Electronics > Projects, Designs, and Technical Stuff
driving electro-mechanical 7 segment displays
ice595:
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?
SiliconWizard:
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.
FenTiger:
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.
ice595:
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.
Prehistoricman:
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.
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