Universal frequency counter & voltage meter


Circuit schematic


PIC assembler program

Photo 1

Photo 2


For the purpose to be used with QRP transceivers I have developed some frequency counters with different types of displays (from simple 8 LED display to 2 x 8 character LCD), but the f-counter described on this page I have found to be the most useful. The requirements I wanted to fulfil with this design, were:

Measurement of the frequency up to 30MHz and of the supply voltage up to 25.5V

4 digit 7 segment LED display (as small as possible)

Programmable frequency offset added or subtracted from measured frequency

Different display modes: XX.XX MHz, X.XXX Mhz, XXX.X kHz

One push-button operation, push-button placed on display PCB

Small dimensions

Display resolution 100 Hz

Measurement resolution 12,5 Hz

Hysteresis of displayed value 25 Hz


After careful study of different types of solutions I have found out as the most appropriate solution to build the f-counter around a PIC16C71 microcontroller, mainly because of it's easy programming, low cost and on-board A/D converter.

The f-counter is built on two PCBs, one serves as a motherboard and the other as a display unit. To achieve small dimensions mostly SMT components are used.

As a display I have implemented two different solutions. First I tried the miniature Siemens DL340M four digit 7-segment LED unit with 2,5 mm digits (looks like LED displays, which have been used in calculators some 20 years ago, but is more up-todate). The advantage of it is simple PCB required and the drawback it's price (ca. 15 US$). So I developed also a display PCB for four HDN1077O (also Siemens) low currant 7-segment LED displays with 7 mm digits, which are also brighter at the same power consumption and have much bigger digits, although they occupy approximately the same space on PCB as DL340. As a display driver serves Motorola's MC14499P and to store the frequency offset a 93C46 EEPROM.

Comment from November 2000: The Motorola's MC14499P (DIL housing) seems to be out of production for some time now, therefore becoming a "hard to find" part. I've decided to construct a new similar counter (maybe even smaller with some additional functionality). In this new product the PIC16C71 + 93C46 + MC14499P chip-set is going to be replaced by single PIC16F876 28 pin device.


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