Help with work with transistors as a switch.

[Fiddy]

G'day,

So i bought one of these dummy cameras that pan when they sense motion.

What i want to happen is:

Once motion is detected, the pan motor turns on and starts panning the camera, at the same time i want a 445nm laser with a vertical line generator lens to 'scan' the room, the camera will stop panning after 20seconds or so and i want the laser to turn off also.

So i thought id use a NPN transistor and tap off the motor that pans the camera, you follow me?

This is what ive come up with.

Would that work as intended? what transistor should i look at for running a ka$io diode at about 1W?

Any help would be appreciated

-Fiddy.

 

[Things]

What voltage is the motor operated at? Keep in mind that if it pans back across the room, side to side, then it's probably being driven by a H-bridge, thus the line you've drawn as positive on the motor won't always be positive.

 

[Fiddy]

it runs from 3xAAA's so ~4.5V nah its too cheap for that, it just runs 1 speed one way, the gearbox sorts out the panning action.

 

[djQUAN]

can you take a hi-res picture of the board that controls the panning motor?

 

[Fiddy]

i can but whats the point of that? it just runs in one direction.

 

[djQUAN]

So we can see how the motor is switched. If you're lucky, we could even run the motor off the same battery as the laser diode.

 

[Fiddy]

its just a small transistor that turns on the motor

 

[Things]

Anyway yeah, your diagram will work. You could use a MOSFET and you probably wouldn't even need the base resistor.

 

[Fiddy]

interesting, mosfet ay? theyd be better for fully on or fully off ye?

 

[djQUAN]

what's the part number on the transistor that switches the motor?

I'd be careful of the mosfet. Since the motor is inductive and it might produce voltage spikes at turn off, which could destroy mosfet gates. The motor controller should have a reverse biased diode connected across the motor to absorb the spike.

 

[Fiddy]

i doubt it would have anything like that, its really cheap. the motor is small and drives the gearbox via a small rubber band/belt

 

[djQUAN]

your circuit in your first post would work but you have to use a different resistor (50ohm is too low) how much current/laser power do you intend to use?

 

[Fiddy]

ill be going for a boost driver so around 1.6A input current.

 

[Fiddy]

might be better off just putting this 5V guy across the motor:

PCB Mount Solid State Relay 12V - Jaycar Electronics

 

[djQUAN]

Ow. that's a little higher than a transistor can handle with minimal losses. As Things mentioned, you can use a mosfet which is better at high currents but you'll need a logic level mosfet due to the low voltage (4.5V) to turn it on.

something like IRL2703PBF should work and use a gate resistor about 4.7kohm just to protect from spikes from the motor.

Edit: yes a relay can also work but keep in mind a solid state relay has more losses/voltage drop compared to a mechanical one.