SparkFun Forums

[MarkS]

I'm making a small three digit counter, just for the fun of it. We use a small mechanical counter at work and I thought I could do that better.

The 4026 looks like the best way to implement this as I want to avoid a microcontroller. The question I have is with the 7-segment driver circuit in the datasheet (pg 7): http://www.bucek.name/pdf/4026.pdf

In my case, that means 21 transistors and 42 resistors. That would be an excessive amount, even if I were using surface mount devices and I'm not! Since I will be driving both the counters and displays from the same 5 volt source, wouldn't a simple current limiting resistor suffice? I can understand using transistors if there was a voltage difference, but that is not the case.

[enggricha]

Just some current limiting resistors will do fine. Some thing like a 470 Ohms or so. U dont need the transistors.

[Mee_n_Mac]

The problem I see is that the output current source/sink spec is only about 1 mA (@ 5V). That's going to be pretty dim driving a normal LED, which generally is in the 10-20 mA range. That's why the typical circuits all show transistor "drivers". Instead of all the transistors and resistors you could use an LED driver IC, something in the ULN series is pretty common. You need to determine if your display is common anode or common cathode and make sure any undesired logic inversion doesn't screw things up.

[MarkS]

I wasn't using the correct search terms in Google. The only 4026 chip available is CMOS and it cannot drive the LEDs directly. If there was a TTL version available, that wouldn't be an issue. I'm going to use three ULN2803A chips in place of the 21 transistors.

Thanks for the help!

[lyndon]

I bought a couple of these and they work nicely for displays. Can't beat the price. Throw a small Arduino into your project (it comes with arduino library code) and you're off.

Actually these from Sparkfun are probably even easier to use since they have an AVR built in and you can reprogram it to count.