A MOSFET is preferred.

Also a BC547 is only rated for 100mA, which is not enough to drive an automotive 12V relay.

Do you have any other transistors?

I have BC337 at hand. I will drive a 12V relay with code SARM-S-112D rated for 15A and 14VDC. It is not exactly an automotive relay, I retrieved it from a remote lock control module.

 

The mosfet is still preferred.
That relay only draws 67ma
Do you have any mosfets?

 

The mosfet is still preferred.
That relay only draws 67ma
Do you have any mosfets?

I have never used a mosfet before and I am keenly interested in using one. Unfortunately I can only get it on Monday due to lack of means in my city.

 

It can be done using a BC337 wired as a voltage follower.
The specs on the relay indicate it will activate at approx 9volts.
Don't have access right now to draw up the circuit, maybe Cruts can help.

 

Here's the LTspice sim of a circuit using the BC337:
I had to add a diode to get the ON and OFF delays close to the same.
May have to tweak the resistor values to get the desired delay times.

 

Another alternative is using two BC337 transistors configured in a darlington configuration.

 

Here's the LTspice sim of a circuit using the BC337:
I had to add a diode to get the ON and OFF delays close to the same.
May have to tweak the resistor values to get the desired delay times.

View attachment 328003

Thank you for the detailed explanation. It is fascinating to get help from you all. I will try to build it tomorrow and share the results.

 

Here's the version using the darlington configuration. Much smaller timing capacitor but does require two transistors.
Resistor values may need to be adjusted as well.

 

Here's the version using the darlington configuration.

The possible problem with that is, the three diode voltage drop from diode and Darlington base-emitter junctions may not be sufficient to turn on the relay.
If you want to use two transistors it would be better to use a Sziklai pair, (AKA complementary Darlington) which has a drop of basically one base-emitter value, plus the saturation voltage of the other.

 

With a 12.5 volt supply the output was at appx 10.5 volts on the breadboard with a 180 ohm load. Should be more than enough to operate the relay that was listed. The circuit was designed around the fact the TS had those BC337 transistors on hand was all.

 

Much smaller timing capacitor but does require two transistors.

Not sure that's a good tradeoff.

 

Bottom line they work the same.

 

With a 12.5 volt supply the output was at appx 10.5 volts on the breadboard with a 180 ohm load.

Is that with the Darlington stage?

 

Is that with the Darlington stage?

Correct.

 

Hi to all, I just tried the first version you suggested and relay opens and closes instantly. Any suggestion on what I might be doing wrong? I even tried to change the capacitor value but to no avail.

 

Why are there 2 capacitors on the breadboard.
Can't see your hand drawn schematic.
Are you using a BS170?

 

If those capacitors are in series, then the total capacitance is less than the capacitance of the smallest capacitor.
Capacitors need to be in parallel for their capacitances to add.

 

Why are there 2 capacitors on the breadboard.
Can't see your hand drawn schematic.

I tried to change the capacitor value and used two in serial to have a value I don't currently have. I tested the circuit both with 1N4007 and 1N4148 diodes. I double checked the pins of mosfet. My diagram is this.

 

If those capacitors are in series, then the total capacitance is less than the capacitance of the smallest capacitor.
Capacitors need to be in parallel for their capacitances to add.

Thank you for the clarification. I tried different ones as singles then added two at a time both in serial and parallel just in case.

 

The schematic is correct but the mosfet doesn't look installed correctly on the breadboard.