John, It was mentioned earlier that the 555 could trigger the SCR. I thought that you were expanding on that. I agree with you both in that it would work to sink current for relay drive and that I wouldn't do it. By the time we add those things we've mentioned to try to establish an initial condition, I didn't see how we were any better off than charging up a cap in a gate circuit. I built several windshield wiper delays arould 555s in the late '70s. That app worked very well and used the whole chip.
Of course, they spent all but one cycle running between 1/3 & 2/3 Vcc where they really shine.
You mentioned some pretty good ptices on your relay approach. Please give me your supplier info. You never know when that will come in handy. I went with the SCR approach because it could be inserted in the negative path to the relay. I've probably had the parts to build one sitting around for 30 years. I thought BigB could just stop at his local RS and pick up a few Rs, Cs, an an SCR to play with. I'm not so sure anymore. A 30 amp automotive realy was a first hit but "SCR" doesn't show up on an online search of their site.
So let's pretend that we know what BigB wants more than he does. We see that he's planning on switching a pretty heavy load. Maybe he wants it to be on with the engine and not risk running down his battery. So let's design him a circuit that pulls in a relay a couple seconds after it sees >13 volts and drops out the relay about a volt < the pick voltage. Any takers? I just like these challenges to knock the rust off.
Scott, I was thinking of what you said about the Tech ref. section. Could this whole thread be shifted over into the Theory area with drawings in the adjacent section?
Joe
