Hacking signal to LCD to control DC Motor

Thread Starter

MisterLevy

Joined Aug 4, 2018
5
Hi.

I have a 9V Laser Line Receiver which has two identical Custom Monochrome Glass LCD's on each side. They both have up and down arrows which indicate when the receiver should be moved up or down.

I have worked out which pin is for up and which is for down. I have connected my multimeter to what I think is a COM pin and the down pin when the down arrow is visible and it reads a fluctuating (possibly square wave) voltage between + & - 60mV (approx). The frequency reads as 33.4 Hz. When I connect the down pin to what I think is the GND pin it reads 2.5V.

I am wondering if it is possible to connect to these pins in some way and have them control the direction of a small DC motor.

Ideas I already have that may be used together or separately but I don’t know how to implement are amplifying the voltage, detecting peak voltage, motor controller, micro controller.

Any help would be much appreciated. I only have a basic understanding of electronics.

Thanks in advance.
 

danadak

Joined Mar 10, 2018
3,628
Could be a static LCD display.

You need a scope to look at signal to see whats actually going on. The 60 mV
sounds wacky.

Yes, could use a comparator if its a static display, or a pulse deetection approach
if muxed.

Regards, Dana.
 

Sensacell

Joined Jun 19, 2012
2,454
LCD's are driven by AC voltages generated by the LCD controller, DC drive damages LCD's via electro-chemical effects.
That's why you are seeing the 33 Hz signal.

You can probably detect the segment being activated by comparing the phase of the LCD backplane drive to the drive connected to the display's arrow segment.
An exclusive- OR gate would do it.

Connect an oscilloscope to the backplane and the segment drive, you will see the correlation.

Here is an excellent tutorial about LCD drive.
 
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Thread Starter

MisterLevy

Joined Aug 4, 2018
5
Could be a static LCD display.

You need a scope to look at signal to see whats actually going on. The 60 mV
sounds wacky.

Yes, could use a comparator if its a static display, or a pulse deetection approach
if muxed.

Regards, Dana.

Thank you so much Dana for your reply. You gave me some search terms for google and this is where I have ended up...

My search led me to checking the COM pin and DOWN ARROW pin voltage but in AC mode this time. It read a solid 5V AC.

My search helped me find out my LCD is Static Drive not Multiplex.

Is it possible to use these up down signals to control the direction of a small motor?

Further searching gave me the idea to use a “rectifier” to convert the 5V AC to 5V DC or less and feed the signals into an L298n which I think are usually controlled by a microcontroller and use a H-Bridge circuit to control the DC motor polarity?

Am I on the right path?
 

Thread Starter

MisterLevy

Joined Aug 4, 2018
5
LCD's are driven by AC voltages generated by the LCD controller, DC drive damages LCD's via electro-chemical effects.
That's why you are seeing the 33 Hz signal.

You can probably detect the segment being activated by comparing the phase of the LCD backplane drive to the drive connected to the display's arrow segment.

Connect an oscilloscope to the backplane and the segment drive, you will see the correlation.

Here is an excellent tutorial about LCD drive.
Thanks Sensacell. You just got your post in befor me. I’ll take a look at that video now.
 

danadak

Joined Mar 10, 2018
3,628
The 60 mV sounded to me quite low, depends on duty cycle employed,
but normally that's 1/2 for a bias of 3.3, so much larger than the 60 mV
you are seeing.

Here is a good ap note - http://ww1.microchip.com/downloads/en/AppNotes/01428a.pdf

Yes, an H bridge makes a good approach to motor control. You are
using PWM I presume to control it ? Normally you want to use dead band
capable PWMs to make sure you have no phase overlap in the bridge
vertical transistor pair (would short supply to ground, typically burn out
H Bridge).

An example of deadband, see attached. http://www.cypress.com/file/131731/download



Regards, Dana.
 

Attachments

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olphart

Joined Sep 22, 2012
78
A production object may have 2 simple LCD drive feeds of the AC: Up & Down.
I would use an active rectifier to not load the LCD driver.
Then it's a couple comparators and some logic to feed the H-bridge.
As previous: an o'scope will answer and help a Lot.
 

danadak

Joined Mar 10, 2018
3,628
If you do not have oscope, but obviously have a PC -

You can start with a PC sound card based scope for free. Will give you basically
audio range scope, spectrum analyzer, and function generator all using your
PC sound card.


https://www.zeitnitz.eu/scope_en


http://www.zelscope.com/


http://www.ledametrix.com/oscope/


http://www.virtins.com/downloads.shtml


But first build a simple circuit to protect sound card inputs so you do not
ruin from transients, overvoltage. Google "protect sound card input".


For example http://makezine.com/projects/sound-card-oscilloscope/

Regards, Dana.
 

Sensacell

Joined Jun 19, 2012
2,454
The voltage out of the display controller will probably be just 0V to - supply voltage pulses, with a high impedance.
Use a CMOS Exclusive OR gate, you don't need all the comparators and such.

The CMOS input will not load down the display driver, the output should be a buffered signal that you can use to control a motor drive.
At worst, the signal might have tiny glitches, which could be filtered out with an RC delay.

This should be relatively simple.
 
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