A frequency counter
using the venerable but now unobtainable

This project was started in 1979 by a fellow student at Caulfield Institute of Technology who had bamboozled me into buying this (then) amazing chip from him for $40.  The project was eventually finished in the last half of 1980 and has been a cornerstone of my radio home brewing since then. The construction  is primitive to say the least but it does actually work.  It was totally scary powering the thing up for the first time.  That Intersil chip cost me several weeks' pocket money and I was somewhat afraid that a wisp of smoke would waft out to mock my impudence. To be sure, it was unheard of in the late seventies that a complex cmos chip should be expected to directly drive a LED display, because after all,  20mA was a substantial amount of current for a complex chip. Being terrified of over driving it, despite the Intersil reference design showing that LEDS could be directly driven, I still choose to use buffers, and in retrospect, that was a wise choice. Not very many of my earlier projects have survived 32 years, let alone still work!

I have long since lost my original hand drawn circuit, so you will just have to believe me. The original Intersil data sheet is presented as is. Intersil has long since ceased to be despite some excellent products and helping introduce the merits of CMOS LSI to a skeptical marketplace.  This data sheet does not appear to be present on any boat anchor or data sheet archive site.  It is still possible to find this chip and some variants in discarded equipment that contained counters and led displays.

This chip also featured in many electronics magazine projects in the early eighties.

Things I would have done differantly.
This chip could have had a 10Mhz timebase.  My oven controlled crystal  worked at 5Mhz, so I divide it down to 1 Mhz. The result is that the non adjustable decimal point is displaced one place.
In retrospect, I should have used a simple frequency doubler and squaring circuit  which would have had the original stability of the 5Mhz timebase.  Maybe one day I will do just that ! This would
give me then a faster aquistion time as well.

Intersil datasheet for the ICM7216 8 digit counter

block diagram of input prescaler boardThe ICM7216 could only accept input speeds not exceeding 10Mhz so this was my front end, using an OM350 hybrid UHF amplifier and an 11C90 UHF prescaler.  I remember handing over something like $50 for this one chip in 1982, they were not rare just amazingly costly, but they could count up to 600Mhz which seemed amazing at the time. I remember that when I had bought this chip , I distinctly felt like I had just been mugged. $50 and all I got was this little white paper bag with just one little chip in it!layout of main board and jumper schedule

The DFM without coverjust visible is the ICM7216 chip and display buffers
timebase using crystal oven and timebase amplifier/prescalerthe prescaler, also my first double sided PCB
homemade double sided pcb display board , resist using pen and tar and FeCl3 etch
display based on 8 FND500 7 segment leds homemade case with a panbrake
obverse view of front end and prescaler


fixed formatting and layout Thu Mar 31 18:56:38 EST 2011