Home » Projects » LED stuff » Pick a number between 0-9

Pick a number between 0-9

While cleaning up the semester’s mess, I came across this pamphlet:

2013-12-18 17.04.05

I have no idea where it came from, but I looked through it, and found some interesting circuits, such as this one:

2013-12-18 17.04.27

It’s a “random number generator”. There are two main components: an oscillator circuit and a decade counter, pretty much split down the middle. About each one…


This involves the two NAND gates on the left. If you’d like, you can replace then with inverters, however, because tying the inputs of a NAND gate together produces the same function as a NOT (check: 1 NAND 1 = 0, 0 NAND 0 = 1).

Putting inverters together in a circuit with feedback creates an oscillator, like so:


And depending on the value of the resistor and capacitor, you can make it oscillate at different rates.

Decade counter

“Decade” means 10, so a decade counter counts to 10. The specific IC used here (4017) has 10 output pins that are driven high depending on the current count, instead of 4 pins that represent 0-9 in binary. Counts are advanced by a clock, so the oscillator output circuit connects to the decade counter at the clock pin, and the circuits are separated by a pushbutton.

To see what pin is being driven high, and hence see the current count, you can put a LED in series with each output pin, and the LED will light according to the count.

When the pushbutton is activated, the decade counter sees the oscillator output and will begin counting. If the oscillator is operating fast enough, seemingly “random” numbers can be produced by letting go of the pushbutton.

Because I’m on break and I just happen to have the components and ICs on hand, I built it.


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