BJT as a switch using power supply current

Other Hardware General Electronics
1

Hi everyone,

Basically,i need to control a small speaker circuit (on/off behavior),and i was thinking of using an arduino (to control timings etc) with a bjt as a digital switch.
The voltage/currents are the followings:
-5V regulated power supply;
-Arduino Pro Mini 5V;

  • BJT [2n3904];
    -speaker circuit [3v, and it draws 100-120 mah under load];

Now, i can't supply the speaker circuit with enough current through an arduino digital pin.
So i was thinking of applying a voltage regulator from the main power supply to have a stable 3V,and use a BJT (controlled by the arduino digital pin) before the speaker circuit to open/close the 3V input.

So,how do i wire the circuit to control the BJT with the 5V digital arduino pin,to open/close the flow from the 3V input?

Or if you have others idea on how i should do it please feel free to share.

Thanks in advance.

2

BJT's aren't rated in milliampere hours.

3

speaker circuit [3v, and it draws 100-120 mah under load];

What does the speaker circuit consist of?

4

groundfungus:
What does the speaker circuit consist of?

I don't really know,the circuit i want to power is a toy [so basically i want to use my switch and power instead of the 2AA batteries so the toy turns on and off on command] inside there's a small PCB connected to the speaker,i don't really know which components are inside it.

I only know that it works at 3V [for the 2AA batteries] and that it draws around 100-120ma because i checked the draw with a multimeter.

5

I was thinking of doing something like this

Does it make sense?

6

2N3904 is NPN, 3V to toy+, toy- to C, E to GND, Arduino pin to 1k resistor, other end of resistor to B, also need a kickback diode in parallel with toy, cathode stripe toward +.

7

Thanks a lot,

Now,a dumb question...since i don't have any diode at home,could i use a led for the kickback?

8
  1. 2N3904 is an NPN... which means it is NOT well suited to switching the Positive voltage rail to the device being powered. Move the NPN transistor so it switches the GND path to the toy.
  2. An LED has very low forward and reverse voltage rating when compared to a proper diode
  3. Is the toy just a speaker? or is there a driver circuit on the board? If there is already a driver... no diode would be required. (my opinion)
9

Does the toy have a motor in it connected to the +/- that the transistor connects to? If not, a diode is likely not needed.

10

Thanks for the answers,

It had a motor connected to it,it's a small train,but i removed the wires that went to it because i just needed the sound of it going.

11

No diode needed then.

12

Perfect,so,just to be sure,this is the final circuit,does it seem ok?

13

Yes. Better. Though I would recommend the use of a proper symbol for components... makes interpretation much faster.

14

Yes. I would use AOI514 vs NPN transistor, will not lose 0.6 to 0.7V across the transistor for better battery speed.

15

Keep the transistor with 1k base resistor, and replace the regulator with two 1N4004 diodes.
2x diode drop and transistor saturation voltage is about 2volt. 3volt left for the toy.
Leo..

16

It's not going to be 0,6V. It's going to be around 0,3V. That's not to bad. If you have a small mosfet you can use that but if not, just go with the transistor you have :slight_smile:

17

4mA base current for 100-150mA collector current.
Could be more than .6volt saturation.
Leo..

18

I only found 1 datasheet that had CE currents above 100mA

Fig 17 show Vce at 150mA to be .45 to .5V requiring 15mA base current. And that parts is not available.
Will have to test and see what the actual voltage drop is. Using a 3.3V regulator to help overcome transistor-as-switch loss would probably help the toy's performance, vs presenting equivalent of dead AA battery voltage to it. Maybe even a little higher. What's the voltage on a fresh battery? Use ~twice that plus 0.5V.

19

This time i have to work with the components i got in house but thanks,i only know few basic ones so it's good to learn about new stuff.

Btw i'm powering everything with a wall power supply at 5v,i'll eventually adjust the resistor in the LM317 if i see that the output voltage is a bit lower than expected.

Thanks everyone for the help,appreciated.

20

3.3volt is all you'll ever get from the LM317 bacause of the high dropout voltage of this part.
~1.7volt@100-150mA.
Leo..