![]() Since the change-over point of the indicator depends on the supply voltage, connected to Ui, this must be regulated. ![]() The circuit uses the NAND gates of a Type 74HC132 IC, which is suitable ‘for supply voltages,Ui, of 2-6 V. If this condition is selected, the connections of D2 must be changed over (as shown in dashed lines). When JP1 and JP2 are in their other position, there is no on/off indication the quiescent current then drops from about 4 mA to a few ♚. This additional indication is often useful to alert the-user to an operational error. The LED will then flash at a frequency of 10 Hz. A second oscillator, lC-1a, is actuated when a high voltage level is applied to terminal a. When the battery voltage drops below a value set by P1, oscillator IC-1b is switched on, whereupon D2 begins to flash at a frequency of about 0.5 Hz. This low-current LED lights constantly as long as the battery voltage is sufficiently high and the optional alarm input (a) is not actuated. With jump links JP1 and JP2 in the positions shown, D2 serves primarily as on/off indicator. And then there are instruments that have no low battery indicator whatsoever. ![]() If, however, the state of the battery must be monitored constantly, that type of check is not very reliable. Even simple instruments are, therefore, provided with a facility (normally a push button) to check the battery voltage. The reliability of readings of battery-operated measuring instruments depends, of course, on the / state of the battery.
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