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.
2 button inputs.
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.
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:
Relay Load : 1A 125VAC;2A 30VDC
Size : 22.86x18x14
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.