Frequency Counter Gate Control Using 74LS123

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Thread Starter

Hobbyist92

Joined Dec 17, 2020
25
Hi:

I have breadboarded a frequency counter gate control circuit using the 74LS123 multivibrator.
I have attached the schematic and a screen shot of the scoped output. The top wave form is off of pin#13
(to control the latching operation) and the bottom wave form is off of pin# 4 (to control the counter
reset).

I plan on using a 1 Hz time base in the final circuit but for now I am feeding in a 1KHz time base. The 1 Hz
doesn't seem to give a proper display on the scope.

Not sure if there is a delay between the two pulse signals.
 

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Papabravo

Joined Feb 24, 2006
17,300
Edit: Is the multivibrator an astable or a monostable multivibrator?
If it is a monostable
, this is a terrible idea for one critical reason. The timeout of a oneshot cannot be precisely controlled. the trailing edge is not synchronized with the leading edge. What you want for the gate in a frequency counter is an interval that is as precise as you can make it. Anything else is just a semi-useless toy.

Question. what were you expecting on the scope?
 

Thread Starter

Hobbyist92

Joined Dec 17, 2020
25
Edit: Is the multivibrator an astable or a monostable multivibrator?
If it is a monostable
, this is a terrible idea for one critical reason. The timeout of a oneshot cannot be precisely controlled. the trailing edge is not synchronized with the leading edge. What you want for the gate in a frequency counter is an interval that is as precise as you can make it. Anything else is just a semi-useless toy.

Question. what were you expecting on the scope?
1. Did you even bother to look at the schematic and the scope image?
2. The design is from MrChips who has perhaps forgotten more than
most of us will ever know about electronics.
3. Instead of criticizing why not offer what you feel is a better solution?
 

dendad

Joined Feb 20, 2016
3,939
Your description is a bit off. Using a monostable for "gate control" is not the way to go, as mentioned above.
I don't think it is actually the gate signal, just generating the latch and reset signals.
The time base signal is the actual gate signal.
Try feeding the second half of the 123 from the NotQ of the first one and see if that helps.
 

MrChips

Joined Oct 2, 2009
24,626
1. Did you even bother to look at the schematic and the scope image?
Did you even bother to read my instructions on how a 74123 works?
You changed the design and got it wrong.
2. The design is from MrChips who has perhaps forgotten more than
most of us will ever know about electronics.
If this was intended to be a compliment it did not turn out that way.
3. Instead of criticizing why not offer what you feel is a better solution?
PB misunderstood. This is not the time-base interval. This is to generate two short control signals to latch and reset the counters. Precise pulse widths are not required.

Did you check on the minimum resistance allowed on the 74LS123?

Why did you start another thread?
 

Thread Starter

Hobbyist92

Joined Dec 17, 2020
25
Did you even bother to read my instructions on how a 74123 works?
You changed the design and got it wrong.

If this was intended to be a compliment it did not turn out that way.

PB misunderstood. This is not the time-base interval. This is to generate two short control signals to latch and reset the counters. Precise pulse widths are not required.

Did you check on the minimum resistance allowed on the 74LS123?

Why did you start another thread?
The data sheet says that you can use a 2K resistor. The 10K did not work. You said to start with 10K and I did.
The 2.2K worked.

The schematic that you sent me did not work at all even though the wiring was correct per the schematic.
I attached the scope image that I got from my circuit and my question this. Is that what I should be getting
out of the circuit? There is a positive pulse on the leading edge on the one output and a negative pulse
from the other output. But is there actually a delay between the + and - pulses?

That PB is always jumping in and not knowing what he is talking about if you read various threads on here.

You said that I could switch the order of the internal units and I did per you instructions.

I started another thread because we are no longer trying to resolve a circuit conflict and there were
responses on items already resolved. Clarity was the reason.
 
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Papabravo

Joined Feb 24, 2006
17,300
1. Did you even bother to look at the schematic and the scope image?
2. The design is from MrChips who has perhaps forgotten more than
most of us will ever know about electronics.
3. Instead of criticizing why not offer what you feel is a better solution?
Yes I did look at it but I have trouble looking at sideways images -- it makes my neck sore!
 

Papabravo

Joined Feb 24, 2006
17,300
Did you even bother to read my instructions on how a 74123 works?
You changed the design and got it wrong.

If this was intended to be a compliment it did not turn out that way.

PB misunderstood. This is not the time-base interval. This is to generate two short control signals to latch and reset the counters. Precise pulse widths are not required.

Did you check on the minimum resistance allowed on the 74LS123?

Why did you start another thread?
That is true. I didn't spend a great deal of time with the schematic which I found hard to read, and thought he was talking about using a monostable to make the gate signal. A more accurate title would have helped (me).
 
Last edited:

Thread Starter

Hobbyist92

Joined Dec 17, 2020
25
The data sheet says that you can use a 2K resistor. The 10K did not work. You said to start with 10K and I did.
The 2.2K worked.

The schematic that you sent me did not work at all even though the wiring was correct per the schematic.
I attached the scope image that I got from my circuit and my question this. Is that what I should be getting
out of the circuit? There is a positive pulse on the leading edge on the one output and a negative pulse
from the other output. But is there actually a delay between the + and - pulses?

That PB is always jumping in and not knowing what he is talking about if you read various threads on here.

You said that I could switch the order of the internal units and I did per you instructions.

I started another thread because we are no longer trying to resolve a circuit conflict and there were
responses on items already resolved. Clarity was the reason.
I was trying to show appreciation for your help!
 
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