I have an existing circuit using 595 shift registers that I have used previously with led’s and it works well.
I would like to modify it so that I can control the segments of a 7 segment display. I have the sketch done and tested with led’s.
The display I’m going to use has 8 led’s per segment (only 2 on the dp) so the 595’s aren’t going to be up to supplying the 7.4v required for each segment.
The display is a Kingbright SC40-19SRWA which is a common cathode device.
I have fabricated pcb’s from the last project I did so I don’t want to change to another shift register like the TPIC6b595.
What is the easiest way of implementing a solution? I was thinking of transistors on the 595 outputs but I’m not 100% on how to do this.
I’ve attached the schematic and the datasheet.
Thanks in advance
7 seg led.PDF (242 KB)
Easiest way is to use the right parts to start. Common anode would have been so much easier to work with.
Needs 7.4 to 10V. 74HC595 can't do that, will blow the outputs.
Unfortunately you bought a Common Cathode part, so you have to switch +7.5V to the anodes.
One method would be to put ULN2803 on the output of 595, have it sink the anode current when you want a segment off, and let the LEDs sink it when you a segment on.
Bit power hungry that way of course, with 7.5V/20mA running all the time.
Other way is to get PNP, or P-channel MOSFETs, and let them be the current source to the anode.
595 is still not the right choice as the Base or Gate of the part you select needs to be able to go to 7.5V to turn the transistor off.
You might look into Allegro's UDN2981 as an package of PNP drivers for source current. You can control them with a 5V signal, but they lose ~1.7V across the output (Vce), so you're supply needs to be higher.
I am waiting on delivery of a similar 12 digit driver board, but using 83 cent TPIC6B595's to drive common anode LEDs, thus I don't need two sets of chips to control the higher voltage parts.
That tutorial is great for Common Anode parts - not much help for Common Cathode parts.
Thanks for the replies.
I had the 7 segment displays already so was trying to use them rather than getting new ones but I see you point about starting with the right parts for the job.
I have had a look at the UDN2981 datasheet and this looks like a neat solution given the existing hardware.
Am I right in thinking I just need to supply the vS pin (pin 9) with the higher voltage and then connect up the 595 outputs to the input (pins 1-8) and the outputs to the 7 seg (pins 11-18).
I was thinking about using 9v as the supply will this be high enough?
58047.pdf (542 KB)
Yes, Vs to +V, Gnd to Gnd. HC595 drives 1-8, 18-11 go to the anode.
9V would be marginal. If the LED or the '2981 was anything past typical, you'd be out of volts.
Thanks for that I can use 12v as the supply.
Thanks again for all your help.