World's Simplest Frequency Counter

Hazmatt_(The Underdog), Thu Nov 23 2006, 02:45AM

That's right. I have made a frequency counter so simple that anyone can put it together (or so I hope).

I needed a tachometer for a motor, and spent 3 weeks going through counter circuits that just did not work. Some of the circuits were untested designs, and some fed pulses into monostables that were too short for the monostable to trigger.

I have done away with monostable triggering, and I have designed a rock solid MSI Frequency counter that really works!

The first step is to setup the counter so that we can see if it will count up, reset and latch the displays. Here is an example using 74XX family logic gates to accomplish this.
I am showing the pinout for the display so you can hook one up. If you have a different type of display you can figure out the segments by using a continuity tester. The resistors should be about 220-330R each. Once you get the display hooked up to the 7447, test your display to see if it decodes properly by jumpering the BCD inputs to logic 1 or 0.

The next step is to get the latch connected. The latch is a quad D flip-flop that stores the old count, or is transparent depending on the latch enable bit. You should be able to jumper the BCD inputs, freeze the display, then change all the jumpers, and the number will stay until you change the latch state. How cool is that!

After this comes the decade counters. These divide the input so that you end up with decades for each digit. The D output spills over into the A input of the next counter, making a decade chain. The 7490 automatically resets itself after 9 to 0, but we need to be able to clear the count, so we trigger the master reset by pin 2. You will be able to count up while the latch is held at logic 1 and reset is at logic 0. Then you can reset the count, freeze the count, and play with it.






So the hard part is done right? Well, kind of. We need to generate the timing structure now. We need to arrange the gate, latch, and reset pulses such that we make a functional counter. I have given you the timing diagram for the sequency generator, another 7490 and a couple of gates. This is where the real 'magic' happens. This sequencer takes your standard timebase input and derives the necessary pulses to control your counter.






The last step of all of this is coming up with your standard. What standard should I use?
Well, if you needed a tachometer, you want the operator to see R.P.M right? And of course for a frequency counter you would want to see counts/ second.

I have set up the divider of 30,000 to divide my 2MHz source so I get 66.6666 Hz. This gives me my timing for the motor.

But likewise, if you wanted 100 Hz timebase for your counter, you could implement a 1MHz crystal or 10MHz crystal and divide it down in decades.



**There's a bit of a mistake on the divider chain. If you look at the last two chips it should be 12 output to 14 input. All dividers are 14 inputs from previous stage, and 12 as the output to the next stage. Hope that clears up any confustion**

This is a great little counter. It will function up to about 40MHz. AND you can add as many digits to the right as you need. Anything more then 8 digits is really un-necessary, but I thought I might like to make a 20 digit counter someday for fun.

The next evolution of this counter is to take it from overly bulky 74XX logic and go full CMOS.
The CD 4026 Decade/latch/decoder replaces the 7490/7475/7447 ALL IN ONE!!!!! How cool is that!
So that will reduce the counter module to 4 chips and 4 displays right off.
Then if you take the 4017 and derive the timing pulses from it, any 3 you like, you have again your timing for latch, reset, and gate. So that will reduce the 'magic gating' even further.
And still yet, if you can find a crystal that matches up with a large 2^n divider chip, you could derive your timebase in the smallest number of chips possible.


A word of caution to builders: This circuit performs perfectly, but be careful about power supply noise. This may be an issue in your circuit. If you're like me, and you built your own regulated power supply, be aware that it may be switching at very high frequencies well in the hundreds of KHz. So if you get some strange results, like the display jitters quickly, that means you are seeing the result of high frequency rail noise.
It took me a while to track my noise down, and it was very high frequency. So I switched to a simple regulated supply unit by Interplex Enterprises. They make a little regulated supply that plugs into breadboards. This supply uses a 7805 and worked fine.
My supply still needs some rework, and that will come later.



Have fun.

Some more old pics showing it working:

Pics of my Tachometer for my DCRSGC