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  • Author Author: scarcesam
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Am I using the 2N2222 correctly?

scarcesam

This is my first use of transistors in a project. I need to control a SainSmart 16-Channel Relay Module using an Arduino 101(3v3 GPIOs). This Relay Module uses 5 vdc through Opto couplers to power the relays. The input pins on the relay board are at 4.4 vdc and need to be brought to GND to switch on.

When I connected the Arduino 101(3v3 GPIOs) directly to the relay module some of the relays would stay on while others would be controlled by the Arduino. It all worked fine when I tried an Arduino UNO(5v GPIOs) in place of the 101. But, I want to use the BLE capabilities of the 101.

So my solution is to use  2N22222N2222 transistors because I’ve read they are good at general purpose low-mid power switching Below is the configuration I think is correct Any feed back would be greatly appreciated I’ve tried reading the Data Sheet but I’m mostly going off of other forum posts

 

Relay Board inputs; +4.4vdc, input to ground 4mA

Arduino 101; 3.3vdc GPIOs, max 20mA per GPIO

T1  2N22222N2222

R1: 10K, to reduce current through the Base to 0.33 mA

R2: 100K, drop down resistor. (is it needed?)




  2N2222.JPG

 

Thanks!

 

Attachments:
image

  • First, thanks for all the feedback!

     

    So, the 2N2222s I ordered turned out to be 2N2222As. I chose them because they were cheap on Amazon. And because this is a temporary project, for learning, I'm not too concerned with quality or having the perfect part. I do want to learn how to find the best fitting parts. So maybe MOSFETS next time. I'm still trying to figure out which specs to search on supplier web sites.

     

    However, I think Rachael had the right idea. The Arduino 101 GPIOs alone can't keep the Relay Module input high enough to keep the optos off. But, it works with a 1k pull up resistor on the input from the Arduino to the relay board. This setup should be draining roughly 9 mA through the Arduino. Well below is max of 20mA.

     

    image

     

     

    I Think I'll use the setup above with the pull up resistor. Do I need a resistor for each individual input? I tried 4 inputs though one 1K resistor and it didn't work. I'll play with the values a bit for power savings but that's not super important. The plan is to use the Arduino and relay module to control the two 12v motors on a kids electric ride-on vehicle. I plan on having a common ground between everything but the Relay module will run off of the cars 12v battery and the Arduino will run off of a separate 9v battery.

     

    Above all else this is a learning experience for me. It's my first dabbling in anything less than 110VAC (former electrician).  So, any other tips or suggestions would be greatly appreciated.

     

    Thanks,

    Sam

  • in reply to D_Hersey

    I agree with Don - these days I always use FETs because they are more efficient - both input driving current and output voltage.

  • I didn't have a  clue to the nature of your load when I mentioned the PU.  In cases where you want some kind of bandwdith from the opto it helps drain the LED capacitance.  A relay isn't fast enough to benefit from this.  If you go to a MOSFET you can eliminate the input resistor

  • in reply to jc2048

    Jon Clift wrote:

     

    I (probably) disagree with Don and Rachael on the need for a pull-up.

    Yes you are right! I (probably) blame that on trying to answer on a forum after 2am when I should really be in bed image

  • I agree with Doug, R2 isn't needed. When the Arduino output is low you have R1 in parallel with R2 to ground, which is about 9k - that's not much different to R1 alone, so R2 isn't doing anything much.

     

    I (probably) disagree with Don and Rachael on the need for a pull-up. The input is an optocoupler and will look something like this

     

    image

     

    You wouldn't bother with a pull-up if you were driving an LED, so why bother with it here [that's based on my guess as to the circuit - though if you look at the photo you can see the optocouplers and the resistor packages next to them, so it's an educated guess].

     

    You don't need to fuss over the transistor too much - when people talk of switching transistors they mean switching at high speed, this is just slowly turning something on and off.

     

    If you were a real cheapskate, you might get it to work with just a signal diode between the Arduino output and the relay board input.