Can I safely wire LED in series with a motor circuit?

I'm working on a project that involves 8 peristaltic dosing pumps. I combined 2 Jebao DP-4 modules into a single 8 head module, but the mounting plate that came with each unit was just too convenient to discard, and it also has an green LED per each pump that I'd also like to work into the project.

As it stands now, I picked up 4 TB6612 dual motor control circuits from Sparkfun, but now I'm thinking a transistor (TIP120?) and a diode (1N4001?) would be more appropriate and safe for the project, and require less Arduino pins. The pumps run on 12 volts, and included with the master pump unit was a 12VDC/2A wall wart, and no added power source came with the slave unit.

My question is can I safely incorporate the LED into each motor circuit? Like +12v/motor/LED/driver circuit/ground? If the outcome is simply just decreased motor RPM, I can live with that. When the motor is switched on, I would also want the LED to switch on from the same transistor.

Also, I am open to suggestions about what to use to drive the motors. I've read N channel mosfets would be good too, but I didn't take notes as to which model number would best work for this and found the TIP120s being used in other threads and videos the same way.


It is not a good idea to wire an unspecified LED in series with an unspecified 12 V motor. Consider for example a 'bog standard red' LED. That has a junction voltage of around 1.6 Volts and a contact resistance in series with that of about 100 Ohms. It never will let through enough current to start most motors and with up to 11 V on the LED, might burn up trying.

A better way is to fit a moderate resistor and LED 'indicator' parallel to the 12V load.

For example a 2 Volt red LED and a 2.2kOhm to let through about 5mA, which the LED datasheet has told you is in the middle of the range for a bog standard LED and I happen to say is 'bright enough'. Those don't effect the motor and do light up while it is powered. If you need a slower motor speed then change your 12V supply line to 11 or 9 Volts; 10% more than the lowest tested which worked in your application.

Putting a led in series with the motor essentially turns the led into a 20ma fuse for the motor, which will sadly blow before the motor can turn.

You could however put the led in parallel, but have to be mindful of back emf created by the motors. Putting protection diodes across the motors should be done regardless of which route you go (mosfet/BJT). Typical N-ch mosfets are used as low side switches, and logic level gates are required to operate with the arduino. P-ch mosfets are used for high side switching, but will require bringing the gate up to the 12v supply, which the arduino cannot handle without using a BJT to control the gate charge.

Are you going for a simple ON/OFF for your pumps, or are you going to try flow control using PWM?

I am still yet to determine the ranges I want the pumps to turn at, and I will need to play around a bit to figure that out, but as of now, my game plan after I get the circuit safely working is to run the motor at full voltage for 60 seconds exactly and measure the amount of milliliters that was pumped, then also run the pump at roughly 75% (PWM) for another 60 seconds.

The application is a hydroponics controller. The pumps will dose in the nutrients according to a weekly schedule and I have a few different ways to calculate how I normally get my nutrients into the system, but I will need the pumps to be able to pump in lots of nutrients in multiples of 10ml at full speed, the system will mix that general mix for a few minutes, then I will measure the Conductivity of the mixture and the pumps will need to more accurately dose in nutrients in multiples of 0.1ml if need be. Over time I hope to code for all of the weekly schedules and completely automate everything, but this is just the first step towards that.

If you only want to drive the motors in one direction you only need a single switching device and a free-wheel diode. I guess we can assume from the supplied power source that 2A is more than the continuous rating of the motors...

As for part number of MOSFET, that's not how it works, there are 1000's of possible devices that are each perfectly capable. The important point is to get an n-channel, logic-level MOSFET with 20V or higher and an on-resistance of 0.1 ohm or less. Make sure its logic-level, or its useless to you, and pick a package you can solder (many are surface mount these days). MOSFET part numbers change year on year as improved models are rolled out - the manufacturers are in intense competition. Stick with a favorite part number and its price will start to rise rapidly when it becomes a legacy device.

The standard circuit for driving a relay works here, n-channel mosfet as low-side switch, free-wheel diode across the motor (cathode to V+). The diode just has to be rated for pulse duty at your motor current (2A??). The diode is essential, it stops the inductance of the motor windings from zapping your circuit.