Circuit automatically switches off DMM

摘要

The circuit in Figure 1 lets you install this automatic -shutoff function in a DMM (digital multimeter). When the momentary pushbutton switch is open, the capacitor discharges through the resistor. The transistor and DMM are off because the gate-to-source voltage is OV. When you momentarily press the button, the capacitor immediately charges up to the battery voltage. The transistor's gate voltage is higher than its source, turning on the DMM. When you release the button, the capacitor begins to discharge slowly through the resistor. When the gate voltage reaches the threshold level, the transistor turns off, thus switching off the DMM.