I started the layout of the controller board last night, but needed to sleep on it a bit — so I could figure out how I wanted to handle start/stop/reset. This morning I had a better idea of how I’d handle it, and started wiring up the board. Here is the end result:

It is very similar to the board I built for the LED Christmas Tree. A 555 in the upper left handles the “ticks” and three CD4026 chips handle the 7 segment displays. You need one chip for each digit. Key differences between the two boards:

• One of the fixed resistors for the 555 timer has been replaced with a 100 K potentiometer. This will allow me to fine tune the ticks to get as close to a second as possible. The 10uF, 10K, 100K pot configuration gives me from about a quarter second up to about 2 seconds of delay / range.
• On the Christmas Tree the Clock Inhibit (pin 2) and Reset (pin 15) are permanently tied to ground. This means that the CD4026s always “listen” for 555 clock ticks and never reset to zero. Since I’m designing a timer, I want to be able to reset to zero easily (without power cycling) and also be able to “hold” the count. So, I bring the Clock Inhibit and Reset pins up to a Molex connector (upper right). This will be connected to a small controller box which will have the appropriate switches.
• This board uses cascading digits, so the chips are connected to easy other to support that. The chain goes like this: tick from 555 goes to CD4026 #1 Clock. #1 Carry Out goes to #2 Clock. #2 Carry out goes to #3 Clock. If you had more digits you would continue the chain (#3 Carry Out goes to #4 Clock). The “Carry Out” ticks whenever the chip rolls over to zero.

Here is what the setup looks like with the LEDs plugged into their Molex connectors. Have I told you how much I love Molex connectors?

To test the unit, I installed the 555 in the socket along with CD4026 #1. Then I plugged in the LED wires for #1. I figured worst-case I’d burn out just those two chips. I powered it on — and — nothing! No fear, though. I just forgot to plug in the LED ground / cathode wire. Whoops! Once that was plugged in, all was well.

Note that I also had to (temporarily) also tie the Reset and Clock Inhibit to Ground (negative) so that it would “listen” to ticks and not keep resetting.

Once the one digit was done, I followed with the next, and then the third. The third was tricky because I couldn’t get the chip into the socket. After almost ruining one chip, and starting to screw up another, I double-checked the socket. Sure enough, one of the wipers in the socket was acting up. I gave it a little heat with the soldering iron, loosened a bit with a tiny screwdriver, and then it was okay. The little things will get you every time!

At this point, I have a display that cycles though 0 to 999 without a problem. Horray! It even handles the rollover cleanly (goes from 999 back to 000). No Y2K problems.

Next up, I’m going to wire in some temporary switches and see if they will work with the Reset and Clock Inhibit properly. My concern is that mechanical switches are usually pretty electrically “noisy” (they bounce between states when the contacts are close). I’m concerned that the CD4026 chips won’t like this. (Meaning that they will reset when I don’t want them to, or start/stop at will.) Worst case, I’ll wire a debounce circuit into the controller. More parts and work, but it will likely be more reliable. We’ll see.

Update: While trying to get the Reset & Clock Inhibit to work I had an “a-ha!” moment. I kept finding that triggering the Clock Inhibit would often cause the 10′s and 100′s positions to increment strangely. I realized that I really don’t need to mess with chips 2 (10′s) or 3 (100′s) since it’s the first chip that feeds them. So, for the #2 and #3 CD4026 I tied their Clock Inhibit permanently to ground. Only chip #1 is controlled by buttons. More on this in the next post. All chips do continue to have the Reset connected, because they all need to get cycled to zero at the same time.