Author Topic: driving electro-mechanical 7 segment displays  (Read 1994 times)

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Offline ice595Topic starter

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driving electro-mechanical 7 segment displays
« on: March 08, 2020, 05:46:06 pm »
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?
 

Online SiliconWizard

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Re: driving electro-mechanical 7 segment displays
« Reply #1 on: March 08, 2020, 06:11:16 pm »
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.
 

Offline FenTiger

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Re: driving electro-mechanical 7 segment displays
« Reply #2 on: March 08, 2020, 06:23:51 pm »
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.
 

Offline ice595Topic starter

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Re: driving electro-mechanical 7 segment displays
« Reply #3 on: March 08, 2020, 07:03:31 pm »
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 idling
I 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.
 
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Offline Prehistoricman

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Re: driving electro-mechanical 7 segment displays
« Reply #4 on: March 08, 2020, 07:49:35 pm »
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.

See https://www.digikey.co.uk/product-detail/en/nexperia-usa-inc/NX3008CBKS-115/1727-1278-1-ND/4386080
This 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.

Offline TomS_

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Re: driving electro-mechanical 7 segment displays
« Reply #5 on: March 08, 2020, 08:07:14 pm »
ok I just thought of another idea, and I think it can't possibly get any simpler.
This would probably produce a nice effect where you can see the display being progressively updated. Worth a try I think.
 

Offline ArthurDent

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Re: driving electro-mechanical 7 segment displays
« Reply #6 on: March 08, 2020, 10:01:32 pm »
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.



 

Online Zero999

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Re: driving electro-mechanical 7 segment displays
« Reply #7 on: March 08, 2020, 10:33:15 pm »
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 idling
I 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.
How 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.
 

Offline floobydust

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Re: driving electro-mechanical 7 segment displays
« Reply #8 on: March 08, 2020, 10:45:36 pm »
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.

 

Offline ice595Topic starter

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Re: driving electro-mechanical 7 segment displays
« Reply #9 on: March 08, 2020, 11:36:08 pm »
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.
 

Offline Prehistoricman

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Re: driving electro-mechanical 7 segment displays
« Reply #10 on: March 09, 2020, 01:28:38 pm »
There are these oddly cheap H-bridge drivers: https://www.digikey.co.uk/product-detail/en/akm-semiconductor-inc/AP1017AEN/AP1017AENCT-ND/5822594

No 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.


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