Circuit Validation - 6.5V Air pumps using NPN Transistors


I have this built on my breadboard and seems to be working but I’d love to get some quick validation on the setup to ensure I’m not missing something.

In short I want to drive X number of Air pumps where each air pump needs 3.5-6.5V. I need to change the voltage of the air pumps from 3.5 - 6.5V (max load is 270ma). I’m using an external power supply (9v DC wall adapter rated at 1A). The DC adapter is connected to the board and lights up the rail for the motors. It is also connected to a linear voltage regulator to drop the voltage to 5v to power the Particle. I’m using 9V b/c the regulator requires a min of 2.2V over 5V to get 5V out. With my breadboard if I drive lower than 9v the Particle doesn’t power on b/c of the voltage drop from the regulator to the VIN pin.

My main questions:

  1. Is this the proper way to power the motors and Particle? I have a capacitor on both sides of the regulator to clean up the power.

  2. Is the circuit for the motors correct and is it missing anything? I drive a PWM signal to the base of the transistors. That PWM controls how fast the pumps operate by opening up the gate to allow the higher 9V to flow through the transistor. I have a capacitor across the motor to soften the PWM signal and a diode to prevent inductive flyback when the motor is turned off.

  3. When I do the PCB design I end up with ratsnest lines that I can’t get rid of… If I remove the ratsnest lines it removes the power wires from the 9v rail to the motors.

I’m still a noob and trying to understand if this is created correctly and if its missing anything that should be included or changed.

Also, where/how would one find an EE (for pay) to help finalize this design?


Here is my PCB… Again total noob so not even sure I’m doing this right. There are ratsnest lines on M3, C2, and C1. If I remove them, my power wires from the 9v rail are deleted from the breadboard view. I didnt know if this was an indication I have something wrong or it just doesn’t know how to handle this type of circuit.


A couple of quick observations on the circuit from experience (I am not a EE).

  1. You might want to add a resistor and capacitor to help debounce the buttons. Many examples available if you search for hardware debounce buttons/switches.
  2. When you use a 9VDC from AC power supply this will be a switched mode power supply and will depending upon the quality of the device produce a very variable output (lots of ripple). If you search on the community for power supply designs you see suggestions for more ceramic capacitors of different values 1nF, 100nF to help filter the high frequency ripple. Using a zener diode to remove possible voltage spikes over 5V. This is all to protect the Photon!
    3.Linear voltage regulators waste a lot energy as heat - if low power isn’t an issue then you are fine.
    4.Fritzing ratsnest - suggest you check and recheck - likely you moved something and it didn’t get updated correctly.

  1. I’ll check on the debouncing… thanks for that!
  2. I still need to do that with a wall wart style wall adapter? So the 9v DC coming out of the wall wart is “dirty” and needs to be cleaned up? Just want to understand the concept here.
  3. Yeah I think I’m ok here… again… noob so not 100% sure.
  4. Yeah, trying to figure out what in the world I’m doing wrong… I’ve recreated it a few times now and still get the ratsnest lines… even though IRL I have this working on my breadboard… so I must be missing some concept.


From my reading, could I just replace my linear regulator with this: to clean up the 9v DC to 5v without anything extra @armor ?


Got my ratsnest issue resolved… dumb error on my part…had vcc gong to wrong pins on motors… #noob


You could but then you really should have a filter since this is a switched mode DC DC converter. I use these or Recom brand which are cheaper. As I said - I think you are OK to keep the linear V reg.