Hi I've been asked to construct an lcd usb controller for an art project. The lcd am using is ST7066. It's spec sheet has told me that the LED. I'm using a 10K pot to adjust the contrast. The 10K pot I have can handle 0.5W power.

[power supply (logic)] vdd 0.3(min)

[power supply (LCD)] V0 Vdd - 13.5 Vdd + 0.3

In my case VDD = 5V

The led forward current is 240ma. At 5V 240ma equals to 1.2W of power so does that mean my pot needs to be able to withstand 1.2W?

The reason I ask is the last time I used a pot, it set on fire as soon as I turned on the circuit. There was no bridges in my soldering so I can only assume I overloaded it.

Correspond to high speed MPU bus interface -- 2 MHz (when VCC = 5V)

Also it's going to be connected to an ATMEGA16-16PU. The spec for the lcd says the logic runs at 2MHZ. So does that mean my avr must also run 2MHZ?


  • 2
    \$\begingroup\$ Can you post your schematic? It sounds like you're approaching this wrong. You want to adjust the contrast by changing the voltage on the $V_o$ pin, which will have a very high impedance (>100k). If your pot is wired correctly, you'll always have $V/R=5\mbox{ V}/10\mbox{ k}{\Omega}=0.2\mbox{ mA}$ of current, or 1 mW of power. \$\endgroup\$ Commented Mar 22, 2011 at 23:22

1 Answer 1


PWM using an output from your MCU would be a much better way of controlling the brightness of the LEDs than using a pot connected directly to them. You could read the pot value with an ADC, and use that to control the PWM. You will need a suitable driver, of course, such as a MOSFET.

The AVR you are using isn't suitable for USB; you need one of the USB AVRs, such as the AT90USB162.

The AVR can run at any speed, but it needs a fast clock to handle USB comms. Use delays to slow things down for the LCD.

  • \$\begingroup\$ This is a character LCD, which has several LEDs behind the screen, and, if it's like the other models I've used, the termination resistor is integrated. The datasheet lists 240mA as typical, not maximum. \$\endgroup\$ Commented Mar 22, 2011 at 23:29
  • \$\begingroup\$ Sick! Thanks reemrevnivek. I just thought because it's adjusting the led it would pull 240ma. Oviously I wired it wrong causing it o set on fire lool. It does make sense now. Thanks for the explanination!!! \$\endgroup\$
    – Ageis
    Commented Mar 22, 2011 at 23:47
  • \$\begingroup\$ if the the led typically uses 240mA of current, does that mean I can supply it 240mA and it be fine? I need to know since my Power source is a 9V battery. So need to work out the resistence needed to supply 5V – Ageis 15 mins ago. [link]img189.imageshack.us/i/lcdl.gif \$\endgroup\$
    – Ageis
    Commented Mar 23, 2011 at 0:57
  • \$\begingroup\$ The avr has a maximum current on vcc and gnd of 200ma hence the resistor \$\endgroup\$
    – Ageis
    Commented Mar 23, 2011 at 1:01

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