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FREQUENCY METER/COUNTER

Modified: 15:10, 27 October 2013

This is a versatile counter timer with 2 inputs and a 5-digit, 7 segment display. It has three main functions:

  1. Frequency counter - input A
  2. Counter - Input A & B
  3. Timer - Input B

It has manual reset for the counter and input B can be inverted for timing low pulses as well as high.

There is indication of function, gate time and overflow. It evens runs from rechargeable batteries with built in charger!

Designed by Phil Townshend 1999

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This piece of test equipment is designed to have as much control over the timing of signals without becoming un-useable. This happened once before when making a similar unit. It has so many controls, it took ages to work out what to set it to. So this is version 2 and I have been using it for about 8 years now and it has never failed.

OK so it has its drawbacks. This was developed when PICs were in their infancy, so no clean multiplexing here. All displays are directly driven from individual counter/driver IC's. Unfortunately the power overheads are considerable. Assuming that all segments are on, all 5 displays at about 25mA equals:

7 x 5 x 0.025 = 875mA....ouch!

but it does mean a nice bright display. I had some spare green 0.56" displays lying about and being quite old, did not have the brightness of modern ones.

CLICK HERE for explanation of the circuit.

SPECIFICATION

Frequency Counter
Gate time: 0.1 / 1sec
Ranges: 10Hz - 99KHz,
1KHz-10MHz

Counter
Range: 1 to 99,999
Max Count Freq: 2MHz, Input A
Modes: Run/Pulse/Cycle, Input B

Timer
Clock speeds: 1Hz/1KHz/1MHz
Gate modes: Run/Pulse/Cycle

CIRCUIT DESCRIPTION
BLOCK DIAGRAM

CLICK HERE for explanation of the circuit.

OVERVIEW

This unit works in the following modes

Frequency Meter
Measures frequencies on InputA from 1/10Hz to 99.99KHz and 1KHz to about 5MHz reliably. It has 2 gate speeds, 1Hz and 10Hz, but with a gate speed of 10Hz the resolution of the reading is 10Hz not 1.

Counter
Counts pulses on InputA in 3 modes, continually, while InputB is high/low (input polarity can be inverted) , or for a cycle on InputB allowing ratios of frequencies to be identified. The lower frequency must be applied to InputB which acts as the gate control.

Timer
Runs as a timer continually, while Input B is high/low, or for a cycle on InputB allowing pulse widths and cycles to be measured. Good for accurately calculating very low frequencies, below 1Hz. The clock speed for the timer can be selected at 1us/1ms/1sec (SW3)

PCB -MAIN BOARD CIRCUIT DIAGRAM
The PCB was designed using Micrografx DRAW very old but I know it well. I now use PCB Wizard 3 which of course is much more suited to the job, just love the freedom of a drawing package. Now its much quicker though.
SIL headers and sockets were used (10 way and 12 way) to connect the rotary switches to the PCB. The remaining switches were PCB mounted and push bank type.
There have been a few modifications to the PCB. I route all my own PCB's and the odd error is inevitable. However in this case all of the changes were through circuit redesign rather than PCB layout errors. The input amplifiers had the bias circuitry removed (top right component side), 2 IC's were changed with the resulting additional wiring shown in green. The circuit diagram has been corrected and is shown right.

CLICK HERE for explanation of the circuit.

PCB -DISPLAY BOARD

Because I hate soldering wires through PCB's, its so clumsy and they break off so easily through wire fatigue, I prefer to solder straight to the underside of the PCB. The wire used was 1.27mm pitch ribbon/rainbow wire, whichever you prefer to call it. Soldering was a bit of a challenge but kinda enjoy the accuracy of it all. Below shows connections to the main PCB.

CONSTRUCTION
The meter was housed in a case from Maplins and featured an aluminium front panel and legs no less! The PCB just fits inside...just, and needed some parts cut away to make room for the display PCB and wires passing underneath to the rotary switches. Note the metal parts of the case were connected to 0v supply of the circuit via a 100nF capacitor.

The frequency range (SW4), Gate time (SW5), Input-B polarity )SW6) and Reset counter(SW7) are all push swiches in a bank mounted directly on the PCB. I hate too much wiring in projects. The Mode (SW1), Gate Source (SW2) and Clock Speed (SW3) are all rotary 3way/4pole switches.

The inputs are connected via BNC panel mounted sockets just below the display. Visible is the rainbow wire soldered onto pads 1.27mm apart - very tricky. This connects the main PCB to the display PCB for the 7 segment displays.

The batteries are located under the PCB. I used AA recheargeables which have a voltage of 1.2volts not 1.5v. So an 8xAA battery holder was adapted to hold 5 batteries. A 6xAA will do also but will have to be adapted.

FRONT PANEL DOWNLOADS
The following downloads are of the original version with the corrections added. If I were to remake this, the PCB would need altering (unles you want to perform all the modifications I had to - they are shown however) so these are for reference only really. The "Functions.xls" file shows a break down of signals required for various funstions in the control section of the circuit. For a detailed explanation of the circuit click here

Phil Townshend 30/09/2007

To add the detail, the front panel design was drawn usign Micrografx Draw, a very old package but one I know really well. Anyway this was printed as a mirror image onto acetate using an inkjet printer and glued down to the front of the panel with spray mount. it leaves a frosted effect but looks pretty good. When pressed down properly its hard to see the spray. The print is on the underside and is protected from wear.
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