Voltage drop

Discussion related to "under the hood" OpenMV topics.
prasap1
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Voltage drop

Postby prasap1 » Sat Apr 20, 2019 6:10 am

Hi, I was wrote a code for shape detection,
After detect a shape in openmv cam p1 value will change 0 to 1.
So I'll get 3.3v from P1.
In ground I connected one wire and p1 I connected one wire. By multimeter I checked voltage it shows 3.3V.
Same I was connected in to 3V relay module after connected I checked voltage by multimeter it's showing only 1.1V so that my relay is not switching over. What can I do for this.
Can any one help me.
Thanks in advance.
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kwagyeman
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Re: Voltage drop

Postby kwagyeman » Sat Apr 20, 2019 5:16 pm

Stop spamming please.
Nyamekye,
jwright
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Re: Voltage drop

Postby jwright » Wed Oct 23, 2019 7:59 pm

I am also trying to connect a small relay to the OpenMV Cam H7. We have code that can drive an output pin high (3.3V), but am not sure if one can even drive a small relay. Maybe I need to try driving a transistor circuit? Anyone have a suggestion - small relays do not require much current. I can drive them with an Arduino or Teensy Microcontroller. Thanks.
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kwagyeman
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Re: Voltage drop

Postby kwagyeman » Wed Oct 23, 2019 8:26 pm

The pin can source/sink 25mA, it might be able to drive a small relay. However, you should not really use microcontroller I/O pins this way. You will end up just destroying the I/O pin. Relays of any type of inductive load are not friendly to I/O pins.
Nyamekye,
jwright
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Re: Voltage drop

Postby jwright » Wed Oct 23, 2019 8:40 pm

Thank you Kwagyeman,

Do you know if you could read the output pin high directly with another microcontroller? Or would you pass a bit another way? I have used relays in the past to try and isolate the controllers (w/diodes on the coil to help reduce CEMF).

More info: I am using a Teensy 3.2 Microcontroller for a small robot and just want a high input when we see a color like red or green. I have that code and am sending the OpenMV output pin high when we detect green, but my tiny relay probably requires more current than 25mA. I will try a small transistor circuit unless someone has a better suggestion. Thanks!
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kwagyeman
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Re: Voltage drop

Postby kwagyeman » Thu Oct 24, 2019 12:12 am

Just use a transistor circuit. That should work.
Nyamekye,
jwright
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Re: Voltage drop

Postby jwright » Thu Oct 24, 2019 2:38 pm

I must be doing something dumb. :?

I have tried to implement the following circuit to interface my teensy 3.2 microcontroller with the OpenMV Cam H7. No matter what I use for a series limiting input resistor, I can not seem to bias the transistor. I am measuring ~.6V on VBE and ~.6VR1 with R1=10K. Where is the remaining voltage being dropped. When I just run a 10K and place P8 to one end and Gnd to the other, I read 3.2V, but when I implement the transistor, I do not see the full 3.2 between VBE and VR1? Any advice would be great! Thanks!
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kwagyeman
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Re: Voltage drop

Postby kwagyeman » Thu Oct 24, 2019 3:29 pm

The OpenMV Cam pins are 5v tolerant. Just directly connect the two devices. You don't need a level shifter.
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kwagyeman
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Re: Voltage drop

Postby kwagyeman » Thu Oct 24, 2019 3:31 pm

I was just saying not to drive a relay with an I/O pin directly.
Nyamekye,
jwright
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Re: Voltage drop

Postby jwright » Thu Oct 24, 2019 3:46 pm

OK, You are suggesting wiring GND to GND and P8 of the H7 to the Input Signal Pin of my Teensy. Correct?
- thanks.
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kwagyeman
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Re: Voltage drop

Postby kwagyeman » Thu Oct 24, 2019 3:58 pm

Yep, that will work.
Nyamekye,
jwright
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Re: Voltage drop

Postby jwright » Tue Oct 29, 2019 5:30 pm

Update:

Well, connecting directly worked - we had to power the H7 (Vin and Gnd) from the Teensy Microcontroller as we had issues with false signals sending driving the teensy high when they each had separate sources with a common ground. Did not expect this to be an issue, but it was.

Thanks for the help! ;)

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