In the past, most monitors did not have a device for economizing power consumption because there was no standardized criteria. Some monitors use devices of hardware circuits or software programs to control the appearance of pictures shown on the screens by way of making cathode-ray tubes stop shooting electrons when no signal has been inputted. When this occurs, there are no pictures on the screens but the monitors are still in a power-on operation state. Therefore, these kinds of devices can protect the cathode-ray tube of a monitor and prolong the lifetime of the cathode-ray tube, but cannot economize power consumption. In some monitors with a device for economizing power consumption, a TTL voltage logic level and other elements are used to construct a circuit in a circuit board but the device occupies too much space. Besides, a device of this kind is not accurate in determining the existence of a signal because determines whether the signal is exsistent only by one reference value, i.e., the monitor is under a power-on operation state when the frequency of an input signal is greater than the reference value but a power-off operation state when the relevant frequency is lower than the reference value. This results in a harmful effect on the monitor if the frequency of the input signal is around the reference value and the operation state of the power is intermittently switched between ON and OFF.
It is therefore attempted by the Applicant to deal with the situation encounted by the prior art.