The reliability of readings of battery-operated measuring instruments depends, of course, on the / state of the battery. Even simple instruments are, therefore, provided with a facility (normally a push button) to check the battery voltage. If, however, the state of the battery must be monitored constantly, that type of check is not very reliable. And then there are instruments that have no low battery indicator whatsoever.
The indicator shown in Fig. solves all these problems. With jump links JP1 and JP2 in the positions shown, D2 serves primarily as on/off indicator. This low-current LED lights constantly as long as the battery voltage is sufficiently high and the optional alarm input (a) is not actuated. 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. A second oscillator, lC-1a, is actuated when a high voltage level is applied to terminal a. The LED will then flash at a frequency of 10 Hz. This additional indication is often useful to alert the-user to an operational error.
Low Battery Indicator Circuit Diagram:
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 µA. If this condition is selected, the connections of D2 must be changed over (as shown in dashed lines). The circuit uses the NAND gates of a Type 74HC132 IC, which is suitable ‘for supply voltages,Ui, of 2-6 V. Since the change-over point of the indicator depends on the supply voltage, connected to Ui, this must be regulated. It is, of course, possible, even preferred, to connect Ui to the regulated supply of the relevant measuring instrument. Here, an independent supply of 5 V is assumed.
Since the regulator, IC2, is a low-drop type, the battery voltage may drop to about 5.5 V before the stability of the output voltage (at Ui) becomes questionable.
The design may be adapted for use with higher supply voltages: if ICS is replaced by a Type 4093, the circuit becomes suitable for an input voltage, U1, of up to 18 V. The series resistor of D2 must be given a different value, of course: for instance, D5 should be 3.9 kΩ when Ui= 15 V. The battery voltage is applied to the input of IC-1a via potential divider R1-P1. Diode D1 protects the gate against too high a voltage from a fresh battery.
Although the function of C6 may not be immediately evident, it is a vital one: the capacitor ensures that the upper switching threshold (about 2.5 V) of IC-1a is exceeded briefly on power-on. Oscillator IC-1b is then reset. After about one second, the potential at junction R2-R3 is a measure of the state of the battery. If that potential lies below about 1.5 V (the lower switching threshold), the oscillator is actuated. If C6 were not in circuit, the oscillator would not be reset at low battery voltages, resulting in an erroneous low-battery indication.
Preset P1 (maximum resistance) is adjusted when a variable power supply is connected to Us+ and set to an output level at which the Indicator is to become operational, say, 6 V. Turn P1 slowly until D2 begins to flash. The alarm input, a, may be adapted for use with negative battery voltages by the circuit shown in Fig. 2. In fact, the two circuits together monitor a symmetrical battery supply: the positive line via Us+ and the negative line via the circuit in Fig. 2, connected to terminal a in Fig. 1. When one of the batteries begins to flash: at a frequency of 0.5 H2 in case of the + battery, and at about 10 Hz in case of the – battery. When both batteries become low simultaneously, D2 flashes with interruptions.
The circuit in Fig. 2 raises the negative battery voltage to above earth level. Note that, because of the loading of its output, R6 in Fig. 1 must be removed. Connect the ++ line to Ui in Fig. 1, the + line to the negative terminal of the symmetrical battery supply, and 0 to 0 in Fig. 1.
Provided all connections are correct, the total current drain rises by only about 10 µA over that of Fig. 1 (assuming two 9-V batteries in series). Preset P1 is adjusted in a manner similar to that described above for P1 in Fig. 1.
When the upper switching threshold of IC-1c is exceeded, the gate oscillates and D2 begins to flash. The power-on reset (terminal a becomes logic 0) is provided by capacitor C2. This capacitor is discharged via D1 and the supply lines.
Diode D2 protects IC-1c against negative input potentials, while capacitor C1 pre-vents the circuit being triggered by possible brief noise peaks on the battery voltage.