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Old 01-24-2021, 07:07 PM   #21
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Default Re: How could I find a FET with specific specs?

Quote:
Originally Posted by eccerr0r View Post
Correct, need a common emitter or common source for mosfets. So that means low side, or use a p channel/pnp transistor. OP can use n channel/npn on the high side if they make a driver for them, which may complicate the circuit considerably.

I'm facepalming because we're seeing the same problems that needs to be solved as the magnetic stirrer problem. Nothing learned... Insisting the final driver must be driven by an analog signal...
That seems to be the case, isn't it?

Does OP actually build the circut? If OP really wants to learn then build the circuit and report the results, the Voltage readings, the condition of the signals, etc. then you can analyze why the circuit is not doing what you want to do.
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Last edited by budm; 01-24-2021 at 07:11 PM..
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Old 01-24-2021, 07:18 PM   #22
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Default Re: How could I find a FET with specific specs?

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Originally Posted by EasyGoing1 View Post
OK, so in terms of learning, I have no idea what "on the low side of the circuit" means at all ... also, "current monitor feedback" - no idea what that is either.
Well, computer and keyboard are your friend so just do the research on those subjects.
You have no idea how lucky you are have internet to access the knowledge world wide, in my days I would be lucky to find the info without waiting for months to get info on pieces of paper or spend lots of money at College book stores.
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Old 01-24-2021, 08:33 PM   #23
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Default Re: How could I find a FET with specific specs?

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Well, computer and keyboard are your friend so just do the research on those subjects.
You have no idea how lucky you are have internet to access the knowledge world wide, in my days I would be lucky to find the info without waiting for months to get info on pieces of paper or spend lots of money at College book stores.
Hey I was there too ... I remember when Al Gore proposed his bill to fund the "Information Super Highway" - which was a federal budget created specifically to expand the Internet from the privileged few (governments and public education) out to the home via the Telco's. I was in my second year of college back then and didn't actually see Internet access until two years after that when I was a tech at Edwards AFB ... then AOL tapped in and started separating the men from the boys... lol

And for the record, I did immediately look up current monitor feedback and was reading eagerly until I realized I started getting completely lost .... what I got from it though, was that a voltage feedback monitor basically takes a voltage and feeds it back to an opamp using a high impedance input on the opamp so that it would be the small voltage differences that guide the output of the opamp causing it to provide a more stable voltage, and current feedback would do something similar only it would be fed back into a low impedance on the op-amp so that it would be the current changes guiding the output of the opamp ... that's about as much as I was able to register before the propeller head tore off into all kinds of hieroglyphics ...
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Old 01-24-2021, 08:43 PM   #24
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Default Re: How could I find a FET with specific specs?

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Originally Posted by eccerr0r View Post
I have to leave it up as an exercise to the reader to convert this to a 'low' side control.
Low side control?




Quote:
Originally Posted by eccerr0r View Post
Thank you for putting the correct polarity/channel semiconductor in the schematic. Very important when dealing with the drive needed to turn on low/high side semiconductor switches.
You're still upset over that little mistake? That was like ... 5 topics ago wasn't it?

:-)
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Old 01-24-2021, 09:03 PM   #25
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Default Re: How could I find a FET with specific specs?

What do you guys think about something like this ... where picking different values for RO1, RO2 and RO3 as current limiting resistors for high, medium and low brightness in the LEDs, turning on only one of the three transistors at any given time?

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Old 01-24-2021, 10:31 PM   #26
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Default Re: How could I find a FET with specific specs?

Before you test that circuit and find it may not work as well as planned, do recall one LED turned on is dimmer than 2, and 2 on is dimmer than 3... so you may also be overthinking the problem.
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Old 01-25-2021, 02:21 PM   #27
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Default Re: How could I find a FET with specific specs?

