I was searching the web trying to see how to use a transistor as a switch for a < 1vdc trigger to completely turn off aquarium LEDs based on a 0 - 10 vdc signal from an aquarium controller. I found several sights, but this one was the most clear and provided a great explanation. I will certainly come back whenever I have electronic circuit questions.

How to I choose an NPN transistor for my application? Are there different specs I need to consider? I will have a 0 - 10 vdc signal on the base. I am considering a small potentiometer to have some control of the cutoff point. Is 10vdc too much for the base? The c - e side is a 5vdc PWM output. It just want it shut it completely off when I get somewhere < 1vdc on the input signal.

 

Yes, 10VDC is too high for base control considering that you only need 0 to 0.7V.
More importantly, you need to limit the current through the base-emitter junction otherwise you will blow the transistor. Putting a resistor of about 1k to 10kΩ in series between the control signal and the base will limit the current to a safe value.

 

Ah.. the old meanwell's do not turn off completely when dimmer voltage is as low as you can go issue.

Does your reef controller not have outlets? I simply plug my meanwells into my controller outlet box and turn off that relay/outlet when the lights should be off.