user10040: I've created a circuit to turn on bulb using GSM module and Arduino. I want a mechanism(or programming) to know whether the bulb is on or off or fused. I've tried taking the input from the pins. But it only tells whether the pin is on or off but not the circuit as a whole. 

user349: This can be done using a few methods.  The simplest and least expensive option is to measure the current consumption of the device you want to verify correct operation of, and see if it's using enough, but not too much, current.
For instance, the following would work well in the arduino case of using an LED:

simulate this circuit – Schematic created using CircuitLab
Connect the Power to 5V or to the pin you're controlling the LED with. Connect the Sense to an analog input such as A0.
If the LED isn't installed, or is broken as an open circuit, the sense would register 5V.  If the LED is broken as a shorted element, the sense would register 0v. If the LED is operating correctly, you would sense the voltage drop of the LED (which varies according to the LED's design), but it would be well above 0V and well below 5V.
For bulbs and other loads, use a much smaller resister for R1 (so the resistor doesn't affect the load as much) and it would be considered a current shunt.  Knowing the resister value, and the voltage at the Sense location will give you a current value using Current = Voltage / Resistance. Again, the load should be drawing current within an acceptable range during proper operation.  If it's not drawing enough current, or too much current, it may not be operating correctly.
This won't tell you if the LED is on, however it's a very reliable method to discover if it's working correctly, and you'll probably never run into a situation where the correct LED is installed and in the right current range, but not emitting light.

10040: <p>I've created a circuit to turn on bulb using GSM module and Arduino. I want a mechanism(or programming) to know whether the bulb is on or off or fused. I've tried taking the input from the pins. But it only tells whether the pin is on or off but not the circuit as a whole. </p>
 349: <p>This can be done using a few methods.  The simplest and least expensive option is to measure the current consumption of the device you want to verify correct operation of, and see if it's using enough, but not too much, current.</p>

<p>For instance, the following would work well in the arduino case of using an LED:</p>



<p><img src="https://i.stack.imgur.com/7ySWi.png" alt="schematic"></p>



<p><sup><a href="/plugins/schematics?image=http%3a%2f%2fi.stack.imgur.com%2f7ySWi.png">simulate this circuit</a> &ndash; Schematic created using <a href="https://www.circuitlab.com/" rel="nofollow">CircuitLab</a></sup></p>

<p>Connect the <code>Power</code> to 5V or to the pin you're controlling the LED with. Connect the <code>Sense</code> to an analog input such as A0.</p>

<p>If the LED isn't installed, or is broken as an open circuit, the sense would register 5V.  If the LED is broken as a shorted element, the sense would register 0v. If the LED is operating correctly, you would sense the voltage drop of the LED (which varies according to the LED's design), but it would be well above 0V and well below 5V.</p>

<p>For bulbs and other loads, use a much smaller resister for R1 (so the resistor doesn't affect the load as much) and it would be considered a current shunt.  Knowing the resister value, and the voltage at the <code>Sense</code> location will give you a current value using <code>Current = Voltage / Resistance</code>. Again, the load should be drawing current within an acceptable range during proper operation.  If it's not drawing enough current, or too much current, it may not be operating correctly.</p>

<p>This won't tell you if the LED is on, however it's a very reliable method to discover if it's working correctly, and you'll probably never run into a situation where the correct LED is installed and in the right current range, but not emitting light.</p>
