Here is a very simple constant current limitter circuit (I never seen before) which can detect the load and indicates by the LED, a clever trick. The LED glows only if the load is connected. So if we used the circuit as a constant current charger then the LED performs as a 'charging is ON' indicator. The circuit has tricky bias so the LED is working there as a reference voltage (Vref) regulator and also as a load indicator. And I made it possible just using four cheap components.
Connecting the load:
The forward voltage of the general red LED with 5V through 1k resistor is around 1.8V. The LED is working as a 1.8V zener diode which regulates the base voltage. So the emitter voltage is around 1.1V (1.8V-0.7V). Now we get the limited current: 1.1V/22 ohms= 50mA. The LED is not a precise voltage regulator so varying the supply voltage would vary the limited output current level too. The LED glows with around 3mA current, indicating that the load is connected and working too.
Disconnecting the load:
When there is no load (open collector) then the main flow of the current changes the way. The emitter steals the base current and due to the current flow through +5V> 1k> BE diode> 22 ohms> Gnd, the emitter voltage decreases to around 92mV [Ve= (5V-0.7V)/(1k+22 ohms)=0.0042A*22 ohms= 92mV]. Then the base voltage also decreases to around 0.8V (Vb= Ve+0.7V), which is not enough to power the LED. So it goes OFF, indicating that the load is disconnected and not working.
I am sure that the circuit is not limitted to just a single current limitter. I think many advanced devices are being controlled utilizing such tricks. So I feel better to say that the circuit is an add-on.
The simplest constant current limiter with 'load indication'
A very simple constant current limiter circuit with a tricky load indicator