GTFT Backlight Control

The GTFT backlight control does not appear to be working for me. I cannot turn the backlight off. The product page says "The LED backlight is dimmable by PWM" http://arduino.cc/en/Main/GTFT (part number A000096)

I only have one unit, so perhaps there is something wrong with the one I have, but I assume the BL pin is supposed to let you turn the backlight off and on, and that the BL signal being high will turn the backlight on. (The Esplora schematic shows the BL pin shorted to +5V).

While investigating this problem, I found that the GTFT schematic symbol for T1 (used to turn the backlight off and on) is for an N-channel MOSFET, but the part numbers (PMV48XP / FDN340P) are P-channel MOSFETs. A P-channel MOSFET would have an integrated diode in the opposite direction of what is represented in the schematic, so +5V would always be connected to LEDA. That's what I'm seeing. The backlight is always on and when I change the BL signal from low to high it just slightly changes the brightness.

So I think that either T1 needs to be changed to an N-channel MOSFET, or (perhaps a better option, but without going into a lot of detail why it is better) the source and drain pins on T1 need to be swapped. In the latter case, the pulldown resistor (R3) should probably become a pullup and you might want to put a BJT on the gate to reverse the signal polarity (so BL high will turn the backlight on).

Sooo.... I guess I'm looking for the forum to either let me know whether this is a widespread problem or just a one-off with my particular unit.

I now have multiple units. They all have the same problem.

Hi, thanks for this information. I received a TFT screen and had the same problem. As you stated, swapping source and drain did the trick. Additionally, I'm using an Arduino Due with PWM outputs of 3.3V. If the power supply used for the TFT is 5V, then using a first stage BJT connected to the 5V power rail is mandatory (3.3V is not enough to block T1). The BJT is not necesary with a TFT power supply of 3.3V.

Thanks for the info DougT, I also bought one and was wondering why the backlight wouldn’t dim (actually mine dims a little when taking the BL input high, as you said). I’ll try reworking it with an nmos,

Cheers

Ray

Just to confirm that swapping out the onboard mosfet with an n channel (i used an IRLML6344TRPBF), makes the backlight behave as hoped - 0V turns the LEDs off, 5V gives full brightness, and it's controllable by PWM