# Building a frequency meter...Input anyone?

#### dyeraaron

Joined Oct 27, 2008
57
Hello...My name is Aaron and I'm a student @ ITT and I was just hoping for a little bit of input regarding a project......Ok, so We have to Build a frequency meter with LEDs for the BCD code also a 2 digit 7 segment display using a step down Transformer 120 VAC to 12 VAC for the input. I have 2 7447 Decoder drivers leading into my segment displays.....I was thinking of using 2 7490 Decade counters for my circuit going to my decoders....the max. Hz input will be 60 Hz, I also have LEDs paralleled between the counters and Decoders.......My question is: can you tie the 7490 counters together to have 2 outputs of 4 (1 from each) I have a 20 MHZ crystal for my 7490s as well....is that too much or OK.....

#### beenthere

Joined Apr 20, 2004
15,819
What BCD code are the LED's supposed to display? That part is unclear.

How are you going to use the 20 MHz crystal? Can you post up a circuit?

#### dyeraaron

Joined Oct 27, 2008
57
The BCD code max would be 60 (Hz) which I know my last 2 LEDs wouldn't ever be on due to 1 2 4 8 16 32 64 128 I'm visioning this in my head....like if my Frequency input is lets say 34 Hz, then my 32 and 2 are lit and my segment displays show 3 and 4 ......Looking at the data sheet for a 7490...there are two inputs which are for a clock and I've seen numerous schematics having a crystal in its circuit, me not knowing which crystal to use and our breadboards only accept few through holes....the best I could find is a 20 MHZ.

#### beenthere

Joined Apr 20, 2004
15,819
A frequency counter normally has an internal time base so it can gate in events for accurate periods of time. For low frequencies, 1 second or even 10 seconds would be good. If you want to use that 20 MHz crystal, you would need to divide it down in frequency to control the gate period. That's a number of counters.

You are using 7490 counters for the event counter. Those are "original" TTL devices. They are hard to find anymore. Many of the logic functions in that family are no longer in any of the catalogs. That means you may have to use 74LSxx chips for some functions. That may cause some fanout problems.

Now would be a good time to go from just talking about the freq counter to actually designing it on paper. You get to learn a lot by doing so. And you can post up the circuit so we can see if there are problems.

#### dyeraaron

Joined Oct 27, 2008
57
The Base resistors for my NPNs are 330 Ohms and My Caps coming from My Crystal are 47 pF. Iknow my Frequency input will give me my Highs and Lows for ABCD, but each counter has 4 inputs....Do I jumper each one?

#### Attachments

• 54.5 KB Views: 31