This might seem like a stupid suction but why not use PWM function ( if this controller has this option) and
only have one output

You do have use the right setup for the transistor or mosfet that you would using

I have done this a few time with a Basic Stamp and using the PWM function but you have to have output setup correctly I have even done this with an optic sensor ( like a 4N25 or something similar )

For give me for saying this but you might be making this hard than it needs to be
unless you have a special reason for doing it this way

One note you do have to set parameters correctly with the PWM signal for this to also work correctly
You might not be able to to go all the way down to 0% duty cycle and see the LEDs light very dimly but maybe down 10% depending on the controller and maybe as high as 75 to 80 for full on brightness in appearance to naked eye

One other note you might even have to put a 1uf ( or less ) capacitor on the output pin of the controller and the negative side of the power supply to smooth out the pulses coming from the controller if they flicker to much ( see below )

I once had to use this method for a solenoid valve coil hook up to Basic Stamp Controller using the PWM output to control a portioning hydraulic valve coil because the controller board for this machine took a crap for controlling this valve and to replace it would have cost to much money

One final note this valve portioning power amplifier version has a feature that you can hook pot control to but because of the way valve works it would not always work correctly all the time and this was the reason for using a controller with a PWM controller function so it would work correctly most of the time

The company that I worked for at the time really appreciate the fact I was able to fix this machine issue without having to spend a lot of money to do it ( this machine was about 20 years old )
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Old 01-26-2021, 12:13 AM   #28
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Default Re: How could I find a FET with specific specs?

The OP was using PWM except was not letting it through to the end device, insisting that the conversion happen early in the chain, thus wasting energy as well as increasing the dissipation requirements of the switching elements. The same problem was encountered in one of the previous projects the OP was working on yet persists on here.

In any case, proper drive needs to be figured out. OP apparently still hasn't figured out how to properly turn LEDs fully on and off - a requirement for PWM.
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Old 01-26-2021, 04:27 AM   #29
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Default Re: How could I find a FET with specific specs?

The one thing that you have to understand using the PWM signal from at least the micro controller ( the one that I am using ) is that full ON and full OFF is not the same thing as what you see with your naked eye with a led light bulb output you have to use a volt meter to measure the average voltage output signal

The way to solve the output issue with a micro controller device is turn on the output and get this to work first then use the PWM signal command and it should work correctly after

If you want to understand how a PWM output works on a micro controller look at page 356 of this manual and it explain it in some detail of how you would use it

Also in this manual it also has a section that shows you how to hook up a transistor to an output pin

I hope this helps
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Old 01-26-2021, 06:23 AM   #30
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Default Re: How could I find a FET with specific specs?

This is the article that has how to hook a transistor to a micro controller output pin

Plus it gives you the resistance values for what type of transistor you might be using

I have even use a 2N3904 it also works very well with a 1k ohm resistor if I remember correctly but this is at 5 volts output from the pin

I have also use ULN2803 before these are very easy to use as well and is also covered in this article below

These values might be different depending on how your micro controller input/output pins are configured

I am using the Basic Stamp Controller as an example because I have used this controller many times in the past and because it well documented and easy to find the information

As far as writing the code for the micro controller that you are using you would have to research for example codes to get a rough idea of how to correctly write the code

I have tried in the past to use a circuit simulator program and was not impressed with the results I have to actually build the circuit to see if it would work correctly or not

EasyGoing

One note I am NOT trying be little you

I am posting this information just in case you do not have a full understanding of how and what it takes to get this project to work

I hope this helps you and I hope you understand what my intentions are

Because I would like to see what you have in mind with this project that is the reason I am following it
Because I might learn something I did not know for maybe a future project

One interesting thing that has happened while looking up this information I found some information about a problem that I have been having using a battery spot welding machine controller board and this might help solve this issue

Here is the website that I found the answer to question about zero crossing and using the right optic sensor to a micro controller

https://microcontrollerslab.com/pc81...les-datasheet/

I found an error with the circuit diagram it does not show the right hook up for a load but it does give a code example which I am going to have to try
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Old 01-26-2021, 09:56 AM   #31
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Default Re: How could I find a FET with specific specs?

Quote:
Originally Posted by eccerr0r View Post
In any case, proper drive needs to be figured out. OP apparently still hasn't figured out how to properly turn LEDs fully on and off - a requirement for PWM.
Remember, the goal in this application is to have three different states of brightness, low, medium, and high.

The circuit where I had three different output pins driving three different transistors (TIP-120 in this case) actually worked.

However, based on the feedback here, I decided to try this configuration:



And after playing around with the RC filter, I settled on the values in the schematic. I used an online calculator which told me to use a 68Ω resistor and a 4.7F cap but when I tried that, there was still significant ripple in the output so I bumped up the values and as you see here, the results are pretty good:


(This is at the medium brightness PWM duty cycle before attaching the MOSFET)

Yellow is channel 1 on the scope attached to the output pin of the Arduino chip and green is channel 2 attached to the output of the RC filter. It's still kinda noisy, but I think that has more to do with the Hanetk USB scope that I'm using, and my final power source will be a battery and not this buck converter that I'm using now which may also be part of the noise.

