450nick:
Thanks very much Paul! Here's what I can see on the diode... It looks like it says 6b... and a symbol.. Any way I can test it to find out?
"6b" does not mean much. Pin 9 is ground, you can put a variable supply on pin 7 through say, a 100 Ohm resistor and increase it while measuring the voltage on the diode. You will detect the point at which the voltage stops increasing which will tell you the Zener voltage. Alternatively, just supply 12 V to pin 7 via a 470 Ohm resistor and measure the diode.
Frankly, that circuit is very peculiar. 10 Ohms seems a very low value to bias a Zener and a 1k resistor in series wit the base of the transistor will severely limit its ability to provide a regulated voltage. That is why I suggested that it would not be appropriate to light more than the minimum three LEDs as the voltage will sag and the brightness vary with more LEDs in play.
450nick:
Are you sure I need a resistor for each LED? On the other board I tried 5v and nothing happened, tried 12v and the LEDs light up perfectly. There is resistance between the transistor the LED and ground so there must be resistance built in somewhere that I can't see, maybe in the wiring or the LEDs themselves? [EDIT] I just tested and this board seems to have 360 ohms between each LED and ground. The other board which uses 12v LEDs does not have any resistance on the LEDs, but the LEDs are all pairs of two in series. Does this mean that the other board will need resistors on the supply for each pair of LEDs?
You always need a resistor in series with the LEDs to control the current. There is really no such thing as a "12 V LED" as while you may have two or three in series to make a higher total conduction voltage, you will still need a resistor to limit the current.
These seem to be just the common variety of LEDs, so I doubt they have hidden resistors built in. I could be wrong!
450nick:
I was undecided whether for the fan speed you lit a number of LEDs depending on speed or just one to show current speed but yes then one for the heat setting and one if AC is on (I think that's what the big one is)
So does that mean then that the transistor emitter is putting out 5-6 volts and the rest of the board (switches and potentiometers) are running on 12?
I rather doubt the potentiometers will be running on 12 V as if they are connected to digital logic, the digital logic will be based on 5 V.
450nick:
If the LEDs are running on 5 volts could I just use the arduino pins to switch to ground? I'm using a teensy 3.2 just for this job so plenty of inputs and outputs available...
Yes, as long as the LED current is limited to 20 mA. I do find that circuit rather odd overall.