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Say we have a controller that the GPIO max is 40mA.
It would be reasonable to derate this to say 20-30mA. -
For a BJT, we will design for GPIO current is 20mA when it is sitting at a HIGH (5V).
5V - .7(Vbe) / 20mA = 215Ω (for a BJT base resistor) . A standard resistor is 220Ω.
5v -.7(Vbe) / 220R = 19.5mA (GPIO current = base current)
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OK, thank you. I was also considering making sure I did not wreck the power transistor (PN2222) as well. I will endeavor to do better math, in more detail, for future projects.
Oh right, yes. I was going to mention that my UNO R4 wifi is actually 8ma maximum on the GPIO pins, but you are actually giving an example, so it is all fine.
- 8mA derated to 6mA.
~680 to 750ohms for a base resistor.
Remember to always confirm, with a DMM, your transistor fully turns ON.
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By the way, this forum and community is soooo good. I do not feel anything but completely welcomed. So many people around, and quick responses. This is exceptional!
Oh, actually with what you just said about fully on, I have a nice little variable resistor to work with. I could use that with my DMM to fine tune it. Although it probably does not need to be that finely tuned.
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Shouldn’t need tuning.
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If you do this, add a fixed resistor (say 680R) in series so the GPIO doesn’t get damaged if you accidentally turn the potentiometer to 0Ω.
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Optionally, when we use a logic level MOSFET (which is a voltage controlled device), the GPIO 5v output turns on the transistor when the GPIO goes HIGH.
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Yes, good point. I have limited components at the moment. I bought a breadboard kit from Amazon. I have also ordered a set of better quality transistors to compliment it (I had ordered a bunch of power relays already, and have them now). Oh, and also I have some strip-board/ vero-board. I will probably make some consistent circuits with what I have, to avoid having to mess around.
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Sweet! Yes, for example, you should always, have that diode across the relay activation coil. You save some time with it, if it is already set on a strip-board. But also, Yes, I would need to check the current rating of the strip-board of course (or maybe look at watts). If I am running high currents, the strip-board may not be rated high enough. So I am aware of that.
That's a yes.
And although the Arduino datasheet gives an absolute maximum of 8 mA per GPIO pin, if I'm reading the Renesas RA4M1 datasheet correctly, then a safer upper bound may be 4 mA (see Table 2.6 on pp. 32–33).
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Yes but as I stated there is no protection circuit. If you draw more current that the micro can handle you will destroy it.
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