Help please. Baffled making tacho with ICM7225

[bigal_scorpio]

Please can anyone shed light on my problem?

I have built most of what I thought would be a Tachograph for my sons Lathe.

I used the parts I had available as you do, and as luck would have it I had some spare 7 segment Common Anode LEDs and an ICM7225 IC which one of the uses listed on the datasheet is a Tachograph, so all seemed well.

I couldn't figure out the pulse count part so I built the circuit hoping to just bench test which of the pins I needed from the 4 that I believe are pertinent to me ie Store, Reset, Count and Count Inhibit.

Now the puzzle begins, when I applied power to the circuit all digits light and the dimming pot I fitted works OK, next I connected Count to a small 555 astable circuit output that I built to test this. Nothing happens, I recheck my wiring and connections and still same. Time for some experimenting now, and I connect Store to Ground! Aha it is counting, but then I realise that count is NOT connected to anything and the counter is still happily counting away at about 4000 per minute

Just to be sure I connect only Pos and neg to the IC and grounded Store, without my 555 circuit in line and the counts still go on - but from what?

I have built a small device that fits on the lathe shaft and pulses a photo transistor/LED device from an old printer, but unless someone can explain the interface needed to make the 7225 count RPM then I'm stuck, and my lad won't get his speed display!

Hoping someone can understand the Datasheet better than I can ( I posted it below) and desperate, please help, assistance would be greatly appreciated.

Thanks for looking..........Al

 

[k7elp60]

It sure would help if you can post a schematic of your circuit including the interface from the lathe and the trial 555 timer circuit. Off hand I would say that when the power is applied the digits will be on but in an random fashion, so a power-up reset my be necessary. This can be accomplished with a capacitor from the reset terminal to ground and a resistor from that same terminal to +5V.