Contrast adjust via PWM?

[NewKleer]

Wondering if its possible to control contrast electronically via PWM (eg off a microcontroller). Designing circuit for in car display (cfa2004k low profile 20x4 display), and want user to be able to control contrast if required.

Is PWM appropriate? is contrast able to be adjusted like led backlight is via PWM? If so am i just looking at a couple of resistors (from pwm pin to ground, with contrast in middle of both)? and maybe capacitor from contrast to ground?

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[cosmicvoid]

You could do this. The PWM out would need to be filtered by a capacitor. Try using a diode (anode to PWM output) and a 470 ohm resistor in series, to a parallel combination of a 10K resistor and a 10 uF cap, to ground. That gives a 100 mS time constant, so your PWM frequency should be greater than 100 Hz. It's a poor man's D-to-A converter. Since the Vo needs to be around 1 volt, your duty cycle will be approximately 20% for typical contrast.

 

[NewKleer]

would increased frequency reduce cap size? eg 10khz for 0.1uf? also, wouldnt the diode/470 mean the cap will charge quicker than it discharges, and with 50% duty cycle, be closer to 4.4V than 2.2v? would increasing 470 decrease the max voltage?

thanks

 

[cosmicvoid]

Yes on all counts. You can leave out the diode and make the 470 ohm larger, if you don't need fast response and voltage above 2.5 volts. Seems like you already understand the circuit and its tradeoffs. Keep the PWM freq at least 10x the RC time constant, to minimize ripple.