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Forum Index : Microcontroller and PC projects : Alternate transistor for Micromite LCD Backpack
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| al18 Senior Member  Joined: 06/07/2019 Location: United StatesPosts: 231 |
The P Channel MOSFET in the BOM is a DMP2215L, which is not a stocked item at Mouser. I ordered a Vishay SQ2351ES-T1_BE3, which I thought was a suitable replacement, but the backlight didn't switch on. I'll admit I'm not experienced at specifying transistors. I don't know if the problem was the part or using a soldering iron on. SOT-23 package. Can someone look at the data sheets and comment? Thanks |
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Mixtel90 Guru  Joined: 05/10/2019 Location: United KingdomPosts: 8298 |
I'm assuming this is the original Micromite backpack... If your display is one of the later ones (Version 1.1 onward) then you can omit the 1k and 10k resistors and both mosfets. The later displays have an on-board transistor (Q1), associated base resistor (R5) and series resistor (R6) to control the backlight. All you need to do is link the LED pin of the ILI9341 to pin 26 of the micromite. You can then set 26 high to switch the backlight on or low to turn it off. Alternatively you can use 26 for PWM to control the backlight brightness. Back to your question - I think those two mosfets are close enough to work in the original circuit but, as I pointed out, you very likely don't need either - look for Q1, R5 and R6 on your display. Mick Zilog Inside! nascom.info for Nascom & Gemini Preliminary MMBasic docs & my PCB designs |
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| al18 Senior Member Joined: 06/07/2019 Location: United StatesPosts: 231 |
I'm using the Version 2 of the LCD backpack at https://geoffg.net/MicromiteBackpack.html Pin 26 uses PWM, along with a N channel MOSFET and 10k resistor, along with a P channel MOSFET and 1k resistor. After I couldn't get this setup to work, I soldered a 47 ohm current limiting resistor at the 100 ohm pot location and unsoldered the transistor. This works, but not I'm not able to adjust the backlight brightness. I was hoping to build another with variable backlight. |
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| al18 Senior Member Joined: 06/07/2019 Location: United StatesPosts: 231 |
Mick, Sorry I misread your post. My 2.8 inch display is Version 1.2 with Q1, R5 and R6. You want me to not populate the two mosfets and two resistors and instead run a jumper wire from the empty 10k resistor hole labeled PWMBL to the Manual BL hole identified with circle-slash? |
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| Mixtel90 Guru Joined: 05/10/2019 Location: United KingdomPosts: 8298 |
With that display you don't need the driver circuit at all. Just leave off both mosfets and both resistors. Then run a wire from LED on the display to pin 26. The display has its own driver. Alternatively, as you have found out, you can fit a resistor but the value makes no difference with this display as the transistor has a high input resistance - the LED pin isn't carrying the actual LED current. Mick Zilog Inside! nascom.info for Nascom & Gemini Preliminary MMBasic docs & my PCB designs |
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