Driving two 7-segment displays with a single shift register and BJT transistors

Question

I'm building a circuit containing 8 switches representing 8 bits in a byte. I have two single digit 7 segment displays to display the two hexadecimal characters that represent the byte value. I'm quite familiar with using 74HC595 shift registers to drive a segment display, having one register to control the segments and one to control the common cathodes.

I figured that since there is no need for a decimal point in hexadecimal values, I can leave out the second shift register and use the 8th output pin that would otherwise control the Dp segment, to toggle between the two displays.

I've come up with this circuit (left out irrelevant parts)

I know it's more of a hypothetical problem, but I know there is a delay between the moment Q1 switches on and Q2 switches off, since Q3 needs to switch too. I tried the following:

• Connect the Dp output pin of the shift register to Q3's base. Q1 was connected with Q3's emitter and Q2 to Q3's collector. This made both displays either on or off (which I could not figure out why)
• Leave out Q3 and connect segment1's cathode with Q2's base. This lights up segment2 very dim when segment1 is on, while segment1 is off when segment2 is on (that must have something to do with segment2 not providing base current to Q1 I suppose?)

Is there a way to switch only one of the displays on at the same time with the same number of transistors for both cathodes? (I can take "no" for an answer)

Answer 1

You can simply bring the segment outputs low, switch the digit, wait a bit for the cathode drivers to settle if necessary (a couple microseconds is probably enough) then correct the segment outputs for that digit. That will eliminate "ghosting".

With the HC595 that means more bytes being shifted out.

You should use 7 resistors not one. One in series with each segment output, and lose the 470 ohm resistors.

Answer 2

Here's a much better part for this job: the TLC5928. It has internal current setting, and can drive 16 LEDs directly so you don't need to multiplex, nor do you need serial resistors. It works the same as the '595, and yes you could multiplex it if that's what you want.

Here's a cheaper, non current-setting one that's 8 channels. You will need resistors.

Another way is to use an I2C expander if your host has I2C capability.