# Circuit Considerations

Note: Can skip all images except last, which is a schematic created using the editor.

I am at a point in the design process where I have created a schematic of the circuit that I want to create. I am using a buck converter to step down a 24V DC battery to 5V and using that battery to power an arduino. This arduino is acting as a speed controller, using PWM and getting input from external switch to set the speed. The input from this switch will be programatically read to set the speed output. The 5V output signal from the arduino then gets run through an op-amp configured as a non-inverting amplifier and drives a motor. The schematic follows: []1

This is the first real circuit I have created, and I don't know where to go from here. It is more than likely that I have made a mistake somewhere or forgotten a part or some heat/power consideration, but I don't have the slightest idea (1) how I could check my circuit for those things or (2) how I would remedy them.

I was wondering how I could go about doing these things and improving my knowledge of electronics in this way. I apologize if this question is somewhat vague; however, I really respect this community and the years of experience you have in the field, and I was hoping that your insights would be valuable in this question, as I don't really know how I could make it more specific. Thanks.

EDIT 1: Ok, after suggestion I am replacing the op-amp with an n-channel MOSFET as follows (Edit 2: added a 10K pull-down resistor on the gate of the MOSFET):

EDIT 3: Added pull down resistor to schematic and flipped MOSFET:

EDIT 4: Adding a full schematic (Edit 5: switched NMOS and Motor position):

simulate this circuit – Schematic created using CircuitLab

• Is this a bad idea? I thought the effective result would be the same as an NPN transistor, but I am just more comfortable with op-amps. Jun 27, 2016 at 19:53
• My biggest doubt thus far about your circuit is whether or not the OP-Amp can actually source enough current to drive the motor you're using. You would be better off using a power MOSFET here. Place the motor between the drain and Vdd, ground the source, and send the PWM signal to the gate, keep the flyback diode. You'll need to find a power MOSFET with a low enough gate threshold to use the PWM signal directly from the Arduino. Jun 27, 2016 at 19:54
• Also, Be aware that you have a small oscillator going with the OP-Amp and the 3.6k resistor. Nothing insane, but it will vary the motor's performance quite a bit.
– user86234
Jun 27, 2016 at 19:57
• I see... I was thinking that the MOSFET wouldn't work because it only worked on or off, but I completely forgot about the PWM of the arduino! Will modify and update, thanks! Will the MOSFET be able to switch on and off quickly enough? Jun 27, 2016 at 20:02
• @louiemcconnell: I tweaked the CircuitLabs schematic a little to compress the width. SE reduces it to a 640 pixel-wide image which makes large schematics difficult to read. Please check that I didn't introduce any errors. +1 for a good detailed question. Please add some text in under the schematic explaining what S1 and the resistors are supposed to be doing. It looks unusual so you may be misunderstanding something. Jun 27, 2016 at 23:10