# Flat Battery Indicator Schematic Circuit Diagram

#### Battery Monitoring Circuit for Model Hovercraft

This compact circuit was designed specifically for monitoring the battery in a model hovercraft. The hovercraft’s lift is generated by an electric motor that drives a fan. To prevent over-discharging of the rechargeable battery pack, the circuit illuminates a prominent LED on the model when a predetermined voltage threshold is attained. The simplicity of the design, employing just a few components, contributes to keeping the overall weight of the model hovercraft at a minimum.

#### Connecting the Circuit for Voltage Monitoring

This circuit establishes a connection to the model only at the two points where the voltage needs to be measured, which also serve as the power source for the circuit. Rather than connecting the circuit at the battery terminals, it is more advantageous to connect it at the motor connections.

#### Setting the Voltage Threshold and Resistance

The circuit is designed to work with nominal battery voltages ranging from 4.8 V to 9.6 V (equivalent to four to eight 1.2 V cells). For instance, if the battery consists of six cells, its nominal terminal voltage would be 7.2 V. An appropriate discharge threshold voltage is around 1 V per cell, meaning the threshold for six cells is 6 V. To determine the voltage UZ across the adjustable Zener diode D1 (an LM431), it should be set to approximately 0.5 V less than the desired threshold voltage at which LED D2 should illuminate.

The choice of resistor R1 controls this voltage. The circuit diagram suggests using a trimmer potentiometer (R1.A) in combination with a fixed resistor (R1.B) in series. Utilizing recommended values of 10 kΩ for both the potentiometer and the fixed resistor allows the discharge threshold voltage to be set within the range of about 5.5 V to 8 V. For lower or higher voltages, R1.B should be adjusted proportionally.

#### Circuit Operation and Hysteresis

Once the desired UZ value is established, the total resistance (R1.A plus R1.B) can be measured, and a single fixed-value resistor of the same value can be substituted for R1. In the example of a six-cell battery, when the battery is charged, a voltage of 7.2 V appears at the emitter of T1. At its base is UZ, which should be 5.5 V (6 V – 0.5 V) for a discharge threshold voltage of 6 V.

T1 will conduct as long as the battery voltage remains at least 0.5 V higher than UZ, causing T2 to block, resulting in LED D2 remaining unlit. Should the battery voltage drop below approximately 6 V (UZ + 0.5 V), T1 will block, T2 will conduct, and LED D2 will illuminate. To ensure the circuit operates stably, R6 introduces a small amount of switching hysteresis, with the ability to vary the amount of hysteresis by adjusting the resistor value between 100 kΩ and 220 kΩ.

#### Current Consumption and Additional Options

The circuit itself draws a current of less than 5 mA (measured with a battery voltage of 7.2 V). When the LED illuminates, an additional 10 mA (LED current) is consumed, totaling around 15 mA. The adjustable Zener diode can be substituted with a fixed Zener set 0.5 V lower than the desired threshold. This allows for the removal of resistors R1 and R2. If desired, a flashing LED can replace D2 (without the need for series resistor R7). For an acoustic alert, D2 and R7 can be replaced with a DC buzzer configured to the appropriate operating voltage.

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