I want to operate relay using iot. Relay operates on 5v but the voltage across GPIO pin is 3.3v . So how can i operate 5v relay using bolt iot module?
I am sure you are not using a dedicated relay module. To trigger a relay using a lower voltage you have to use a transistor generally a npn transistor like bc546 and a dedicated power supply like a battery. A transistor has three pins
connect the positive terminal of battery with one of the input pins of relay, then connect the other pin of relay with collector of transistor, then connect both the emitter and the negative terminal of the battery to the ground of bolt module.
Now you left with the base of the transistor which you can connect to any of the digital pin of the bolt module in series with a 10k resistor. Whenever base of the transistor goes high the circuit gets completed and the relay gets triggered.
But this circuit is not a good circuit to trigger a relay, as relay has a coil inside it which can store energy and this stored energy can burn your transistor and may be the bolt module because of fly back effect. So to deal with this you have connect a diode (a in1470 diode can work) in parallel to the relay input pins.
**After all this your circuit should look like this ** ---------> https://electronics.stackexchange.com/questions/145504/triggering-a-12-v-relay-with-an-arduino
You can choose a relay that can work with low voltage/low current. Your best bet would be a “solid state relay”, since these do not require a coil to be energized and may work on low voltage/current. Check the specs to find one that fits your needs.
The best way to go about this, is to use a relay module instead of using a standalone relay.
Here is a link to the some relay modules available online.
Here is a link which shows you how to use a relay module with the Bolt WiFi module.