# Help with Drive and Control Circuit for a LC Plate

I am looking to make a drive circuit for a Liquid crystal plate with the ability to adjust the drive AC voltage/ / amplitude.

The hard part is that I have to have a stable +/-5V max signal with no bias to either side and a strong drive ballast of at least 3 times the peak current. Was looking at a RS232 MAX type Transever but I don't know which one to choose, I also saw people suggesting op amps, or an H bridge which could work too. I only need minimum 80Hz of square wave. I just don't know which avenue to try for a pure Square wave that I could switch on an off from say one pin and perhaps control amplitude from another. Let me know what you would suggest.

-Nathaniel

MorninGlory453: I am looking to make a drive circuit for a Liquid crystal plate with the ability to adjust the drive AC voltage/ / amplitude.

The hard part is that I have to have a stable +/-5V max signal with no bias to either side and a strong drive ballast of at least 3 times the peak current. Was looking at a RS232 MAX type Transever but I don't know which one to choose, I also saw people suggesting op amps, or an H bridge which could work too. I only need minimum 80Hz of square wave. I just don't know which avenue to try for a pure Square wave that I could switch on an off from say one pin and perhaps control amplitude from another. Let me know what you would suggest.

-Nathaniel

Welcome to the Arduino forum. As a point of reference, all the MAX232 boards I have worked with all gave +_ 8.3 volts, or thereabouts. Makes sense since 5.0 volts is the top end of the RS-232 invalid window.

Nothing you can do directly with an Arduino will give you the -5.0 volts. Perhaps someone else can offer engineering solutions.

Paul

Are you sure about that ±5volt.
The datasheet mentions an amplitude of <=5volt (could be peak/peak, ±2.5volt).

If so, then a simple capacitor/resistor could make that from a 5volt PWM signal.
Leo…

Wawa:
Are you sure about that ±5volt.
The datasheet mentions an amplitude of <=5volt (could be peak/peak, ±2.5volt).

If so, then a simple capacitor/resistor could make that from a 5volt PWM signal.
Leo…

Later on the data sheet explains the ± requirement:

“The transmittance of the variable ND filter reacts to the RMS voltage. In order to prevent ion migration within the LC layer that might impair variable ND filter performance and lifetime, it is recommended to ensure that there is no net DC bias present in the drive signal. This is best achieved via use of one of the two AC square waveforms illustrated below. When the top alternative is used, the recommended minimum frequency is 80Hz if visual flicker is to be avoided.”.

Paul

Yes, I knew it had to be positive/negative, but OP asks for 10volt peak/peak (±5volt). A series circuit of a 10uF cap and a 10k resistor (cap to pin, resistor to ground) can give ±2.5volt across the resistor from a 5volt pin. Change the 10k resistor for a 10k lin pot, and you can adjust transparency. If you need to do this digitally, then an external DAC might be needed. Leo..

According to the device datasheet, the OP needs a [u]variable[/u] +/- 0 to 2.5 volt peak to peak square wave with a fixed frequency of 80 to 1000Hz. It is the voltage amplitude that controls light transmittance through the device, dark at 0 volts, full transmissibility at +/- 2.5 v.

Thanks everyone!

Looking at the Spec sheet they said that they had a drive amplitude of approx 5V I assumed this is only the positive component because in section 9 they show +Vd and -Vd. They really need to clarify if the amplitude is 5 V peak to peak or 5V peak amplitude ie 10V peak to peak. I'm going to ask the manufacturer.

For reference i'm building an adjustable ND time lapse module with intervalometer. Might have to control the voltage with a pot to adjust the resistance/ transparency as Wawa and Watts suggested. Maybe thinking a Digital Potentiometer to control. I'm just in trouble if i need 10V peak to peak amplitude as Paul said.

-Nathaniel

Ah, things do become clear after reading datasheets for the sixth or seventh time…

I’m fairly certain the drive requirements are 10 volts peak to peak where Vd = 5 volts and -Vd is -5.

There are several hints for this, the clearest being the Vd voltages listed in table 7. What confused me were the voltage specs in table 8.3, I thought that was the P-P value but no, it is the Vd value. It have been crystal clear had they included the definition in the table.

Update: it is indeed a 10V Peak to Peak. Planning on using a frequency controlled inverter, a H bridge analogue switch, and an opAMP and DAC to achieve this. voltage doubler needed as well to bring up the 5V rail to the necessary 10.

looking at a similar build for a LC plate driver here: http://www.diyphysics.com/wp-content/uploads/2015/10/DOLPi_Polarimetric_Camera_D_Prutchi_2015_v5.pdf

10V peak to peak only needs a 5V supply if you drive differentially.

Good point. +1.

Leaves the problem of amplitude control (if needed). Leo..

duty cycle does the amplitude surely? LCDs are low-pass filters in effect. Just clock the waveforms at several kHz and should do something useful.