I need a small size solution e.g. resistor and small transistor to switch on a tiny 0.11 amp 5V solenoid. Grateful for any suggestions. (NB I don't want to use a relay - dimensions of relay too big for space I have!) Many thanks Andrew
So... Why not a resistor and a small transistor?
well thanks for quick reply but hmmm I need advice on which transistor - can't be physically big Can you advise? or MOSFET? I would hugely value any suggestion for an actual solution that can happily switch 5V 0.11amp. I appreciate this may seem a trivial question to some - but (as I have discovered) getting the right transistor is not simple (for me anyway!)
5V / 110 mA is not a big deal, there are plenty of transistors that can easily handle such a load in full saturation, even BJTs.
What do you mean by "can't be physically big"? Do you want a SMD transistor? Or a thru hole one like in a TO-92 case?
You're right, choosing the right transistor is not an easy thing! But the specs you need are not that huge, almost any transistor would be able to do it. Just make sure that the max collector current is at least 150 mA and the VCE at saturation is low and everything should be fine! Don't forget the freewheeling diode though.
Something like this should work. The 10K resistor is optional - it ensures the transistor is off and doesn't pick up stray signals while the output pin is floating during power up of the Arduino.
10k isn't really needed with a BJT, but is very important if you use a MOSFET, so use it and the circuit will work with either NPN or logic-level n-channel.
1N4148 diode will work fine for the free-wheel diode at 110mA, and its tiny!
If you are driving several of those solenoids, there are driver chips made just for that purpose. The ULN2003 has 7 drivers in a 16 pin DIP (SMD I don't know). Includes base resistors, and flyback diodes with Darlington drivers. There may be more modern parts but these I know.
Many thanks to all
and also thanks for advice about diode - I would have forgotten that - so I am on my way! I need something very very light as being lifted by helium.
Best wishes Andrew
I usually use a 2N7000 for this. With just a 100k from gate to ground. Leo..