How to drive a Small 5 * 7 Seg - Page 1
Forum section: electronics / projects

author: Monostable
author_rank: Contributor
timestamp: July 15, 2015, 03:44:03 pm
content: Hello all !My company recently threw out a lot of old electronic component and in it there were some 7 seg LCD with their associated ribbon.I'd like to include them in one of my home project to display some temperature or other stuff.I have toyed with it a bit, and apparently in order to turn on a segement I need to apply a +26V on the pin, and since there are 5 seven seg that means that there is 35 pins to drive, plus one (the ".").Do you have any IC to recommand in order to drive them ?I've though of using arrays of transistor like this, but using 36 transistors seems a bit much in order to drive a small display.

author: BennVenn
author_rank: Regular Contributor
timestamp: July 15, 2015, 09:11:53 pm
content: That looks like its LCD? If it is, you'll want to drive with an AC waveform at a much lower voltage. Or it could be LED...

author: Monostable
author_rank: Contributor
timestamp: July 16, 2015, 06:51:01 am
content: Thanks for the adviceIt doesn't seems to be LED, it's a bit too old to be anyways (it's at least 15 years old).I'm curious, with what kind of waveforme people usually drive such LCD(squarre ? and if so is it to decrease the aging of the segment ?), and around which frequency ?

author: bktemp
author_rank: Super Contributor
timestamp: July 16, 2015, 08:38:57 am
content: Quote from: Monostable on July 16, 2015, 06:51:01 amLEDs exsist for around 50 years. They are older than LCD.QuoteI count 37 pins. You show 5x7 segments + 1 dot = 36 segments. This matches perfectly. So it is a non multiplexed display. The larger pin seems to be common.The typical voltage is in the range of 3-6Vrms. The frequency is around 30-500Hz.One possible approach using only 74HC oder 4000 logic is making an oscillator with 50% duty cycle. This signal is beeing fed into the common input and into a xor gate for each segment. Depending on the other logic level applied to the other input of the xor gate, each segment either sees 0V or +/-5V. 0V switches each segment off, +/-5V on.

author: Monostable
author_rank: Contributor
timestamp: July 16, 2015, 09:21:31 am
content: Here is a more detailed picture of the LCD without the ribbon.QuoteI just tried to turn on a segment with a Function Generator, placing the common on the big pad and configured it for a 50% duty cycle 0-5V at 500Hz, and well no segment appeared.I then tried pushing the amplitude a bit, and it appeared ! (note on the picture I was touching multiple pin with my wire comming out of the Function Generator, which explain why there several segments turned on)It seems that in order to turn on a segment I need to apply an amplitude greater than 15V.

author: bktemp
author_rank: Super Contributor
timestamp: July 16, 2015, 09:59:51 am
content: Did you use 0/15V, +/-7.5V or +/-15V?

author: Monostable
author_rank: Contributor
timestamp: July 16, 2015, 10:04:30 am
content: 0/15V.I'm afraid that I will have to make a board containing a lot of transistor in order to drive it, I was hopping there would some kind of 1-3 chip solution.Also, I should had said this earlier, but those screen were designed to be mounted on Aircraft.

author: bktemp
author_rank: Super Contributor
timestamp: July 16, 2015, 10:49:52 am
content: 0/15V is wrong. You must use a voltage with no DC offset. Try +/-5V to +/-10V.For a single chip solution you could use a LCD driver like PCF8576. It allows upto 9V supply voltage and generates +/-9V at the outputs for the LCD. If +/-9V square wave is enough this is probably the best solution with the least number of parts.Another solution would be using 4055 CMOS ICs. It needs a clock for generating the ac signal and converts 4bit BCD into the 7 segment display and drives the LCD. So for 5 digits you need 5 ICs + clock generator.

author: Moshly
author_rank: Regular Contributor
timestamp: July 16, 2015, 11:02:08 am
content: This circuit looks like a good place to start ->http://www.lxdinc.com/application_notes/lcd_direct_drive_circuit

author: Monostable
author_rank: Contributor
timestamp: July 16, 2015, 11:24:21 am
content: QuoteThank you for the website, I get now why I shouldn't use a DC voltage with LCD, plus the references of the components on the website are quite interesting.So I tried the +/-9V, works like a treat, so I guess I'll go with the PCF8576 solution, all in one chip, plus it has a I²C interface which is a plus for using it with a micro.Thank you all

author: Isad
author_rank: Frequent Contributor
timestamp: July 16, 2015, 06:20:24 pm
content: Quote from: Monostable on July 16, 2015, 11:24:21 amEver consider using a uln2003 or uln2004 generally goes up to 50V.