Vcc[l—e-(t/R1C1)]

When the voltage of pin ⑥ reaches 2/3 of the power supply voltage, the internal trigger of the time base Ic flips again, connecting pin ⑦ to the ground (GND), Vx becomes 0v instantaneously, and pin ③ changes to low level "L", Tr1 Cut off, turn off the power supply. From the time from power on to power off, you can replace Vx with 2/3VCC in the above formula to get the following formula:

t=loge3R1C1≈1.1R1C1

Therefore, the time to maintain power supply is determined by the product of R1C1.

If the setting time is 60 minutes, then R1 takes 10MΩ. The capacitance of C1 requires 327μF, which requires the leakage current of the capacitor to be as small as possible; however, large-capacity, low-leakage capacitors are difficult to obtain. Therefore, the circuit in Figure 1 is suitable for working for several minutes.

In response to the above problems, by changing the circuit structure, counting the output of the low-frequency Rc oscillator, and turning off the power supply when it reaches a certain value, a long delay can be achieved.

The circuit is shown in Figure 2. The time base Ic is used to form an astable multivibrator. It works in the time period between VCC1/3 and VCC2/3. Its period:

τ=0.69C1(2R1+R2)

The output of the low-frequency oscillator is fed to the input of a frequency divider. The frequency divider uses the asynchronous 14-level binary counter TC4020B. The maximum frequency division can be 213. If you want to obtain a delay of 60 minutes at this time, the input pulse period τ = 0.44 seconds.

It can be seen that the time constant of RC is small, so a small-capacity film capacitor can be selected to avoid instability caused by leakage current.

Two gate circuits of the four-NAND gate TC4001BP form an RS flip-flop, and the other two gate circuits form an inverter. They reset IC1 and IC2 and drive Tr1 to turn off. The output of TC4020B selects Q14 terminal.