I’m pretty much a beginner in electronics so I’d be glad if you could check my schematic to spot any mistakes. It’s a plant watering project and I’m mainly concerned about the setup of the two watering pumps and one LED strip. All three components are controlled by mosfets which are connected to digital pins (D7, D8, D2) of an arduino nano (see images below). The pumps will be active about 5 seconds every two days and are rated 3V-5V / 100-200mA. The LED strip is about 30cm long and is powered by 9V (measured current draw is around 25mA). The arduino is also connected to an LCD display, 2 RGB LEDS (not using the blue leg), a thermistor and to a number of buttons via the analog pins A1, A2, A3 (to control LED strip, display, manual watering and plant parameters). As a voltage regulator from 9V to 5V I’m using an LM2596S-ADJ, which gives power to the Arduino, pumps and other 5V components.
So here are my questions:
Do you think the IRL540 mosfet is a good choice in this case?
Are the mosfets wired correctly and are the resistors in the correct place and of a suitable resistance? I copied the layout from different sources on the internet.
Are the capacitors wired correctly and is the capacitance suitable? (to smooth the current draw at pump/LED start and filter noise) Are the capacitor needed/ recommended for the display and the thermistor?
Any other errors in the schematic? Other recommendations?
I have a similar breadboard prototype, which uses relays instead of mosfets and doesn’t have any capacitors. It works, but I wanted to improve the overall electrical design and eventually put it on a PCB.
The images are pretty small and hard to read on my tablet.
The pull down resistors on the MOSFET gates should be on the Arduino side of the current limit, not the gate side (small thing). The gate current limit resistors are too high in value. 180 Ohms is what I use.
The LED strip needs no caps across it.
The caps should be just across the motors. I can't read the values, but you only need 1 cap, a 0.1uF ceramic.
Certainly does, every high current switched load needs decoupling to reduce generation of EMI and transients on the supply. LEDs are particularly noisy in practice as they switch current very fast both on and off (motors only have rapid transients on switching off).
Sorry about the small images, on PC I think you can enlarge them by clicking on it.
I used 0.1uF and 10uF caps at the motors. I thought a larger cap could be useful for smoothing out the current draw at the start up of the motors and that is also why I placed the caps around the motor and the MOSFET. I thought the 10uF cap could not charge when placed only around the motors while the MOSFET is in "off"-position, but please correct me if I'm wrong.
So with 180 Ohms you would get a current of around 28 mA, which would still be okay for the Arduino pin, because its limit is 40mA, is that correct? Aside from slowing down the charging of the MOSFET are there any other disadvantages when using a larger resistor?
Would a 100uF cap for C6 be suitable? Just for clarification: Could I also use a capacitor of at least 9V since the LED strip is powered by 9V? (I'm guessing the 24V is recommended for security reasons?)
Would you use a 100uF around the motors as well, in addition to a 0.1uF ceramic cap?
I think my I2C unit has those already but thanks for the suggestion.
if you take capacitor 10V and attach 10V power then capacitor will explode. usually capacitor should be little over twice voltage. but i have seen on PC motherboard 6,3V caps on 5V line, 16V caps on 12V line.
I would use 100µF on power line just like you and probably an additional 0.1µF (100nF or 0.1uF if µ is not on keyboard) solder direct on motor.
I'm just used to using these on the fly.
Alt-234 is useful though, Ω