By varying the duty cycle of the PWM output, I am able to get three states of brightness ... basically, in the low state, I drive them at 90mA then at 170mA then on the highest setting, I get 270mA and the LEDs are rated at 280. And that is all from a 3.3V source.
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Old 01-26-2021, 11:31 AM   #32
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Default Re: How could I find a FET with specific specs?

This looks ok but I would have the one ohm resistor on each LED and remove the one the mosfet to negative side of the power supply

I might be wrong about this part it might not work correctly ——>
Or put the LEDs on the other side of the mosfet and have a resistor on each LEDs that goes to the negative side of the power supply

The reason why is so each led has the same current

Have you tried a raising value capacitor on the output pin to the negative rail of the controller and what results do you get ( or maybe put a 5 UF more before the resistor) with higher value ( just curious what the difference is with capacitors values I kind-a of know what might do but I only have experience doing it on the solenoid valve controller but have not tried this on LEDs before ) see if this makes a difference the only problem might be that response time for changing in brightness level will take longer ( this might be able to corrected this in the PWM code routine )

Can I ask you what are you making

Quote:
Originally Posted by EasyGoing1 View Post
By varying the duty cycle of the PWM output, I am able to get three states of brightness ... basically, in the low state, I drive them at 90mA then at 170mA then on the highest setting, I get 270mA and the LEDs are rated at 280. And that is all from a 3.3V source.
I am glad you are making progress on this project

Is the mosfet getting warm or hot driving it at full brightness or at low brightness what happens then

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Old 01-26-2021, 12:05 PM   #33
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Default Re: How could I find a FET with specific specs?

Quote:
Originally Posted by sam_sam_sam View Post
This is the article that has how to hook a transistor to a micro controller output pin

Plus it gives you the resistance values for what type of transistor you might be using

I have even use a 2N3904 it also works very well with a 1k ohm resistor if I remember correctly but this is at 5 volts output from the pin
A 3904 wouldn't work in this application because it has a rated maximum current that it can handle at 200mA and I need to push around 280. I think I'm close to the final circuit that I'll be using. I'm not sold on the IRF510N MOSFET YET ... I'd like to find something that can be driven with lower voltages - but only in the interest of that component consuming as little power as possible since this circuit will be running on a battery.

Quote:
Originally Posted by sam_sam_sam View Post
Because I would like to see what you have in mind with this project that is the reason I am following it
Because I might learn something I did not know for maybe a future project
Well, this project is what I affectionately call a "Hat Light" ... it's a flashlight that clips onto the bill of a baseball cap. I've already designed and printed the housing for it - in a mock setup at the moment, I haven't finalized the design yet, but here it is ... and YES, that's tinfoil epoxied into the cones ....



HOWEVER, I found some reflective PLA material that I want to use to print the cones with as you can see here:



After some testing with the reflective material, I'm not too happy with the way it works, and have been considering using this mirror finish spray paint that they have for plastic model projects which would no doubt work much better, BUT ... I had also planned on getting these thin sheets of clear acrylic that has a melting point tens of degrees C lower than PLA, which means if I print the cones onto the plexiglass, it could possibly bond with it in the print process creating a protective transparent cover for the cones - but if I do that, then I can't spray paint the mirror finish on the inside of the cones ... so I'm not yet decided ... but the LEDs I've been experimenting within this post are significantly brighter than the LEDs I have in the prototype hat light now, which means they COULD be bright enough so that the lack of performance I get from the PLA reflective material might be offset and I could use it anyways and then print to the plexiglass sheet.

