Will measure and display frequency or period.
Also useable as an event counter.
Upper frequency count limit is about 2 MHz
(or 15MHz in the divide by 10 count position.)
50Hz Mains frequency is used as a reference for all needed frequencies and timings.
We are informed that AC mains frequency is held to close tolerances, therefore the frequency counter should display a similar degree of "accuracy".
Uses 6 off 7 segment displays for readout.
There are many good circuit designs that have a much higher frequency limit,
but this basic design uses only CMOS and TTL family IC's (cheap and readily available).
Inspired by the work of Onno PA2OHH this is my build of the LED display frequency counter.
The circuitry and operation is very simple, the only trade-off being that the display must be interpereted from the state of 8 adjacent led's: the frequency count being the aritmetic "sum" of the illuminated LED's.
This you must do mentally or with pen & paper,
but your eye/brain adapts suprisingly quickly to this presentation.
My example was constructed from mostly reclaimed componentry and made to fit in a small metal enclosure. (mustard tin again!)
The need for such a device arose from having to measure (in particular) the VFO frequency of the SDR detector shown elswhere on this site.
A 4020 14 stage ripple counter, which gates on/off the 4040 12 stage ripple counter is used to count the incoming signal frequency.
Half of each cycle is used for counting the incoming signal.
On the second half of the cycle, the count is latched in a 74HC574 IC and the 4040 is reset to zero.
The state of the latched count in the 74HC574 is displayed on a row of LED's.
The 8 LED's indicate a frequency value of 4, 2, 1, 0.5, 0.25, 0.13, 0.06, 0.03 (MHz pos'n) and 125, 62, 31, 16, 8, 4, 2, 1 (KHz pos'n)
Because I have used 'B' 4000 series logic IC's the maximum frequency limit (at 5V rail) is about 4MHz.
I really should have used the 'HC' (high speed CMOS) family variant; for which the upper frequency limit would not be a consideration. (c 50 MHz)
However; once again I have used that which I had readily at hand!
A 74LS74 configured as a divide by 2, D flip-flop; acts as a "prescaler" for both the "clocking" reference (4MHz xtal oscillator) and also the input signal.
The 4MHz quartz oscillator module is used for timing, and 'accuracy' depends on the stability of this unit, which for my purposes is really quite good enough.
Upper freq' limit spec' for the LS family is about 25 MHz.
This then means; an upper freq count limit of about 8 MHz (sufficient to cover the 7~7.5 MHz range of the VFO) can be acheived.
Limited by the capability of the IC's and also the frequency readout.
In the MHz switch pos'n the counter displays the most significant 8 highest 'bits' of the count.
Accuracy here would then be 1 part in 255 (about 0.4%)
In the KHz position the counter is over-run (several times) however provided the input signal is stable we read the truncated frequency count accurate to 1KHz. Arithmetic addition of the two readings will give a figure of accuracy of 1 bit in 2 power 14 or 1 part in 4096 ( 0.02%)
So you can see; that this very simple idea is indeed quite "powerful"
My first reaction when I studied the design was:
"I am really freaked out by this freq' counter" !!
I must set-to and build an example !
Note that the input signal must be at TTL-level .
Since there are a wide variety of possible signal levels from different equipment this counter might find use with; an external VFO buffer amp must then be utilized.
I also recommend that you read Hans Summers excellent follow-up designs and notes (based on Onno's circuit(s) at Hans Summers G0UPL
As inspired by the work of Onno PA2OHH and Hans G0UPL
this is my variation of the 2 chip LED display frequency counter.
The circuitry and operation is very simple, the only trade-off being that
the display must be interpreted from the state of 6 adjacent LEDs:
the frequency count being the aritmetic "sum" of the illuminated LED's.
This you must do mentally or with pen & paper, but your eye/brain
adapts suprisingly quickly to this presentation.
A 4060 xtal osc/14 stage ripple counter, which gates on/off a 4040
12 stage ripple counter which is used to count/store the incoming signal frequency.
Half of each cycle is used for counting the incoming signal.
Other half displays the frequency "count" on the LEDs
With a 4MHz xtal reference (4060 on-chip oscillator):
Mhz position: 15625 Hz sample 64uS period - 32uS count, 32uS display
Khz position: 244.1 Hz sample 4.096 mS period - 2.048 mS count, 2.048 mS display
No display "flicker" is seen.
During the sampling interval; input transistor-preamp/buffer is switched/gated on
and 4040 ripple counts incoming signal-frequency.
4060 rising edge thru 100pF capacitor spike resets the 4040 to zero, just prior to counting.
All LEDs will be dark because their cathodes are at +5v
During display interval; input transistor is gated off,
and LEDs can illuminate (cathodes now grounded thru 4060) to display count that is stored in the 4040
Cycle then repeats
The 6 LED's indicate a frequency value of 8, 4, 2, 1, 0.5, 0.25 (MHz switch pos'n)
and 125, 62, 31, 16, 8, 4 (KHz switch pos'n) See diagram.
I used small (SMD) hi-intensity blue LEDs; unsoldered from LED ribbon strip light.
These are well under-run but still bright whilst drawing only 0.5mA each (4K7 limit resistor)
I have used the 74HC 4xxx series (high speed CMOS) family logic IC's
The useable maximum frequency limit (at 5V rail) is 40 MHz.
The LED display upper limit capability is just< 16MHz (8+4+2+1+0.5 ....etc)
In the MHz switch pos'n the counter displays the most significant 6 highest 'bits' of the count.
Accuracy here would then be 1 part in 64 (about 1.5%)
In the KHz position the counter is "over-clocked" 64 times (15625 ÷ 244)
however provided the input signal is stable we read the truncated frequency count accurate to 4KHz.
Arithmetic addition of the two readings will give a figure of accuracy of 1 bit in 212 or 1 part in 4096 ( 0.02%)
So you can see; that this very simple idea is indeed quite usefull!
Test mode: counting action of 4040 ripple counter: