Add push button switch to TIP120


I have a simply circuit for controlling a solenoid valve (the valve connects to the RCA socket). Can I add an on/switch as per the picture or will this cause problems for the TIP120?


Not a problem to do that, as long as the switch can handle the current of the solenoid.

If you have/want to use a small switch, you could also use a second 2k2 resistor on the base of the transistor.
And connect the switch between 5volt and that resistor.

I was expecting the switch in another position: between D9 and the TIP120 base... but it really depends on the switch use: if you want the switch to turn the solenoid constantly on then your placing is correct, but you will practically exclude the TIP120 which will have no effect any more.

If that is the intended purpose I wouls use a DPDT switch to effectively disconnect one circuit in favor of the other: in one position would be ON, on the other would be AUTO (or arduino controlled). If you need a third OFF position then you might go for an ON-OFF-ON dual pole switch, quite common and cheap on ebay.

If you switch on the TIP120 base rather than on the ground you will not need the switch to handle the solenoid current and you can use smaller switches, but you will have to go for DPDT or it will won't work.

Thanks for the replies.

The valve is normally controlled by an Arduino and the switch is to allow manual draining for cleaning.
I do have a drain function in the software but it is proving inconvenient to use. A physical switch will makes things much easier.