# Frequency display

#### elecbeg

Joined Sep 7, 2009
30
Hi all,

I am fairly new to electronic design, but have worked with electronics for a couple of years. I have created a simple PWM with a 555 using a couple rotary switches, I can change the caps and resistor values over a wide range.

The prob I have is, the only way I can display the frequency is with an oscilliscope, which is not ideal. I would like to make a frequency counter to display whatever frequency is being output.

If you could give me some options on how to do this, I would be gratefull.

#### Wendy

Joined Mar 24, 2008
21,848
Many DVMs nowdays come with the feature built in. Radio Shack has one for around $70, among others. They come a lot cheaper. You can buy a full fledged freq counter for just over$100 (I checked, my source sold out).

You can build your own, but it is cheaper to buy them in this case. You can get panel meters, kits, and more for this feature.

Googling around, I found this for \$120.

What part of the world are you in?

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#### elecbeg

Joined Sep 7, 2009
30
I'm in the UK, was hopefully looking to build my own, the more things I can build, the more I will understand how they work.

I can read schematic diagrams, if you happen to have one that would work for what I need it for, or could provide a link to somewhere that could show me how to make one, I would be gratefull.

Preff with a how and why it works that way, if not, just the schematic would be more than usefull.

#### davebee

Joined Oct 22, 2008
539
It's not too hard to make a simple frequency counter from discrete chips.

Wire up two chains of cascaded digital counter chips, like the 74LS160.

Make a stable crystal oscillator.

One chain of counters divides the oscillator output down to a slow value, like 1 pulse per two seconds, so it is high for exactly one second.

The other chain connects to a series of parallel loading latch chips.
The latch outputs feed a display of some sort, maybe 7-digit LEDs driven by 74LS47 chips.

In operation:

When the gate pulse is high for one second, it enables an AND gate.

Your signal under test passes through the AND gate into the chain
of counters attached to the latches.

As soon as the AND gate turns off, pulse a latch to capture the count
that has accumulated, then clear the accumulated count.

The captured count will be displayed on the LEDs.

Repeat

I've glossed over some of the details, but this is enough to make a basic counter.

Depending on the chips you use, you might run into high-frequency limits pretty quickly.

It's a little tricky but not too difficult to make the logic that pulses the count capture chips at the correct time, then clears the counters; you can use several cascaded one-shots like the 74LS123 to generate a sequence of pulses when triggered.

If you want to measure analog input then you'll need to digitize the signal; a 74LS14 can do this. The analog signal may need amplifying or limiting, depending on its strength.

An exactly one-tenth second or one-second on time is handy if you want the count to display frequency exactly.

This is all from long-ago memory, so I may have gotten some of the chip numbers wrong, but I think this is generally an accurate description.

Many years ago, I made a digital frequency display for my tube-type shortwave radio in this way. The radio's local oscillator ran 455 kHz greater than the received signal, so instead of clearing the counters, they were pre-loaded with the correct value so they'd count 455,000 up to zero, then continue to count until they displayed the exact tuned frequency. It worked!

#### elecbeg

Joined Sep 7, 2009
30
Thank you for both your comments, I now have something to work from to make it work.
I just needed a push in the right direction, now I can go off and research all the little bits to make it work.

Cheers.

#### Wendy

Joined Mar 24, 2008
21,848
The core to any freq counter is, well, the counter. If you come up with a simple counter with an LED readout you are more than 70% there.

#### THE_RB

Joined Feb 11, 2008
5,430
You can make a reasonably accurate analog freq counter (or tacho) using a 555 timer as a monostable, with the output fed to a panel meter (moving coil meter). You can put it together in an hour for a few bucks and a meter. You can even use an old multimeter for the case and meter, and use the rotary switch as a "range" switch.

Just remember to run the 555 from a regulated voltage like +5v from a 7805 regulator.

#### k7elp60

Joined Nov 4, 2008
555
Here is a schematic from an old Radio Shack publication. The IC's are still available.

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#### Wendy

Joined Mar 24, 2008
21,848
So where are the IC's available?

#### k7elp60

Joined Nov 4, 2008
555
So where are the IC's available?
Bill,
Both Digikey and Mouser list the MC14553BCPG and the MC14545BCPG, and the Mouser lists the CD4011BEE4, and Digikey lists the CD4011BCN.
Ned

#### KMoffett

Joined Dec 19, 2007
2,770
Confusing reigns! Because there was originally no image in your post, I and Bill assumed you were talking about the IC that rjenkins and I were discussing. I think.

Ken

#### k7elp60

Joined Nov 4, 2008
555
I am sorry for any confusion that my posts of caused on this thread. When I reviewed my post prior to submitting it the diagram was there.