0
\$\begingroup\$

Inside of a calculator. Image from: http://www.explainthatstuff.com/calculators.html

Hello, so I'm trying to use a calculator's LCD for a project of mine and I want to put a current through it. However, the LCD is powered by the "sheet of plastic" shown in the picture, circled in red. Does anyone know what that "sheet of plastic" is and how does it work? In addition, how can I put a current through it with just two wires?

Thanks!

\$\endgroup\$
5
\$\begingroup\$

That is a thermally bonded ribbon circuit.

You can remove it from the existing PCB with a hair dryer, though you'll probably destroy the first one you try. Also be careful the hair dryer blast does not hit the plastic housing if you have plans for that - they can warp at surprisingly low temperature.

You can then re-attach the ribbon to a custom PCB or breakout, using something like a solder station with a large tip set to absurdly low temperature (try it against the edge of the ribbon to make sure it doesn't melt it) or possibly a small sealing iron like used for model airplane covering; likely the factory uses some sort of heated bar press.

An LCD display requires an AC voltage between a select and a segment pin. If you look around, you'll probably find some app notes on driving them from an MCU by leveraging the 3-state capability. Essentially you toggle the select and the segments you want, feeding them a square wave in opposite polarity from each other, select in same polarity with the segments you don't want, and assign the other selects as inputs. Then you cycle to the next select and drive the segments that should be on for that, etc. A temporary firmware with serial input can let you probe around the possibilities and find the mapping.

It is possible to do some crude tests with a single flashlight cell and some wires, but with DC excitation the segments will only pulse briefly before fading, and continued application of DC is undesirable as there are electrochemical degradation concerns. Setting up an MCU to pulse two pins in opposition would be a better test source.

\$\endgroup\$
3
\$\begingroup\$

The 'sheet of plastic' is a flex-circuit. Basically a very thin bendable PCB. It is probably connected at each end via a "zebra-strip" aka "Elastomeric connector"

If you want to try applying voltages to the LCD use a fine point probe to touch the connection points on it. It will however be wired up internally as a matrix and may require specific voltage patterns, NOT DC, so figuring out how it works may be harder than you imagine.

\$\endgroup\$
  • 1
    \$\begingroup\$ No zebra strip on that one. Also, DC voltages are inappropriate. \$\endgroup\$ – Chris Stratton Mar 27 '17 at 16:37
  • \$\begingroup\$ @ChrisStratton, ya hard to see how the flex is attached. Could be just fold and compress too. \$\endgroup\$ – Trevor_G Mar 27 '17 at 16:39

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.