Low battery indicator. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Low battery indicator. Encyclopedia of radio electronics and electrical engineering

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

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The proposed indicator monitors the voltage on a 9-volt battery and displays it in the form of a light column made up of six LEDs of different colors. With it, you can evaluate the overall condition of the battery: "fresh" or discharged (at minimum or maximum load current). The indicator does not require setting a voltage threshold, it is powered by a measured source and is made on widely available components.

The indicator circuit (Fig. 1) is built on six voltage drop detectors (microcircuits of the KR1171SP series). These microcircuits are specially designed for voltage control in microprocessor technology. Microcircuits of various markings have their own threshold switching voltage. So, the KR1171SP87 microcircuit switches at a voltage of 8,7 V, and the KR1171SP53 - at a voltage of 5,3 V.

(click to enlarge)

Microcircuits are produced in the TO-92 package and look like a transistor. They have three outputs: voltage measurement input, common (minus) and open collector output. If the controlled voltage falls below the switching threshold of the microcircuit (depending on its letter index), the output is set to a low potential. This property is used to measure battery voltage.

The controlled voltage coming through connector X1 is loaded by the discharge circuit R1-R2. Trimmer resistor R2 can set the load current from 10 to 90 mA. Resistor R1 protects the battery from a short circuit when the trimmer slider is in the extreme position.

It is best to test a standard 9-volt battery at a recommended discharge current of about 10 mA for it, by unscrewing the trimmer resistor to maximum resistance. But you can set the load current based on the actual current consumed, and thus evaluate the performance of the battery.

The measurement inputs of six detectors DA1...DA6 are connected in parallel with the discharge circuit. Field-effect transistors VT1 .VT6 are connected to the outputs of the microcircuits, which control the switching on of the HL1 ... HL6 LEDs. Each of the detectors is designed for its own response voltage, therefore, with a "fresh" battery, the outputs of all detectors have a positive potential passing through the "pull-up" resistors R3...R8 to the gates of field-effect transistors VT1...VT6. All transistors are open, and the HL1.HL6 LEDs are lit. The LEDs form a luminous column, which is used to visually control the voltage on the battery. If all LEDs are lit, the battery voltage is normal. When the LEDs HL4...HL6 are off (green) and luminous HL1...HL3, the charge level is about 50%. If the HL2, HL3 LEDs (yellow) also go out and only the HL1 LED (red) lights up, then the battery is almost completely discharged (its voltage is only 2,8 V).

You can also make a more accurate assessment - for each extinguished LED. For example, if only the HL6 LED goes out, this means that the DA1 chip, designed for a threshold voltage of 8,7 V, has switched.

Consequently, the voltage is less than this level and with luminous HL5 is about 7,6 V. The switching ranges of the KR1171SP microcircuits are indicated in Table 1.

When using the module, you need to set the battery load current.

This can be done approximately by moving the tuning resistor engine to one of the extreme positions. Then the load current will be either minimum or maximum. You can set the load resistance with a DMM and calculate the battery current.

The current that the indicator consumes is determined mainly by the current through the LEDs HL1 ... HL6. To reduce this current, you need to increase the resistance of resistors R9 ... R14.

With the details indicated in the diagram, the current consumption is about 50 mA (with the maximum resistance of the trimmer resistor). The indicator is assembled on a printed circuit board made of one-sided foil fiberglass with a thickness of 1,5 ... 2 mm. PCB dimensions - 25x50 mm.

The drawing of the board is shown in Fig. 2, the location of the radio components is in Fig. 3, and the ratings and types of elements are in Table 2.

To connect to the battery, a connector is soldered to the printed circuit board. It is more convenient to take the connecting connector from another, similar battery that has already served its time.

Use a metal paper clip to connect the connector and the board. The paper clip is cut in half and cleaned, and then twisted into the fastener holes.

The excess ends are cut off and soldered to the PCB and connector. When soldering the ends of the paper clip, use a neutral liquid flux. This will make it easier to tin and join the metal parts. The connection securely and rigidly fixes the board with the parts in a vertical position on the connector.

Now you can connect the battery and check the performance of the indicator.

Author: A. Lechkin

See other articles Section Chargers, batteries, galvanic cells.

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