Help with a digital frequency meter

[Bosinis]

Hello everyone, ive been reading some threads over the forum and i tought this was the perfect place to learn more about eletronics and to help with some doubts i have.
It was proposed to make a frequency meter using the following components:
74ls47
74ls273
74ls193
ampop - zero detector
ne555 - monostable
some inverters to delay the clock over the 193 and 273

So far i got this done:

I got in to the following problems, from the right to left the display counts but it apears the hex code after the 9 digit and adding the ampop with the 555 to detect the frequency and show on the display it only shows me trash over the display and not the frequency i added on the signal generator.
I would apreciate if anyone could help me with this since ive been going around this for a while and i need it done by the end of the year. NOTE: im new to the eletronics but im willing to learn and absorb any knowlege you guys could have.
See you soon.

 

[ericgibbs]

hi Bosinis,
You should be using 74LS192 IC' which are BCD ,, not 74LS193 which are binary.
Eric

 

[audioguru]

The LM741 design is 46 years old and it performs poorly above 9kHz.
With your plus and minus 12V supply its output goes to about +11V and to -11V which will COMPLETELY DESTROY the 74LS08.

The NE555 is not connected properly as an oscillator. Its output IS NOT pin 2.

The 74LS48 LED drivers have low output currents so the display will be dim.

 

[MrAl]

Hello there,

I agree that there are a number of problems here.

The 741 severely limits the usable input frequency to be measured. Start with just a gate for digital signals and add a high speed op amp for lower level signals perhaps later on once everything else is working right.

Also, is the "TR" of the 555 actually an input? I would think the output that the gate signal should come from the output of the 555. Also, depending on the accuracy, the 555 is not the best choice for a gate signal oscillator you probably want to use a crystal oscillator for that, but yes that can come later on once you get everything else working properly.

I like the idea of the little delay circuit to delay the latching pulse. That gives the counters time to settle.

As Eric says, you should be using the LS192 counters not the LS193 because you want a BCD output not a binary output. However, if you are going to bother using synchronous counters then you might as well also CONNECT them as synchronous counters. The way you have the connected now looks like you have them connected as asynchronous counters which is not as good because the last bit takes N delay periods to flip, where N is the number of flip flops and this delay can be significant. Connecting them as synchronous counters means all the output bits change at very nearly the same time which means the latches can catch the count faster (less delay required). They dont function as perfectly synchronous counters this way, but are much better than ripple carry counters which do require that N bit delay. With the synchronous connections the delay is only an N package delay.

 

[ronsimpson]

Do you need resistors to limit the LED current?

Data sheet attached for a CD4026 counter 7-segment decoder. I think there are other versions where the counter and decoder are in one part.

 

[audioguru]

Do you need resistors to limit the LED current?

The 74LS48 has 2k ohms output pullup resistors so the LED currents will be a whopping 1.5mA.

 

[KeepItSimpleStupid]

You sort of need three things here:

1) A counter to start, that you can stop, start, latch and gate and zero.
2) Some sort of signal conditioning. Simplest schmidt trigger.

So, once you can count events, you need a timebase.

Then you need a bit of control logic.
Rising edge - clear counters, gate ON,
Falling edge - strobe latch ON, gate off