Transistor Help


Wondering if someone can chime in with some advice on a circuit I'm trying to design for my Arduino Mega to do some controlling & monitoring a battery source. Below is the schematic I currently have, but I'm having an issue with the transistor. The current circuit experiences a voltage drop over the transistor which makes perfect sense to me given the setup. I understand that I would need to use a high-side switch to get this to work, but then I have trouble with the analog input (A6) being always on which I don't want.

Anyways, here's the goal...

  • Camera batteries remain completely off until SWITCH2 is activated (preserving batteries)
  • Power to the cameras is being supplied via the two USB outputs
  • When SWITCH2 turns on CAMERA1 power, it should also activate CAMERA2 power, which uses a separate battery source, via transistor switch.
  • Both battery sources are being monitored using A6 and CPI-1 so these should have 0 voltage when SWITCH2 is off.
  • Side note: Do you think think the diode's are necessary? I was worried about power from the Arduino trying to "charge" the camera battery sources if/when the battery sources were depleted, which I do not want.

Thoughts? Thanks in advance! This is also my first time using Eagle CAD, so I'm sure the sketch could be better. :)


For further clarification... here was my second attempt at the circuit. This gives the camera the correct voltage, but then I don't know how to add in my A6 monitoring so it too gets the correct voltage, but only when SWITCH2 is activated.


The Top of R2 needs to connect to CPI-1, or to the other side of Diode1. R2 & R3 are kind of low, they will draw a lot of current if you're running on battery: I = V/(R2 + R3), with 7.5V for example that results in 7.5V(250 ohm) = 30mA, with A6 = 7.5V x 100/(150+100) = 3V. A6 only needs 1uA, so you could multiply R2 & R3 each by 5 to 10 and reduce current flow a lot.

CPI-1 is monitoring Camera Battery #1’s voltage. R2 is a voltage divider for Camera Battery #2 so R2 and CPI-1 would not connect. I’ve got to connect R2 to Camera Battery #2’s transistor somehow and still get the actual Camera Battery #2 voltage.

In terms of the resistor values, I’ve heard this before, but was never really clear on this so thanks for the equation, that clears that up nicely. You think if I used 1.5K and 1K resistors instead that would work better?

Here’s a quick edit on the R2/CPI-1 clarification I was referring to. CPI-1 is relabeled ALARM-OUTPUT-CIRCUIT to eliminate confusion and I moved the monitoring circuit into this section so you can see that it is heading to A1 like I mentioned. I also upped the resistors to 15K and 10K.