Show us a photograph of the actual real circuit. That is probably where your problem lies.

 

G5V1 series 'must operate' is 80%, 'must release' is 10%. Operation on 5v with a BC547 is marginal as Vce(sat) needs to be <0.2v.

Odd thing is that 6v version is 25mA/240ohm not as stated by TS.

Is it a true OMRON G5V-1-dc6 or a cheap Chinese copy?

 

Odd thing is that 6v version is 25mA/240ohm not as stated by TS.

The OP never specified what relay he was using. I just used a convenient relay component to show a more conventionally drawn schematic.

 

What is the collector-emitter voltage of the real circuit when the transistor is on?

 

The OP never specified what relay he was using. I just used a convenient relay component to show a more conventionally drawn schematic.

My bad, thats what happens when you're half asleep...

Mind you, 80% of rated volts isnt uncommon, so still might be marginal on 5v.

I'd swap the BJT for a decent logicFET, problem solved (maybe).

 

Mind you, 80% of rated volts isnt uncommon, so still might be marginal on 5v.

I've used 5V relays that would pick up at about 3V.

 

Your schematic drawing style is very strange, and the simulator is crap. Are the meter widgets ideal?

View attachment 268290

You're operating a 6V relay at 5V. What's the pick-up voltage?

I agree with that. 4 mA of base current and the transistor has a Vce of 1.2 volts.

 

Even if the transistor is a dud from ebay or it has its emitter and collector pins swapped, the measurements show a Vce of 1.2V and a collector current of 38mA which make it warm (not hot) at 1.2V x 38mA= only 45.6mW. Its heating is allowed to be 13.7 times hotter at 625mW.

 

yes. I replaced the transistor but at 5mA base current ( which means the transistor is on ) it don't work properly.

it seemed your transistor is on through out. Bipolar transistors turns on for very little base currents (mA, uA) where your Vbe > about 0.6v. To switch it off, the base voltage would need to be really low, closer to GND.
And you may be right about a possible faulty transistor. But before replacing it again, try connecting the base to GND (0v) to see if you can switch it off. That'd at least give you a feel if the transistor is after all working.

Adding a resistor at the emittor (to GND) would help limit the currents in the transistor Vce and the relay. This would prevent straight out burnout of the transistor. But if that resistor is too large, the currents may be too small to operate your relay. Hence, choose an appropriate resistor if you want to place an emittor resistor.I think the emitter resistor may incidentally raise your base on voltages, so that it is easier to switch it off.

and at the MCU side, check if after all some pull up configuration is done or that it is part of the mcu's gpio. that would be really bad if the pull up can't be turned off as your transistor would be always on. Some MCUs can configure pull up resistors at the gpio pins, among them stm32 has that, but that pull-up resistor can be configured as enabled or disabled. I used that (configured internal pull up) for a few times for being lazy to place an external pull up resistor. and the gpio pin should be configure as push-pull and not 'open collector/drain', i.e. if that pin is 0 (low), it should be shorted to gnd