Electronic – 4026 + 7-segment display: do I need transistors


I'm a beginner and, as an exercise, I'm building a digital clock using without a microcontroller. The main IC that I'm using is a 4026, which counts and converts do 7-segment led format.

The leds in the 7-segment glow very dimly. I'm reading the 4026 datasheet, and trying to make sense of it. The diagram (reproduced below) seem to imply – if I'm reading correctly – that I would need a transistor for each one of the leds in de 7-segment display. My display is common-cathode.


Since I'm building a digital clock, this would mean that I'd need 6*7=42 transistors, which seem like overkill for such a simple project. I also saw some similar projects on the internet that don't use the transistors.

Should I put all the transistors in the circuit? If not, then how can I make the LED glow more brightly?

Best Answer

Life can be tough. CMOS ICs from the CD4000 series, like the CD4026 can only source and sink very little current, typically 1 mA at 5 V and that will be much too little for a typical 7-segments display. So I'm afraid you'll need the transistors, especially since you're now already asking for more brightness.

But You don't necessarily need all these discrete components. A ULN2803 replaces 8 transistors, so you'll need 1 IC per display. Important note: I just read that your displays are common cathode. The ULN contains an array of NPN transistors, switching to ground, so they can only be used with common anode displays.

starblue found a high side version of the ULN2803 in the UDN2981, so this is suitable for common cathode displays. I don't know about availability; Digikey lists only two versions, both from Allegro, as as non-stock, call for price.
Inputs are active-high, so it can be driven by the same devices which would control NPN transistors or the ULN2803.
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Alternatively you may use low-power Schottky ICs, but then you'll need the counter and the display decoder separately; I don't think the combination like the 4026 exists in LS-TTL. The 74LS90 is a decimal counter, and the 74LS247 a 7-segments decoder, which can drive LEDs directly. This is also an active low output IC, which means common anode:-(.

Other solutions you've seen requiring less transistors may have used a microcontroller and a multiplexed display. Then for 6 digits you'd only need 6 + 7 transistors, instead of 6 \$\times\$ 7.