Low battery indicator

Encyclopedia of radio electronics and electrical engineering / Chargers, batteries, galvanic cells

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For the attention of readers, we offer an indicator of the battery status, which signals by giving light and sound signals about its discharge when the voltage drops below the threshold level. The device is assembled on the basis of a voltage drop detector of the KR1171 series.

The device described in the article belongs to the group of the simplest indicators, which only signal a decrease in the voltage on the battery (AB) below the set limit. Only a device consisting of a zener diode, an LED and a resistor can be considered simpler than the proposed indicator, however, it has an inverted indication, that is, it turns off when the voltage at the battery is less than the threshold level.

The basis of the proposed AB discharge indicator is a low voltage detector - a microcircuit of the KR1171 series. These microcircuits are specially designed for the control of supply voltage reduction in microprocessor technology and are ideally suited for this task.

The microcircuit includes a reference voltage source, a comparator that compares the supply voltage with a reference voltage, and an open-collector output transistor. It is made in a three-pin, "transistor" package KT-26. To implement the simplest indicator, it is enough to connect an LED and a current-limiting resistor to the output of the microcircuit. The dimensions of such a device are almost equal to the dimensions of the microcircuit and the LED (you can take the smallest resistor). The only drawback of such an indicator is a rigidly fixed number of manufactured microcircuits in this series, each of which is designed for a very specific threshold voltage.

The last two digits in the name of each microcircuit of the series indicate its threshold turn-on voltage. The main characteristics are given in the table.

Although the fixed threshold turn-on voltages create some difficulties, they still make it possible to make indicators of the discharge of different batteries. So, for example, on a KR1171SP20 chip with a threshold voltage of 2 V, you can make a very compact indicator for devices powered by two nickel-cadmium batteries (children's toys, photographic equipment, audio players, radios, flashlights, etc.).

Such an indicator can be supplemented with a simple sound signaling device. A diagram of one of the variants of such a device is shown in Fig. 1.

The use of SHOP microcircuits makes it possible to obtain high efficiency and a large operating voltage range. In practice, in order to convert the indicator to a different voltage, it is enough to put a KR1171 series chip with a different threshold voltage. The only exception is KR1171SP20, since at a voltage of 2 V the generator on the K561LN2 chip does not work. However, this is easily explained. For microcircuits of the K561 series, the supply voltage interval is 3 ... 15 V. In practice, this generator worked stably up to a voltage of 2,5 V, however, with a reduced signal volume. The current consumption of the indicator in the off state (the LED is off, there is no sound signal) at a voltage of 6 V does not exceed 20 μA, and in the on state (the LED is on, the signaling device emits tones) at a voltage of 5,2 V - 6 mA.

Such a low current consumption in the off state allows the indicator to be built into devices that require continuous monitoring of the battery voltage. In this case, it should be connected to the power switch of the device, directly to the battery terminals.

The printed circuit board of the indicator is made of one-sided foil fiberglass. Its drawing is shown in Fig. 2. It was developed for the most common elements and therefore has increased dimensions. By using the 564 series chip and other surface mount parts, a much more compact design can be achieved. To reduce the dimensions, instead of the ZP-18 piezoceramic emitter, it is desirable to use a more miniature one with an acceptable sound volume. The remaining parts are MLT, OMLT, S2-ZZN, etc. resistors with a power of 0,125 W. Capacitor C1 - any oxide with a minimum leakage current, the rest - K10-17 or KM. Diodes can be used from the KD522, KD521 series. LED - any with a rated current of not more than 10 mA. Its color, brightness and dimensions are chosen based on specific conditions.

Establishing the indicator is reduced to the selection of resistor R6 to ensure the maximum volume of the applied piezoelectric emitter.

In general, any sound signaling device can be used in the indicator. The main requirements for it are the minimum current consumption in the off state and performance at the required threshold voltage. It is also permissible to use a sound emitter with a built-in generator, for example, HPM14AX from JL World.

Author: S.Malakhov, St. Petersburg

See other articles Section Chargers, batteries, galvanic cells.

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