INDEX

The relays mostly available are of 12V,196 ohm relays, if you use D880 transistor for driving it then remember the resistance at the base of the transistor should be around 1Kohm. I will explain this in Transistor section briefly You can hear a sound when the relay got activated.
Checking a relay circuit:
1. First check the relay is good and test whether your relay work with the Vcc you use. So first you connect Vcc and gnd between two ends of the relay. If it is activated you can hear sound. If not see the voltage rating of the relay and increase voltage. This is the most problem occurring with relays.For a 6V,100ohm relay it required 6.86V to make it work. If Vcc=5v then u can hear a small sound that means that magnetization is not enough.
2. See the connections properly because on the other side of the relay you might be using 230V, so be careful when you touching the relay.
3. See the voltage of the other circuits and sensors when you connect relay(whether they are getting proper voltages).
4. Remember to put the protection diode
5. Touch the heat sink of the transistor to see if the transistor is getting heated or any faults.
6. See the value of the resistor connected in the base of the transistor. I will explain about it in Transistor section.

Here it is a small relay representation (a diagram of relay i have). The other side of the relay can be 230V or even 5V (no restriction), but we normally  get 230V relay, means voltage<=230 (on the 230V side). You can use this in the last stage of a line follower(assume that line follower has more than 2 LDR's), when last sensor go out of the line, you can use relay mechanism to provide Vcc to the other lead of the motor so that motor start to rotate backward.
NOTE:
The switching speed (around 100ms)of relay limits its application in high power circuits(so SCR and Triac came).