fetswitches
I have an audio amplifier that draws about 200-300mA max that I would like to switch on/off using either an I/O line or a the switch pin from the headphone connector. I was thinking about using an npn transistor or an N-type FET but I've never really done anything like that before so I was hoping for some tips.
The I/O is from an ATTINY85 and the amplifier I'm switching is an LM386
Neat idea. I just finished building a Bluetooth controlled Canon SLR remote, but it's controlled from the phone, not by the phone.
I think you can use this idea to drive the input to a MOSFET (a 2N7000 should do). When the signal from your phone drives the opamp high, the MOSFET will act like a switch, shorting your shutter release pin to the camera/circuit ground. If it turns out that the signal is too short in duration for the shutter to fire, just use a 555 in monostable operation in between your phone and the MOSFET, I'll bet the audio signal will work as the trigger input. If it doesn't, use both ideas together, have the square wave from the opamp trigger the 555 timer.
To return to your transistor solution (as others have opted for a different mechanical switch), it might look like this. I included Wouter's hint for using a diode instead of the second transistor. Also I changed the bipolar transistor to a MOSFET. Reason for a MOSFET is because the base current of a bipolar transistor gets rather high with regular power transistors. Reason for P-channel high side switching is that this way the load can be attached to ground. If this is not a requirement, the circuit can be slightly changed to support an N-channel MOSFET.
simulate this circuit – Schematic created using CircuitLab
Best Answer
My preference is to switch the high side.
simulate this circuit – Schematic created using CircuitLab
Look for a P channel MOSFET with a low \$V_{th}\$. The NPN transistor could be substituted with a logic level N channel FET if desired.