The microcontroller I'm using is an ATTINY85, here's the code that seems to work pretty well:

Code:
#include <Arduino.h>
#include <avr/sleep.h>
#include <avr/interrupt.h>
#include <BlockNot.h>

#define BUTTON 3
#define P0 0

#define LO 0
#define MED 1
#define HI 2

int mode = 0;

#define BUTTON_PRESSED digitalRead(BUTTON) == LOW
#define TOO_HIGH mode > 2

BlockNot offTimer(2500);

void setPins() {
    pinMode(BUTTON, INPUT);
    pinMode(P0, OUTPUT);
}

void sleep() {
    GIMSK |= _BV(PCIE);                     // Enable Pin Change Interrupts
    PCMSK |= _BV(PCINT3);                   // Use PB3 as interrupt pin
    ADCSRA &= ~_BV(ADEN);                   // ADC off
    set_sleep_mode(SLEEP_MODE_PWR_DOWN);
    sleep_enable();                         // Sets the Sleep Enable bit in the MCUCR Register (SE BIT)
    sei();                                  // Enable interrupts
    sleep_cpu();                            // sleep
    cli();                                  // Disable interrupts
    PCMSK &= ~_BV(PCINT3);                  // Turn off PB3 as interrupt pin
    sleep_disable();                        // Clear SE bit
    ADCSRA |= _BV(ADEN);                    // ADC on
    sei();                                  // Enable interrupts <---THIS IS THE PROBLEM
    setPins();
}

void allOff(){
    analogWrite(P0, 0);
}

void ledLow() {
    analogWrite(P0, 193);
}

void ledMedium() {
    analogWrite(P0, 208);
}

void ledHIGH() {
    analogWrite(P0, 253);
}

void setup() {
    setPins();
    allOff();
    ledLow();
}

void loop() {
    if (BUTTON_PRESSED) {
        offTimer.RESET;
        delay(100);
        while(BUTTON_PRESSED) {
            if (offTimer.ON_TRIGGER) {
                allOff();
                delay(1000);
                sleep();
                mode = -1;
            }
        }
        mode ++;
        if (TOO_HIGH) mode = LO;
        switch(mode) {
            case LO:
                ledLow();
                break;
            
            case MED:
                ledMedium();
                break;
            
            case HI:
                ledHIGH();
                break;
            
            default:
                break;
        }
        delay(1000);
    }
}
Though I'm having a problem with the code in that while I can put the tiny85 to sleep just fine, It's not waking up properly so I'm trying to figure that out. One other option is just to add a button to the reset pin and use that to power it on, then the mode toggle button to power it off by holding it for about three seconds.

Quote:
Originally Posted by sam_sam_sam View Post
One interesting thing that has happened while looking up this information I found some information about a problem that I have been having using a battery spot welding machine controller board and this might help solve this issue

Here is the website that I found the answer to question about zero crossing and using the right optic sensor to a micro controller

https://microcontrollerslab.com/pc81...les-datasheet/

I found an error with the circuit diagram it does not show the right hook up for a load but it does give a code example which I am going to have to try
That optical-transistor device looks interesting ... I've never worked with anything like that before, but I'm wondering how tightly coupled output would be to any input signal since it's translated by light ... like how sensitive is it to small input fluctuations... which I assume would be dependent on how sensitive the receptacle of the light is inside the device.
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Old 01-26-2021, 12:05 PM   #34
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Default Re: How could I find a FET with specific specs?

Yes the proper way is to have individual resistors for each LED unless your LEDs are perfectly matched. In a prototype environment this is possible; in a production, not so much.

I think the OP got lucky that the mosfet likely won't get warm or hot at all due to the low/limited currents involved here. 270mA times negligible voltage drop in an ideal situation is not many watts of power.

However one thing that we're being lied to about is that the IRF510. These are not "logic level" FETs and will barely turn on if at all at 3.3V gate drive, again maybe you can cherry pick a device that will turn on, but in a production environment you'll get a lot of failures. A BJT here will guarantee turn on in all cases but it will limit LED voltages to 3.1V or so due to saturation voltage drop.
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Old 01-26-2021, 12:19 PM   #35
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Default Re: How could I find a FET with specific specs?

Quote:
Originally Posted by sam_sam_sam View Post
This looks ok but I would have the one ohm resistor on each LED and remove the one the mosfet to negative side of the power supply I might be wrong about this part it might not work correctly >
Or put the LEDs on the other side of the mosfet and have a resistor on each LEDs that goes to the negative side of the power supply

The reason why is so each led has the same current
Based on the LED Array calculator, that is actually the preferred way to do it (one resistor in each branch of the LED portion of the circuit) I just didn't have any 1 Ohm resistors to test with so I made one out of coil wire and connected it to the emitter. When I create the PCB, I'll use surface mount resistors and do it the right way.

Quote:
Originally Posted by sam_sam_sam View Post
Have you tried a raising value capacitor on the output pin to the negative rail of the controller and what results do you get ( or maybe put a 5 UF more before the resistor) with higher value ( just curious what the difference is with capacitors values
Are you talking about the RC filter portion? In an effort to smooth out the signal even better?


Quote:
Originally Posted by sam_sam_sam View Post
Can I ask you what are you making

I am glad you are making progress on this project

Is the mosfet getting warm or hot driving it at full brightness or at low brightness what happens then
See my post before this one on the details of what I'm making. And thank you! I'm glad to be making progress as well :-)

and NO, the MOSFET doesn't get warm at all but it's a pretty beefy MOSFET rated I believe at 5 amps - which I couldn't imagine driving it that hard especially without a heat sync. It would be nice to scale it down for the final PCB, but then again ... I do have a lot of real estate in this project for a relatively simple circuit.

:-)
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Old 01-26-2021, 12:26 PM   #36
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Default Re: How could I find a FET with specific specs?

Quote:
Originally Posted by eccerr0r View Post
However one thing that we're being lied to about is that the IRF510. These are not "logic level" FETs and will barely turn on if at all at 3.3V gate drive, again maybe you can cherry pick a device that will turn on, but in a production environment you'll get a lot of failures. A BJT here will guarantee turn on in all cases but it will limit LED voltages to 3.1V or so due to saturation voltage drop.
I take great offense to being called a liar. I don't lie, what would be the point? How could I possibly get the right help in here if I lie about what I'm doing?

Here is a picture of the exact MOSFET that I used in the test. I also uploaded the pic file itself so you can check the date and time on it or whatever to confirm that I just took that picture.

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Old 01-26-2021, 12:29 PM   #37
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Default Re: How could I find a FET with specific specs?

Yeah you got lucky, the threshold voltage of these FETs are not cut out for 3.3V usage and if you mass produce, you'll find a lot that don't work as planned.

And again you MUST COMPLETELY LOSE THAT CAPACITOR if you want to save energy from battery use. That sole capacitor is causing you to waste a lot of the energy stored in the battery.
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Old 01-26-2021, 12:34 PM   #38
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Default Re: How could I find a FET with specific specs?

Quote:
Originally Posted by eccerr0r View Post
However one thing that we're being lied to about is that the IRF510. These are not "logic level" FETs and will barely turn on if at all at 3.3V gate drive, again maybe you can cherry pick a device that will turn on, but in a production environment you'll get a lot of failures. A BJT here will guarantee turn on in all cases but it will limit LED voltages to 3.1V or so due to saturation voltage drop.
And here is a video of it actually working...

https://youtu.be/gLwZeiql5uc
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Old 01-26-2021, 12:39 PM   #39
EasyGoing1
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Default Re: How could I find a FET with specific specs?

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Originally Posted by eccerr0r View Post
That sole capacitor is causing you to waste a lot of the energy stored in the battery.
But the capacitor is part of the RC filter which smooths out the PWM and eliminates flickering which you wouldn't think would be detectable but I can "feel" it when the LEDs are lit, I can't look directly at them because they are too bright, but I can sense the flickering when I don't use the RC filter. I'll see if the flickering doesn't show up on video if I take that portion out of the circuit.

Besides ... I don't see how the cap could cause any more loss than not having it since it's only mitigating the signal coming out of the microprocessor which can only put out a max of 40ma at the 3.3V that it's being fed.
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Old 01-26-2021, 12:49 PM   #40
sam_sam_sam
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Default Re: How could I find a FET with specific specs?

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Originally Posted by eccerr0r View Post
you MUST COMPLETELY LOSE THAT CAPACITOR if you want to save energy from battery use. That sole capacitor is causing you to waste a lot of the energy stored in the battery.
You could do that but what you would have is the LEDs pulsing
If you do not want to have this effect then you have to put this capacitor on the output pin

Trust me I have run into this problem before

Now you are probably right trying to run the mosfet at 3.3 volts might have issues I would agree with you about this

And if this is the case then use a ULN2803 which I mentioned earlier and if I can remember correctly I think you use it all the way down to 3 volts but you would have to look at the data sheet to make sure

Last edited by sam_sam_sam; 01-26-2021 at 12:58 PM..
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