Relay should work , but the LED, not so much… The LED in that cct is the wrong way around.

Transistor is wired OK – it is an NPN, so diagram is correct.

Also, the resistor from the I/O pin ( 13) to the base is very low in ohm – The poor arduino has to provide around 10mA to drive the base( if the arduino is a 3.3v job, else 20mA if its 5v)  – 2k2 ohm is quite adequate.

So, I am dismayed that the led flashes in this circuit – it cannot when reverse biased…

Likewise dismayed that the relay does not activate, it should..unless the relay needs a higher voltage. What voltage does the relay coil require? Does it activate when you apply your 9v across the coil? If so..

Correct the LED connection direction

Remove the resistor from the arduino pin 13 and momentarily connect it to the arduino 3.3v or 5v supply.

The relay should activate. If it does not, did the LED come on?

If not, the voltage on the collector should be 9v with the base resistor unconnected. When connected to 3.3v/5v the collector voltage should be close to 0v. if not, transistor is dud..

I re-drew the circuit to make sure it’s OK, and got this:

This is correct except:

• make sure the LED is the right way round.  If it flashes, it probably is!
• check the transistor connections.  James shows EBC, I think a BC545 is CBE

Transistor is a bit small.  Max rating of the BC545 is 100mA.   Wasn’t able to find a blue miniature relay of the Arduino sort and measure it , but found a web source saying the 12V version has a 54ohm coil, which is 160mA at 9V.   Probably OK if the relay doesn’t stay on for long.

Test the circuit without the relay and make sure the LED flashes correctly.   Then connect the relay.   If the relay doesn’t stir, it may be there isn’t enough energy in the time it takes to flash the LED to power the magnet.   Try a longer flash!  A few things may combine to reduce the energy put into the relay’s electromagnet:

• Running a 12V relay at 9V is probably OK, but marginal
• A 9V battery may not be able to provide enough current. A PP9 has more beef than a PP3.  Also, the battery technology matters – a cheap Zinc Carbon PP3 produces much less current than a pricey Alkaline PP9.     If necessary, put two or more batteries in parallel.
• Pulse length too short    (I’d start with this one!)

Never forget Sod’s Law: if it can go wrong it will, and in the worst possible way.   The relay could be faulty…

Dave

If you can find some 12v,you can get little boards with 4 relays each with it’s own drive circuitry and freewheel diodes designed to be driven off Arduino or similar

You could buy a ready made relay board. This is one I have used from CPC.

https://cpc.farnell.com/whadda/wpm400/4-channel-relay-module-for-arduino/dp/SC17702?st=relay%20board

It also shows the circuit diagram on the website.

or these 2 from Hobby Components:

https://hobbycomponents.com/relays/75-4-channel-5v-relay-module

https://hobbycomponents.com/our-brand-exclusives/1090-mlink-4-channel-i2c-parallel-relay-module

They don’t charge much for delivery and usually arrive next day.

Stuart

Having read on the other thread that the object is to drive an electromechanical clock I’d echo someone else’s suggestion to ditch the relays and use logic level fets to drive the clock coils direct.

Just to be pedantic (who me) a relay is not a solenoid. Solenoid has a hollow coil and the iron bit is pulled down the hole.

Given that the opening post included Solenoids … This is worth a look:

https://youtu.be/nwVRMU9grSI?feature=shared

MichaelG.

.

Edit: __ The pace is a bit laboured, but none the worse for that … his third circuit is the good one.

.

The problem with testing with a 9V battery is that it will likely run down quickly and start to cause other problems, possibly resetting the Arduino for example, which can be difficult to find unless you have a ‘scope.  This happens because the supply voltage dips when the current switches on, and the regulator on the Arduino board can’t handle it.

Much better to use a proper PSU from the get-go.  It’s amazing how many people ask about problems using motors, relays etc on Arduino support who are using such a battery.

That first picture is what Duncan calls a solenoid.  The second picture might be an electromagnet, but neither is a relay.  See https://en.wikipedia.org/wiki/Relay#:~:text=A%20relay%20allows%20circuits%20to,of%20controlling%20industrial%20electronic%20systems. for lots of pictures of different types of relays.

DataSheet for the MOSFET mentioned in the video:

https://protostack.com.au/download/FQP30N06L.pdf

Possibly overkill … but they are cheap enough.

MichaelG.