Digital frequency meter circuit latches

Thread Starter

Avikaar Sonlall

Joined Mar 14, 2017
11
Hi all

I created this frequency meter circuit on Multisim 14.1. How it basically works is that as high pulses come in from a source, the counters are incremented accordingly and the 555 timer is configured such that after every second, a high pulse is sent to the reset of each counter and then the entire display is reset to 0. For example if I have an input signal of 100 Hz frequency, this circuit will count from 0 to approx. 106 and then reset.

What I need is for the value of the display to remain even after the counters are reset (i.e. with an input frequency of 100 Hz, the display should remain at 100 +- 10Hz ). I know that I have to use a latch IC but the problem is I am not familiar with latches and how to connect them. I tried using a 4543 but I could not get it to work properly.

Any help would be appreciated.avizz1.png
Thank you
 

Thread Starter

Avikaar Sonlall

Joined Mar 14, 2017
11
Also in case the picture is a bit unclear, the ICs used are the 7490 counter, 7447 decoder and of course a 555 timer. I can also consider changes to the design.
 

Dodgydave

Joined Jun 22, 2012
10,088
Use a CD4543be seven seg latch decoder, Make the Latch Enable High to load data, then low to save, can be used on common Anode or Cathode.
 
Last edited:

LesJones

Joined Jan 8, 2017
3,571
Use an SN7475 between each 7490 and each 7474. Clock the latch at the end of the 1 second gate period just before you reset the counters to zero.

Les.
 

ian field

Joined Oct 27, 2012
6,539
Hi all

I created this frequency meter circuit on Multisim 14.1. How it basically works is that as high pulses come in from a source, the counters are incremented accordingly and the 555 timer is configured such that after every second, a high pulse is sent to the reset of each counter and then the entire display is reset to 0. For example if I have an input signal of 100 Hz frequency, this circuit will count from 0 to approx. 106 and then reset.

What I need is for the value of the display to remain even after the counters are reset (i.e. with an input frequency of 100 Hz, the display should remain at 100 +- 10Hz ). I know that I have to use a latch IC but the problem is I am not familiar with latches and how to connect them. I tried using a 4543 but I could not get it to work properly.

Any help would be appreciated.View attachment 124463
Thank you
You're using a 555 for what?!!!

Most people use at least 10MHz crystal and dividers for an accurate timebase.

You can simplify the load and rest pulse generator circuits by using simple CR differentiators.

Its in the CMOS cookbook by Don Lancaster.
 

Thread Starter

Avikaar Sonlall

Joined Mar 14, 2017
11
You're using a 555 for what?!!!

Most people use at least 10MHz crystal and dividers for an accurate timebase.

You can simplify the load and rest pulse generator circuits by using simple CR differentiators.

Its in the CMOS cookbook by Don Lancaster.
This circuit is for a project and the requirement was to use a 555 timer
 

AnalogKid

Joined Aug 1, 2013
9,493
Are you required to use 7400 series TTL parts, or can you change to CMOS? As noted above, there are CMOS parts that have the counter, latch, and display decoder all in one device. If you are stuck with TTL, then adding one 7475 is the cleanest way. There are 8-bit and 16-bit latches that would decrease the body count, but if this is only for simulation then there is no advantage there.

ak
 

Thread Starter

Avikaar Sonlall

Joined Mar 14, 2017
11
Are you required to use 7400 series TTL parts, or can you change to CMOS? As noted above, there are CMOS parts that have the counter, latch, and display decoder all in one device. If you are stuck with TTL, then adding one 7475 is the cleanest way. There are 8-bit and 16-bit latches that would decrease the body count, but if this is only for simulation then there is no advantage there.

ak
Thank you so much for the reply, I did not realize the 7490 was a TTL IC. Is there a CMOS version of this IC that provides the same function as the 7490? If so I would rather use that because I read that going from TTL to CMOS does not usually go well.
 

ian field

Joined Oct 27, 2012
6,539
I would use an old analogue clock module as the 1Hz clock source..not a 555, or use a 32768Khz Xtal.
If a 555 is a required specification - someone has gone out of their way to make things tough for the TS.

As other's have mentioned; there are CMOS latch/decoder/driver chips that do pretty much most of it for you.

A circuit in Donald Lancaster's CMOS cookbook shows how to generate load and reset pulses with a CR differentiator, the narrow pulses eliminate dormant cycles.

AFAICR: there isn't a similar circuit in the TTL cookbook, but I've seen the same trick done in a TTL based Cedenco industrial tachometer.
 

Thread Starter

Avikaar Sonlall

Joined Mar 14, 2017
11
Update:

I got the circuit to work, the ultimate goal is an RPM measurement circuit so this is what I have working so far.
The units display is wired to display 0 which is kind of a 10x multiplication.RPM circuit.png
 

ian field

Joined Oct 27, 2012
6,539
Update:

I got the circuit to work, the ultimate goal is an RPM measurement circuit so this is what I have working so far.
The units display is wired to display 0 which is kind of a 10x multiplication.View attachment 124865
A frequency counter usually measures Hz (pulses per second). RPM is 60x that many - you might need a PLL to multiply your tacho pulses................or you could make your latch time 60x longer.
 

Thread Starter

Avikaar Sonlall

Joined Mar 14, 2017
11
A frequency counter usually measures Hz (pulses per second). RPM is 60x that many - you might need a PLL to multiply your tacho pulses................or you could make your latch time 60x longer.
The full setup will be... 10x by setting units to 0, 4 blades on the motor for 4x and 1.5 seconds counting time. This combination gives 60x Mult
 
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