Hi Guys,
Having struggled over the last few days to get what is my first project of the ground, have come to the conclusion I need your help!!
The circuit I am trying to build is a simple analog to digital convertor from a variable dc voltage, nominally 4 – 18v. Basically I am using a multi- diode ladder array to pull of voltage triggers (inputs) at differing voltages. i.e If the input supply voltage does not overcome the V drop (at differing) levels the input drops out. I should say that this will be interfaced to TTL logic and hence to a computer. With me so far, good! Initially the idea worked using a standard AN425 opto- isolator( to decouple DC voltage ground from Pc).However as the voltage was variable (Irrespective of the diode ladder) in- determinate states were experienced causing a rethink. The use of a Schmitt trigger was thus used to ‘ square up’ the pulse which is ok. One problem to overcome is in the use of a zener diode ( to hold voltage at a set level ) . Obviously, as the input voltage to the zener (prior to the opto) is variable with the dc voltage, a way to regulate the current is required thus the use of a LM 334 Constant current device to source the opto at these differing voltages. I hope you are following this!! Now he problem I have is that I decided to try the H11L1 schmitt opto isolator to replace the an425 and subsequent Schmitt trigger. Simpler, but no!! Can I get the h11l1 to work,no! I even set it up as per Isocoms pdf as a test switch all to no avail. All voltages correct Vcc Vd and currents Ic max etc,no good. Ok I initially thought it was a duffer and having purchased ten of them tried another, still no joy. The output pin (4) rises to Vcc and stays there irrespective of input. Any ideas you guys??
I can hear you all say that a A to D device might well be simpler , however have not managed to source one that fits the bill. I hope therefore that maybe you guys can help and I would really apprciate your input, Thanks in advance,