Hi Mike, thanks for your reply, ahem, the voltage difference was an error on my part, the supply voltage is common to the whole circuit at about 6.13v. I also hadn't realised that the HCT used a lower supply voltage either, so I've added a resistor to the +v for the counter that brings the voltage down to 4.8v.
Not sure if I was clear before, if I power the solenoid directly with the pulse from the 555, then it seems to work ok, in that the pulse is the right length and the counter progresses as it aught to, during, before, and after the actuation, except that it's inverted. That is, the solenoid is energised continuously, and only goes limp for half a second. I understand why that is (I think? - because only during the pulse is a parallel circuit made through the 555 that is of a lower impedance than the solenoid?), and hoped I could use one of the channels of the 2803 to do it properly.
I have also tried using a discrete NPN transistor (3904) instead of the darlington pair package, with the same result.
I have rigged my arduino as a little scope, and here is a couple of screen grabs showing the voltage I see going into the clock input pin of my counter, and also the voltage on the QA pin of the first counter (
). It's right-to-left. Whenever the 555 circuit triggers the solenoid there's another little peak or trough as long as it's on. It's during this pulse that the counter is going crazy. Measuring the supply voltage to the counter shows me that there's a corresponding dip in voltage to about 4v while the solenoid is energised.
Is this a supply conditioning / isolation from the big draw of the solenoid type thing? Hopefully I have explained a little more clearly now. I am getting a clearer picture too, I think. Perhaps the counter is effectively being turned off, and when it comes back on, then it's lost count, or has counted randomly?
Thanks again, it's very much appreciated!