All I know is that it’s supposed to be AC, so one could give it a proper sinewave I guess, but I don’t doubt that this works and am just wondering what the ideal waveform is for it
– The 555 won’t oscillate at all when wired like that.
– Shorting pin 7 on the 555 to the positive rail is not recommended.
– It’s important to avoid DC on the LCD, for longevity. A 555 can be wired to give approximately 50% duty cycle (although it isn’t here). The 555 (unless you’ve chosen a CMOS one) doesn’t give symmetric drive either. Most designs use some sort of digital circuit (e.g. a flip flop wired as a divide by 2) with a CMOS output that can give exactly 50% duty cycle (EDIT: and the output voltage averages to exactly half the supply voltage).
– This type of LCD drive circuit is only suited to “direct drive” LCDs. Multiplexed LCDs (with > 1 backplane) require multiple drive voltages and can’t be driven with simple digital logic. Active matrix LCDs (e.g. TFT) require something else again.
I suppose that’s one method of doing it.
All I know is that it’s supposed to be AC, so one could give it a proper sinewave I guess, but I don’t doubt that this works and am just wondering what the ideal waveform is for it
Note only one resistor on the 555. This is to give a symmetrical 60Hz square wave.
The 1μF capacitor can be either electrolytic or ceramic.
You can use either 74LS48 or ‘248 to drive the LCD, depending if you want tailed 6 and 9.
Comments:
– The 555 won’t oscillate at all when wired like that.
– Shorting pin 7 on the 555 to the positive rail is not recommended.
– It’s important to avoid DC on the LCD, for longevity. A 555 can be wired to give approximately 50% duty cycle (although it isn’t here). The 555 (unless you’ve chosen a CMOS one) doesn’t give symmetric drive either. Most designs use some sort of digital circuit (e.g. a flip flop wired as a divide by 2) with a CMOS output that can give exactly 50% duty cycle (EDIT: and the output voltage averages to exactly half the supply voltage).
– This type of LCD drive circuit is only suited to “direct drive” LCDs. Multiplexed LCDs (with > 1 backplane) require multiple drive voltages and can’t be driven with simple digital logic. Active matrix LCDs (e.g. TFT) require something else again.
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