This circuit is to illustrate how to detect a lower voltage in a voltage source. As example here, I am using a nominal 9V source. Although I have seen lots of similar circuits like that, none of them have a good explanation on how it works. So, I will try.
The circuit is composed of, from the left to the right skipping the voltage source, 3 segments
- a voltage dividir. The 100K resister in combination with the 10k forms a voltage dividir that will be applied to the first transistor in the next Section
- A transistor driven by the voltage divider.
- A led and a transistor that will be driven by the current flow when the transistor 1 is not conducting.
If you play with the voltage, you will see that the voltage divider will have a ration of 0.091 of the source voltage. This is given by the formula for voltage divider. Which is
Vout = (Vin*R2)/(R1+R2)
So, Vout will 0.091 of Vin. That will establish at which voltage threshold the first transistor will keep conducting enough current to keep the LED off. For this example is around 7.5. At 7.5 Volts, the amount of current passing through the LED is not yet enough to get it a bright lit. But, it will start to present some light at 7.4. If you keep decreasing the voltage, you will see that the LED is coming to life and it will reach the maximum current around 6.66 V. But that will still be a 11.3 mA. So, it doesnt even require a resistor to limit the current to the LED.
This circuit is best illustrated in speed of 171 ms/s or higher. Some examples out there also shows a capacitor between the output of the voltage divider and ground. I decided to take it off for illustration purposes but it would be a good idea in a final product
This circuit while in high voltage of the battery will consume very little current. Around 270 uA. But, it will start to use more current when it the LED starts to lit. That will make sense for its use once it is supposed to only show that the battery need to be changed.
To get the best of it, you will need to play with the 47K resistor. That is the one that will prevent the circuit to use lots of current when the LED circuit is off. But, it cannot be too big tp the point that will prevent the LED to lit when it needs to drive the LED transistor
If you want to deal with different voltage ranges, you will need to play with the voltage divider. That is what will tell the circuit which transistor to drive. For smaller voltages, you will need bigger voltage ratios.
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