driving electro-mechanical 7 segment displays - Page 1
Forum section: electronics / projects

author: ice595
author_rank: Regular Contributor
timestamp: March 08, 2020, 05:46:06 pm
content: Hi guys, I got a bunch of electro-mechanical 7 segment displays for Christmas and now I am trying make a clock out of them.At first I thought I could just drive them like LEDs but it turns out they need a 12V 250mA pulse to flip each segment and to un-flip them I need to reverse the current pulse polarity.So I would need something that can source AND sink current so the good'ol ULN2003A is out of the question.High current op amps are too expensive, same with H-Bridge ICs.SPDT SSRs are pretty much non-existent, but miniature mechanical relays like the RM699 series go for about $2 a pop on ebay.Theres gotta be an easier/cheaper way of multiplexing them. any thoughts?

author: SiliconWizard
author_rank: Super Contributor
timestamp: March 08, 2020, 06:11:16 pm
content: If you need to reverse the polarity, the common approach is to use a H-bridge. You can either find them all integrated, or implement them with discrete transistors. If the integrated option is too expensive, use cheap MOSFETs (you'll need 4 per segment - a lot to put on a circuit, but can still be pretty cheap.)If it's acceptable to first toggle off all segments before changing the full digit, you may be able to find a trick to reduce the number of required transistors.

author: FenTiger
author_rank: Regular Contributor
timestamp: March 08, 2020, 06:23:51 pm
content: Can you get away with only 2 transistors per segment if you use a split +12/-12 volt supply?That is, tie one end to 0V and put a half bridge on the other end.

author: ice595
author_rank: Regular Contributor
timestamp: March 08, 2020, 07:03:31 pm
content: ok I just thought of another idea, and I think it can't possibly get any simpler.I could drive 7x6 matrix with only 13 transistors and only 1 supply rail. (in theory at least)since these displays only need a pulse (<100ms) to change state and doesn't require any power when idlingI just need to make sure those transistors aren't switched on for any longer than necessary, maybe a capacitor in series with the base so that even if my microcontroller crashes it won't set my house on fire.

author: Prehistoricman
author_rank: Regular Contributor
timestamp: March 08, 2020, 07:49:35 pm
content: If you figure out how much current/voltage/time they take, that can inform some decisions on what circuit can and can't work.I'd have thought that making an H-bridge of SOT-23 MOSFETs wouldn't be hard or expensive.Seehttps://www.digikey.co.uk/product-detail/en/nexperia-usa-inc/NX3008CBKS-115/1727-1278-1-ND/4386080This is a convenient combined P and N-channel MOSFETs in one package. If you want to make a clock with 6 numbers, that would require 84 of these. It's cheaper to buy 100 and I think it's a pretty reasonable price.

author: TomS_
author_rank: Frequent Contributor
timestamp: March 08, 2020, 08:07:14 pm
content: Quote from: ice595 on March 08, 2020, 07:03:31 pmThis would probably produce a nice effect where you can see the display being progressively updated. Worth a try I think.

author: ArthurDent
author_rank: Super Contributor
timestamp: March 08, 2020, 10:01:32 pm
content: I have a 'pinclock' like the one shown below. It uses solenoids to either push the pins for the 7-segment displays out or to retract them to display the time. The idea is somewhat similar to the display you have.

author: Zero999
author_rank: Super Contributor
timestamp: March 08, 2020, 10:33:15 pm
content: Quote from: ice595 on March 08, 2020, 07:03:31 pmHow about using high value resistors, with reasonably large capacitors in parallel? That would give enough kick to activate the solenoid, yet limit the continuous current to a safe level.

author: floobydust
author_rank: Super Contributor
timestamp: March 08, 2020, 10:45:36 pm
content: You can stagger the segment pulses in time, so pulsing only one segment then the next and so on- instead of hitting them all at once. This lowers the peak current needed.I would look at old flip-dot display driver circuits. We used to use capacitive-discharge and not need such a large power supply and no smoke if a driver shorted.

author: ice595
author_rank: Regular Contributor
timestamp: March 08, 2020, 11:36:08 pm
content: ok yeah, I will definitely experiment with the 13 transistor driver design.Driving all 84 transistors in a discrete H-bridge design will be a nightmare in itself let alone the PCB layout.

author: Prehistoricman
author_rank: Regular Contributor
timestamp: March 09, 2020, 01:28:38 pm
content: There are these oddly cheap H-bridge drivers:https://www.digikey.co.uk/product-detail/en/akm-semiconductor-inc/AP1017AEN/AP1017AENCT-ND/5822594No fancy features here (unlike the 10x more expensive ST parts). Unfortunately it would still be hard to route (good point) as there's 2 signals per driver to control.