Problema contrasto LCD

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mi trovo ad alimentare un display lcd hd44780 con una tensione variabile tra 5v e 4 volts o meno (+5 da un lm7805, oppure da batterie nimh 4.8v, che naturalmente si scaricano e la tensione va giu), ed il contrasto o lo regolo per 5 volts, oppure per tensioni inferiori....ma in questo modo è spesso poco leggibile!
come posso fare per ottenere una regolazione piu o meno costante? grazie!

io ho scoperto che in realtà il contrasto è regolabile con un trimmer usato come reostato, cioè una resistenza variabile tra il pin del contrasto e massa; in pratica collego il centrale del trimmer sul pin del display ed un dei laterali del trimmer su gnd; l'altro capo lo lascio libero, non lo collego all'alimentazione. prova così.


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