Current availability indicator

There is a need to track the presence of a current flowing in a circuit in two states: either it is or it is not. Example: you are charging a battery with a built-in charging controller, plugged into a power source, and how to control the process? You can of course include an ammeter in the circuit, you say, and you will be right. But you will not do it all the time. It is easier to integrate a charge flow indicator into the power supply once, which will indicate whether the current is flowing into the battery or not.
Another example. Suppose there is some kind of incandescent lamp in a car that you do not see and do not know if it is on or has burned out. In the circuit to this lamp, you can also turn on the current indicator and monitor the flow. If the lamp fails, it will be immediately visible.
Or there is a certain sensor with a filament. Tapa gas or oxygen sensor. And you need to know for sure that the filament has not broken and everything is working properly. Here the indicator will come to the aid, the scheme of which I will give below.
There can be a lot of applications, of course, the basic idea is one - control of current availability.

Current indicator circuit


Current availability indicator

The scheme is very simple. Resistor with an asterisk is selected depending on the controlled current, it can be from 0.4 to 10 ohms. To charge the lithium ion battery, I took 4.7 ohms. A current flows through this resistor (if it flows), according to Ohm’s law, a voltage is released on it, which opens the transistor. As a result, the LED lights up indicating the charging going on. As soon as the battery is charged, the internal controller will disconnect the battery, the current in the circuit will be gone. The transistor will close and the LED will turn off, thus making it clear that charging is complete.
Diode VD1 limits the voltage to 0.6 V. You can take any, for a current of 1 A. Again, it all depends on your load. But you cannot take a Schottky diode, because it has too little drop - the transistor can simply not open from 0.4 V. Through such a circuit, you can even charge car batteries, the main diode can be selected with a current higher than the current of the desired charge.
Current availability indicator

In this example, the LED turns on during the current flow, and if you want to show when there is no current? In this case, there is a scheme with reverse logic of work.
Current availability indicator

All the same, only an inverting key is added to one transistor of the same brand. By the way, the transistor of any of the same structure. Suitable domestic analogues - KT315, KT3102.
In parallel with the resistor with the LED, you can turn on the buzzer, and when there is no current when we control, say, a light bulb, a beep will sound. What will be very convenient, and do not have to display the LED is not a control panel.
In general, there can be many ideas where to use this indicator.

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Current availability indicator

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