Shown in Figure 1 the electric circuit both simply and to spend not many, may use for to protect in the mobile equipment one of most valuable parts: Battery. This electric circuit’s application scope including all need time limit operation’s mobile equipment, like the measuring instrument, the guitar tunes a stringed musical instrument and the electrical toy. Presses down the make-and-break instant soft switch then to start for one cycle, this electric circuit for equipment power supply. When you when any time presses down the switch once more, the electric circuit shuts off, enters “the sleep” the condition until the next cycle. You forgot accidentally shuts off this electric circuit power source, in the electric circuit has an automatic power failure function, the time delay cycle is the preinstall time-constant function.
IC1 and the related part have one kind of bistability cut function, and prevents the switch contact rebound. The IC1C buffer cut’s signal, and isolates the R1-C1 charging current. This signal according to the formula t=1.1×RTCT present for the IC2 timer, IC2 disposes a list steady multivibrator, is at the state of activation until its suspension. t is the automatic off time. In this example, this time-gap probably is 6 minutes. The timer output signal present gives the Q1 phase inverter. Q1 activates power by-pass type transistor Q2. This electric circuit disposes a PNP block, guaranteed that the load loss is very small. The power loss only comes from VCE (SAT), approximately 100mA×0.2V, namely 20mW. If an equipment need bigger electric current, may choose an appropriate transistor in addition. If you request the static state to lose are young, or between the battery and the application electric circuit the loss of voltage is small, MOSFET is possibly one highly effective method.
Under the shutdown condition’s static state loses may ignore, because the electric circuit only from is at the non-state of activation the CMOS electronics grid to obtain the power. LEDD1 indicating circuit’s on-off state. Does not need is LED provides the extra power from the battery, because it connects in actuates in transistor’s current supply leg. In suspension period, outputs to the 0V transition, this guaranteed that comes fixed time the power failure with the C5 feedback loop, because the C5 feedback loop cuts the two-state circuit the shutdown condition, plays with you may forget uses on-and-off switch same role. This simple electric circuit when the equipment does not need to use the micro controller is very useful.
