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Simple battery voltage detector

by:dcfpower     2021-04-03
People often encounter many devices that use batteries. When the battery voltage gradually drops to a certain threshold, it should issue instructions to remind people to replace the battery or charge it quickly. For example, in mobile phones, there is a battery-like symbol on the screen, which displays the battery voltage at any time, which can remind people to charge in time. However, many devices do not have such devices, such as radios, rechargeable hair clippers and so on. In fact, this battery voltage detector can be self-made. Here are five circuits: u003cpu003eu003c/pu003e Figure 1 is a circuit made of transistors, which can be used to detect 3V to 15V battery-powered equipment. Q1 and Q2 two tubes form a two-stage inverting amplifier. Adjust VRl to make the circuit light up the LED at a preset threshold. For example, when testing a 9V battery, you should set it to 8.2V to give a warning (that is, the LED is lit). Since the two-stage inverter amplifier is basically linear, the battery LED lights are on between 8.2 and 8.0V, but the brightness is different (the current flowing through the LED is between 3mA and 15mA). The effect of this circuit made with discrete components is not ideal. The model used in Q1 and Q2 is BC547, which can be changed to 9014. u003cpu003eu003c/pu003e Figure 2 is a circuit dedicated to detecting 9V batteries. Its main component is a thyristor (SCR), and the entire circuit is basically a bridge circuit structure. The voltage of the Zener tube ZDl is fixed at 8.2V. When the 9V battery voltage is normal, point B is higher than 8.2V, that is, the control electrode voltage of SCR1 is higher than the cathode, and the thyristor is triggered to turn on, so the LED lights up immediately. Once the battery voltage drops to 8.2V, the thyristor turns off and the LED goes out, indicating that the battery has undervoltage. u003cpu003eu003c/pu003e Figure 3 is a circuit specifically used to detect 1.25V Ni-Cd batteries. The nickel-cadmium battery is charged by the solar cell. If it encounters continuous cloudy days, the battery charging current will decrease and the battery voltage will be lower than 1.25V. At this time, the integrated block LM3909 will be triggered and the LED will be lit. It means that the nickel-cadmium battery is undervoltage. Note: The integrated block LM3909 is a low-voltage power supply integrated block. u003cpu003eu003c/pu003e Figure 4 is a circuit made of integrated block 8211. This circuit can be used to detect batteries powered by 5V to 25V, adjust VRl to make the circuit light up the LED at a preset threshold, similar to Figure 1. For example, when testing a 9V battery, you should adjust it to 8.2V and let the LED light up (that is, give a warning). Since this circuit is made of integrated blocks, of course the effect is very good. u003cpu003eu003c/pu003e Figure 5 shows the internal structure of the integrated block 8211. It can be seen that it is mainly composed of a 1.15V reference power supply and a comparator. When the battery voltage is lower than a certain value, the circuit flips to turn the LED on and light up . In addition, because the integrated block 8211 has a wide operating voltage range, a constant current power supply is used as its power supply system. At the same time, constant current power supply is also required to keep the LED brightness constant. Therefore, a constant current source circuit is specially provided inside the integrated block. . u003c/pu003e
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