Saturday, May 22, 2010

Electronics, or, why didn't I use a GREEN light?

Back in Ye Olde Days (okay, 1984), before computers existed (according to some kids) and before the Internet existed to distract us from work, those of us of a slightly nerdy disposition would do Electronic Stuff.

Whilst perusing an elderly Everyday Electronics magazine, I spotted a project which - gasp - I could make use of today! How cool is that? After a quick blast around the virtual aisles of Farnell, and an impatient 24 hour wait, I got my sweaty mitts on a shiny new µA747CN dual op-amp, the heart of the circuit.



Above is the article, all 2-and-a-bit pages of it, and the rather optimistic £5 estimate. Below, is the schematic:



Sharp-eyed viewers will note the use of a 7-segment LED display to craftily give the user a "middle" reading, whilst only using a "low" and "high" sensor. Clever. No good for me, though, 'cos I had no 7-seg display; and besides, I wanted 3 LEDs, as I'll make a nicer panel than the one pictured in the book.

Poking around in the scrap pile revealed an elderly 74LS02 chip, quad 2-input NOR. Perfect! When both main LEDs are extinguished (logic zero), the NOR will be on. So I just found my "middle" lamp. And, after faffing around with some pull-down resistors and other bits, I ended up with this schematic:



Breadboarding the circuit took no time at all (ahem, who am I kidding, it took ages...) - actually, I made the circuit and then the schematic. You could never tell I was a computer programmer, right? Here it is assembled & working with green LEDs:



Above, there are no probes in the water, so the LOW lamp is lit. When probes A and B (the two orange jumpers) are popped into the water, the MID lamp lights (on the original circuit, LOW and HIGH would both be out, leaving just the two elements lit to give an "I" character):



And, finally, with probe "C" (blue) in the water as well, the HIGH lamp comes on:



Perfect, right? Except I'd like the LOW level light to be RED. After all, it's supposed to be a warning. But substituting a single red LED didn't have quite the effect I wanted. In the following pics, all three LEDs are red, but the outcome is the same:



Here the LOW level light is on. But also, there's a faint glow from the HIGH level. Why? Do I need to up the 3M3Ω resistors to 4M7Ω, as per the original schematic? Does this also explain why the current draw rises to a bit over 9mA (from 7.5mA with the green LEDs)?

With the two probes in the water, the only lamp on is the logic-driven middle lamp:



Except, that is, for the faint glow from BOTH low & high lamps...

Finally, the high level:



The LOW level is glowing slightly as well...

Any suggestions gratefully received.

PS: I would like to make it clear that I don't own a caravan.... but I do own a racecar transporter with a water tank... which runs out at the least convenient moment...