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| Introduction | construction-1 | Construction-2 | Construction-3 |
NoteThis " 6 Digit Frequency Counter " circuit and PCB was designed by Laurier Gendron. It is being made available to hobbyists for personal development only. It cannot be used for commercial purposes of any kind without previous written permission. ( 10 Feb.2001 )
Introduction
BBased on the application of the three digit decoder driver chip MC14553 published in the Motorola data manual I undertook the task of designing a 6-digit frequency meter and the results were excellent , simple enough and at a cost of well under $ 50.00 Canadian.
Since the Frequency counter may require as much as 250ma of current when all digits are illuminated it was designed as a bench instrument complete with a regulated power supply as described later. The transformer used was retrieved from an ancient digital alarm clock . Although a clocking signal could have been derived from the AC supply line , a digital clock oscillator was incorporated into the design to accommodate a battery pack supply instead of an AC supply source as an alternative. .
Following the design application of the Digital Capacitance Meter , the digital display read out section needed to be expanded by adding a second set of three digits for a total of six digits to accomplish my goal of being able to count up to 1MHz without adding many stages of frequency division .
Once this accomplished I decided to increase the capability by adding one dividing stage to obtain a reading up to 10 MHz minus one count ( 9.99999MHz ) and this was accomplished by adding only on IC and one switching stage, but with the basic CMOS Ic's it tends to bog down at about 4mHz . This problem was later solved by a hobbyist who used the Phillips HCFxxxx Cmos and kindly suggested the change and the much improved results . See modified part list . .
You can make a search and download application data sheets for all the IC's used in this project from ON SemiconductorsThe circuit
In addition to the six digits counter circuit for display , the measurement of a frequency can be achieved with only four additional CMOS IC's ,MC or CD4521, two MC14093 ( Motorola ) , one MC or CD4017 . The six digits counter is made of two CMOS IC's MC14553 and two MC or CD14543 , six common Cathode digital displays and a few other parts . A complete list of parts is provided .
Logic Circuit Description
As a reference you may wish to open up a new window to view the circuit while reading the description.( The window size is adjustable)
a ) Condition the input stage to accept a sine, square, pulse or triangle signal.
b ) Amplify a weak signal voltage to a level required for good processing.
c ) Attenuate any high level signal to a pre-determined level so as not to overload the permissible logic input voltage level .
d ) Shape most frequency signals to be acceptable by the counter section for stable processing .
e ) Provide for a timing sequence interval to enable the counter to accumulate a total count that accurately reflects the frequency being measured.The input is fed to Q1 base through R26 and C15, the amplified signal from the collector of Q1 is decoupled by C6 and connected to pin 8-9 of IC3 . This circuit gain is set to enable a linear amplication from 1Hz to behond 5 MHz into a MC14093 . On its own , the amplified signal level is not sufficient to trigger the MC14093 and and R6 connected to the positive bus as a pull-up resistor is used to raise the voltage reference level sufficiently to trigger the MC14093 . This set-up is very stable and immuned to noise that long leads can cause or from other sources and also insure thast the counter will auto zero when not signal is present .
The output of IC-3 is taken from pin 3 ( x1 ) and its one output is all that is require for a maximum count of 999,999 hertz which in this case can be connected directly to IC-2 gate D pins 12/13.Important Notes :
The output of IC-3 from pin 3 is sent to IC-4 pin 14 . C-4 is a CMOS 4017 Decade Counter and is used as a frequency divider , it is configured to divide any frequency by 10 .The maximum operating frequency of IC-4 ( CMO4017 ) is 12 MHz . The divided frequency is taken from pin 12 ( x10 ) and connected via S1a to gate D pins 12/13 of IC-2 .
Switch S1 is used to select the output of either IC-3 pin 3 for a maximum count of 999,999 Hz or the output of IC-4 pin 12 for a count of up to 10 MHz minus one which in this case would be displayed as 999,999 kHz .The selected output ( x1) is fed to the input of gate D of IC-2 as mentioned earlier , IC-2 is used to shape all the input signals required by the three digit counters CMOS MC14553 described below .
In order to be able to provide timing pulses to the counter an oscillator is required , IC-1 CMOS 4521 with its appropriate crystal (see parts list - xtal ) delivers a one ( 1 ) pulse per second taken from pin 14 and is delivered to pin 1 & 2 of gate A .As we can see IC-2 and IC-3 are used with the combination of C3,C4,C5,R3,R4 ,R5 to condition and invert the pulses where required to the counter as it requires exact timing and pulses shape for the stable operation of the counter.
Range Options
Perhaps all you need is a 1Hz to 1MHz counter ! While still using the same Layout and PCB all you need to do is the following simple modifications :
1... Omit IC- 4 ( 4017 ) and connect IC3 ( MC14093 ) pin 3 (S1a -x1 ) directly to pin 13 of IC2 .
2... Change S1 from a DPDT to a SPST for power ON only .
3... Better sensitivity and lower frequency can be obtained by using values shown for the logic input circuit to be used for 1Hz -1mHz or the 1Hz - 10mHz counter as shown below for R6,R7,R8,R26 and C6 , C15 , observe the polarity indicated for polarized capacitors.
Test Oscillator
I include below a small oscillator circuit that can be used to test your high frequency range , any crystal up to 50 MHz can be used , also very handy as crystal checker which could be incorporated in your project with a means to plug in your crystal for direct frequency reading with a simple connecting switch to the input .
The Counter Display
As a reference you may wish to open up a new window to view the circuit while reading the description.( The window size is adjustable )
To do this the MC14553 needs three input signals, a positive (high) pulse to the Latch input ( pin #10 ) to enable storage of pulses to be stored into the latch and a Reset ( pin # 13 ) pulse ( high ) to reset the counter . The total count desired is controlled by the Latch action which sets the time we require to insert the amount of pulses to the Counter ( pin #12) to be displayed.
When the latch is high the count starts and when the Latch is low (zero) the count is stopped and the total of pulses accumulated in the latch are displayed then a positive pulse is required to the Reset to clear the Latch , the speed of this process is controlled by an internal 100 kHz oscillator which is determined by the .001 capacitor C7 connected between pin # 3 and pin # 4 of IC-6 .
To display six ( 6 ) digit we simply add an exact duplicate of the three digit circuit and connect the overflow from pin 14 of IC-6 to pin 12 of IC-5 as well as carry the oscillator signal from pin 3 of IC-6 to pin 4 of IC-5 and duplicate the inputs of the " Count, Reset, Latch " as well.
Notice that only one set of driving transistors of three is still used but they are now connected to the second set of digit as well BUT the segment connections of each three digit must be kept separate and connected to its dedicated digit driver IC-7 and IC-8 as shown.
Introduction construction-1 Construction-2 Construction-3
