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Topic: Boosting Logic Voltage (LED Strips on Teensy) (Read 7692 times) previous topic - next topic

Qdeathstar

Jul 11, 2015, 11:55 pm Last Edit: Jul 11, 2015, 11:56 pm by Qdeathstar
I apologize ahead of time if we are only supposed to talk about Arduinos. But, the Teensy is super small and has more ram than the mega.


My issue is that I want to control LED strips using the teensy, but it seems the logic level outputs are only 3.3v which wont work, so I need a buffer to increase the voltage.

What do you recommend. Ideally, id like something easy to solder (through pin)


Also, I need 5v for the teensy and a 433mhz receiver from a 12v supply (the leds run on 12v) so could I use a voltage divider here? Thanks!
A creaking creeping shadow
stiff against the freezing fog
glares at a tickless watch.

Time has failed him -- all things shall pass.

Grumpy_Mike

Quote
What do you recommend.
It depends on the current that the LED strip takes. If < 500mA use a transistor, a TIP120 or similar. If >500mA then use a logic level FET.

Quote
so could I use a voltage divider here?
No use a voltage regulator to drop the supply voltage. You can only use a divider to cut down signals.


CrossRoads

Can use a low voltage level gate like this one
http://www.digikey.com/product-search/en?pv959=13&pv69=80&FV=fff40015%2Cfff8007d&k=n-channel+mosfet&mnonly=0&newproducts=0&ColumnSort=0&page=1&stock=1&quantity=0&ptm=0&fid=0&pageSize=25
for the teensy to turn on to connect cathode of LED strip to GND to turn the LEDs on.
LED strips are typically 3 LEDs in series with a current limit resistor, drawing about 20mA.
So this MOSFET is good for ~ 15 LEDs worth of strip.

5V regulator to as GM says for the 12V to 5V supply.
Designing & building electrical circuits for over 25 years.  Screw Shield for Mega/Due/Uno,  Bobuino with ATMega1284P, & other '328P & '1284P creations & offerings at  my website.

Qdeathstar

I should have been more clear,

The leds are LPD8806's and have a clock and data line....
A creaking creeping shadow
stiff against the freezing fog
glares at a tickless watch.

Time has failed him -- all things shall pass.

CrossRoads

Ah, so addressable LED strips then, such as
http://www.adafruit.com/product/306
3.3V to 5V adapter would be good - could be a simple 74HCxx gate, such as
http://www.digikey.com/product-detail/en/SN74HC08N/296-1570-5-ND/277216
Designing & building electrical circuits for over 25 years.  Screw Shield for Mega/Due/Uno,  Bobuino with ATMega1284P, & other '328P & '1284P creations & offerings at  my website.

Grumpy_Mike

I have used a 74LS14 to step up from 3V3 to the 5V system. You need to use a 510R seriese resistor on the signal and a hefty capacitor on the output.

CrossRoads

Seems a bit overkill Mike. How much current does the control line into a CMOS-based LDP8806 need?
https://frack.nl/w/images/3/33/LPD8806_datasheet.pdf
Designing & building electrical circuits for over 25 years.  Screw Shield for Mega/Due/Uno,  Bobuino with ATMega1284P, & other '328P & '1284P creations & offerings at  my website.

Grumpy_Mike

#7
Jul 12, 2015, 07:53 am Last Edit: Jul 12, 2015, 07:54 am by Grumpy_Mike
Not sure what is overkill? The resistor is there to stop reflections on the line. The capacitor is for the led supply.
You need two non inverted signals so a 74LS14 will provide six inverters but you only need four.

CrossRoads

I was thinking running the Clock and Data lines thru 2 gates to get them from 3.3V out of the Teensy to 5V into the first LPD8806 on the LED strip.
Not sure reflections would be a big problem there if the strip is connected with decent wiring to whatever the 3.3 to 5V buffer is. Couldn't hurt I suppose.  I could see a cap on the supply line if the LED strip didn't have that already.

Designing & building electrical circuits for over 25 years.  Screw Shield for Mega/Due/Uno,  Bobuino with ATMega1284P, & other '328P & '1284P creations & offerings at  my website.

Qdeathstar

#9
Jul 13, 2015, 01:57 am Last Edit: Jul 13, 2015, 02:06 am by Qdeathstar
Thanks for your comments guys.

After reading the comments, it seems like the 74LS14 is the "best" option


I was trying to get this set up in eagle, and I think I found the chip, but when I add it to my schematic, it only adds a pair of "pins" at a time...  but the vcc and gnd pin don't show up?

Also, VCC in this case would be 5v correct? And I'm taking the signal from the eagle, to an input pin on the 74LS14, then taking the output and putting it back to another input pin? To invert the signal twice so it becomes "uninverted"?

Lastly, the datasheet
http://www.futurlec.com/74LS/74LS14.shtml
is limited... how do you know where the capacitor and resistors need to be, and the correct value?
A creaking creeping shadow
stiff against the freezing fog
glares at a tickless watch.

Time has failed him -- all things shall pass.

jack wp

Just a question about level shifters from 3.3 to 5v.
Would a 74LS14 really be much better than a pullup resistor to 5v, and a diode from the 3.3v output, allowing the output to only pull the signal low?

CrossRoads

In Eagle, right click and select Invoke to grab the power pins.

Diode with pullup - that won't keep 5V off the 3.3V output pins.
Designing & building electrical circuits for over 25 years.  Screw Shield for Mega/Due/Uno,  Bobuino with ATMega1284P, & other '328P & '1284P creations & offerings at  my website.

Wawa

It seems that these chips have internal pull-up resistors.

Try a 2N7000.
Gate to teensy port, source to ground, drain to LED data pin.
No other parts.

Leo..

Qdeathstar

A creaking creeping shadow
stiff against the freezing fog
glares at a tickless watch.

Time has failed him -- all things shall pass.

Wawa

The pull-up resistors inside the first LED driver IC should pull-up the data/clock lines to it's own 5volt supply.
The mosfet just pulls that pin to ground when data is flowing.

Data/clock goes only to the first IC in the LED string.
The first chip passes things on to the next one.

NO long lines between teensy and LED strip....
Too much wiring capacitance, and data will be corrupted.

You could try a transistor (and 1k base resistor) if you haven't got a small mosfet.
But transistors could get saturated, and have slow switching speeds.
A schottky diode across B/C (baker clamp) could help.
Leo..



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