Other Radio Topics
Guest Project Gallery
|LCD Frequency Counter|
|Written by Hans Summers|
|Saturday, 05 September 2009 22:31|
This frequency counter module is intended as a "digital dial" frequency readout for portable QRP radio tranceivers. It uses only six standard discrete logic IC's, i.e. no PIC or other microcontroller, no programming to be done, etc. The readout is designed to show kHz from the band edge with 100Hz resolution, i.e. 00.0kHz to 99.9kHz. The display updates ten times per second, which I have found to be a comfortable rate for tuning. The current consumption is 1.4mA, very suitable for portable (battery powered) operation. The size of the counter is 2.3 x 1.6 x 0.8 inches (58 x 41 x 20mm). I included an LED backlight module, which consumes approximately another 100mA (4 LED's, 25mA each). In a battery powered situation, to save power one would probably disable the backlight or have it operate on push-button command only.
I also included a x10 mode, which reads 000 to 999kHz from the band edge to 1kHz resolution. Finally, I included a x4 mode, in which the displayed frequency must be multiplied by 4 to get to the actual frequency. The purpose of this mode was to allow both x4 and x10 modes to be used simultaneously, for setting up the transceiver's VFO. In this mode, the counter covers 4MHz of the frequency spectrum to a resolution of 4kHz. Once the VFO is setup correctly, I will probably leave the frequency counter on the intended 100Hz resolution mode.
The LCD is direct drive (non multiplexed) and actually has 4 digits, only three of which are used in this application. It would be relatively straightforward to add further IC's to expand the counting range to 4 digits or more, using exactly the same circuit but repeating the circuitry for each digit. Non-multiplexed LCD's are available from Farnell and Rapid with matching LED backlights. The LCD has real pins (not elastomer contacts), and is as easy to use as a 40-pin DIP chip.
Many thanks to Ingo DK3RED who provided the nice computer drawn circuit diagrams. A German translation of this project will appear in a forthcoming 2006 issue of QRP-Report, the quarterly journal of the German QRP club.
The complete circuit diagram (schematic) is shown above, or CLICK HERE to open in a new window.
The timebase for this frequency counter uses a 32.000kHz crystal. This is NOT the usual 32.768kHz crystal which is found in clocks and wrist watches by the billion. 32.000kHz crystals are quite hard to find. They are not available in the UK but can be purchased in the US from Digikey or Mouser. 32.768kHz crystals divide down to 1Hz, but this produces at best a display update rate of once per second, which I find too slow for comfortable tuning. 32.000kHz divides to 1000Hz, but this can be further divided to easily create timing for 10 updates per second and 100Hz resolution count, which I consider optimum for this application. The reason I chose such a low frequency crystal rather than a crystal in the MHz range, was twofold. Firstly, the lower frequencies require less division, which means lower parts count (in this instance it is all done within a single 74HC4060). Secondly, current consumption. The lower frequency crystal oscillators consume only some 30uA of current, compared to 3mA for a 4MHz crystal for example. I wanted to keep the overall current consumption of the frequency counter as low as possible.
Photographs (click the thumbnails for larger images)
|Last Updated on Sunday, 13 May 2012 13:42|