Looking for low standby current LCD 7-segment display and MCU - Page 1
Forum section: electronics / projects

author: Zipdox
author_rank: Regular Contributor
timestamp: February 12, 2023, 10:02:11 pm
content: I'm looking to build a laser tag system that uses programmable magazines for physical reloading. I need to pick a display and an MCU to build these with. I'm thinking about making a custom PCB with the MCU on it, and possible having it assembled by JLCPCB, but I'll prototype it first.So far I have these requirements in mind:Inexpensive (Would need many of these)7-segment display (only need 2 digits)Always displays ammo countMomentary switch for backlightLow operating currentSerial communication with gun while connected (UART maybe)Remembers ammo count either by:Internal EEPROMRAM with MCU always poweredI know that LCD displays don't draw any current when static, but that's not always true for the driving IC or the MCU. Using an FPGA would also be acceptable.So far I have this LCD in mind:https://www.aliexpress.com/item/1005003516468280.htmlIt has a supposed standby current of 0.4mA, which would mean it lasts 21 days on a 210mAh CR2032 battery. But it requires minimum 4.7V to operate, although this might just be for the backlight, the HT1621 can go down to 2.4V.Any recommendations for MCUs or displays?If I get an MCU with enough IO I can drive this LCD:https://buy-lcd.com/products/10ieces-2-1-2-digit-lcdanel-eds819-wide-temperature.Edit: It appears that some PIC MCUs have LCD charge pumps. I'll look into this.

author: tooki
author_rank: Super Contributor
timestamp: February 12, 2023, 11:12:01 pm
content: It is not true that LCDs require no power when static. It’s very little power, but nonzero.The displays that truly require no power when static are e-ink displays, and they could be a viable option for your application.

author: Zipdox
author_rank: Regular Contributor
timestamp: February 13, 2023, 02:12:38 am
content: I originally wanted to use the PIC16F1933 but its USART and ICSP pins overlap with LCD segment pins, so it wasn't suitable. I think I'm gonna use the aforementioned LCD and the PIC16F1937-I/PT, which has more I/O.But another problem I'll run into is the maximum I/O input voltage for the MCU. It's Vcc+0.3V, which is very risky. The button cell will output ~3V, and the MCU would be connected to 3.3V USART, which is living on the edge. If the battery drains anymore, the MCU could fail. So should I use a level shifter for this purpose. Or can I use the dirty trick of attaching input resistors and a Zener diode clamp?

author: Zipdox
author_rank: Regular Contributor
timestamp: February 13, 2023, 03:15:25 am
content: Hmmm I've been doing some research and now I'm starting to worry that level shifters might have trouble if the voltage difference between the A and B side isn't high enough. Would the TXS0102 have a problem with shifting between 3.0V and 3.3V? If the coin cell is fresh it might even have a slightly higher voltage than 3.0V.

author: tooki
author_rank: Super Contributor
timestamp: February 13, 2023, 03:46:39 am
content: Why would you have that worry? Each side of the level shifter is simply referenced to its supply voltage. (So the level shifter has two supply voltage inputs.)

author: Zipdox
author_rank: Regular Contributor
timestamp: February 13, 2023, 02:45:14 pm
content: Quote from: tooki on February 13, 2023, 03:46:39 amThere are some caveats with the LSF0102. I don't know if this applies to other shifters, I might just contact TI to ask.

author: edavid
author_rank: Super Contributor
timestamp: February 13, 2023, 03:22:44 pm
content: 1. Non-multiplexed LCDs use less power and have higher contrast at low voltage, so if possible you should avoid multiplexed parts like the first one you mentioned. They can also be driven directly by plain MCU GPIO pins, although that may cost power compared to a dedicated driver.2. Instead of using level shifters, you could have the gun power the display when it's connected. You would lose a little power in the isolation diode, but avoid the whole level shifting issue, and possibly extend batttery life.

author: Zipdox
author_rank: Regular Contributor
timestamp: February 13, 2023, 03:45:47 pm
content: Quote from: edavid on February 13, 2023, 03:22:44 pmI intend to use the other LCD I linked, which is static and has one pin for each segment. The PIC has a builtin LCD driver.Quote from: edavid on February 13, 2023, 03:22:44 pmHow would I do this? Put a Schottky diode on the battery?

author: Zipdox
author_rank: Regular Contributor
timestamp: February 14, 2023, 12:37:20 am
content: Ok I cooked something up. I chose what segment to connected to what pin of the PIC in order to make routing easier.https://oshwlab.com/zipdox/laser-tag-magazineIt's supposed to slot into this edge connector slot:https://www.lcsc.com/product-detail/Card-Edge-Connectors_WingTAT-ER10BGFBK_C5173327.html

author: james_s
author_rank: Super Contributor
timestamp: February 14, 2023, 12:42:45 am
content: Almost any microcontroller with enough pins can be made to do the job, they all have low power sleep modes these days. I know some AVRs have LCD drivers built in but I don't recall which ones. Take a look at the AVR Butterfly, it's quite old and obsolete at this point but it does just what you're proposing, directly driven LCD with a battery powered microcontroller. You should be able to scale that down using a much simpler display and lower pin count part.