Help looking for possible problems in physical components and layout

I would like some help to look for "gotchas" in my project.
I have the code doing everything I want but I want to ask the group to give this a reasonability check on the components and layout.
I was asked to make a project for the local Dad's club who does things for the grade school.
Using only pieces/parts I had on hand, I am on track to deliver this ahead of schedule this weekend.

But almost all of my projects and this one so far, have been workbench projects. I do not need to worry about making things work in the real world. So I am asking for help looking for design snafus.


Arduino Uno
2 button inputs.
MP3 (YX5300)
2 DR21A01 DC 5V DPDT Relay Module Polarity reversal switch Board for Arduino UNO (limited spec at bottom of post)
One of the relays will power 4 x toy police beacons (separate power supply). I don't have a current measure for these toys but each is an LED and a small motor that is barely loaded during operation)
The other relay will power 2 x toy motors (even smaller than the beacons)

The power wires from the relays to the toy beacons will be about 5 feet each.

Are there any gotchas in my layout?


Edit: Oh, I am guessing on some of the pin numbers. I have them in code but as I draw the diagram I do not have access to my code.

Relay Description:
DR21A01 5VDC 1 Channel DPDT Relay Board
Operating Voltage : DC 5V
Operating Current(Relay ON): 35MA
Standby Current(Relay OFF) : <1MA

Input control signal voltage:
0V - 0.6V Low stage (relay is OFF),
0.6V – 2.4V (unknown state).
2.4V - 24V High state (relay is ON).
Input control signal high state current:
2.4V: 0.15mA
5V: 0.4mA
12V: 1.2mA
20V: 2mA
Relay Load : 1A 125VAC;2A 30VDC
Size : 22.86x18x14
Weight :7g


I'd use MOSFETs instead of relays, and use the 5V from the battery to power the Arduino as well.

Easier, smaller, cheaper.

I'd use MOSFETs

This was my first thought. And I even purchased 20 "logic level" mosfets some time ago to experiment with.
Then I checked out the details and realized that I had fallen into a trap that so others fall into.
Apparently even though "logic level" is in the description, IRF530N TO-220 100V 17A MOSFET only pass higher current with 10v.
I have some IRL540 in my shopping cart. (Edit: but cannot be part of this project due to time constraints)

You are correct, its threshold starts at about 4 volts, you want something that starts under 2 volts. Look at the RFP30N06LE, RF1S30N06LESM There are a lot of devices that will do what you want. Look for Avalanche rated as well, these are.

The VGS, TH is rather unimportant. I rarely if ever look at it when selecting a MOSFET, maybe a sideways glance out of curiousity.

What I am looking for is an RDS, ON value for a VGS value that is lower than the value I intend to drive the thing at. So for a 5V drive you’re looking for an on resistance at 4.5V. For 3.3V, typically a value for 3.0V. If not given, the part is not suitable for use as a switch at your intended gate voltage. That’s the rating that counts.

In case of Infineon parts, as a rule of thumb IRF prefixed parts require 10V gate; IRL prefixed parts are supposed to be logic level (5V typically for those in TO-220 packages, possibly 3.3V or even less for those in SOT-23 packages) but always check the datasheet.

but always check the datasheet.

Here is where I have to admit a shortcoming of mine. I simply cannot make heads or tails out of data sheets.
I read through your description (and many many similar posts before) and I cannot determine which chart/graph/table I should be looking at.
I have failed enough times that I gave up trying. Instead, I try to go with what works for other people.
IRF is bad for switching on and off my toys.
IRL is good.
I will buy a few dozen IRL mosfets for future tinkering!

IRL520 or IRL540 is a fine general purpose MOSFET but fairly old design; you may be better off with the newer IRLZ44N.