Need info/help with Electronic Switching!!

[R.G.]

i'm still in the dark on where to keep my V+  9v, 5v, 7v??  Of course, everything at 9v being the easiest - but adjusting wouldn't be too much work.

The biggest issue is the relay availability. If what you have is 5V relays, then everything is easiest if you use a supply of about 5V. The transistors eat up a little of that, maybe half a volt, but the relays will still work as the "must operate" voltage on a 5V EA2 is 3.7V or so per the datasheet. 12V relays will operate at 9V as long as it's really, really 9V, not a battery which sags under 9.

If you have 9V available already and also have 5V relays already, consider either putting a resistor in series with each relay coil so that the steady state current from +9V is still 28ma per coil, or putting in a 7805 three terminal regulator to drop 9V down to 5V just for the relays. Either way works. If you go with resistors, the added resistor in series with each coil is R = V/I = (9V-5V)/0.028 = 143 ohms. A 140 ohm resistor is a standard value and will work fine. The power is P = I^2*R = (0.028*0.028)*140 = 0.109W, which is pretty hot to run a 1/4W resistor over the long term. I'd use two 270 1/4W in parallel or two 75 1/4W in series, or go to 140 1/2W.

There's an all-electronic one-and-only-one-of-eight circuit at GEO under the programmable footswitch system. A single 74C373 CMOS latch and some diodes does this.

[wokeupfalling]

Thanks RG,

I did read your CMOS setup on GEO - after this thing, I am going to try that one, especially if the 74C373's can be linked up for more than 8 - was that mentioned?  This project is more of an experiment and learning tool more than anything, but you know, hopefully it's a useable one too ;-)

Thanks again for all of your help, thanks for GEO too!  You are seriously awesome.

[R.G.]

You're welcome. Glad I could help.

I have not tried this, but yes, I think the 74C373 latch can be extended by using two latches and paralleling the clock inputs.

That particular circuit only works with the particular chip, the 74C373, not the TTL or other CMOS variants like 74373, 74HC373, 74HCT373, etc. That's because there is a quirk of the clocking in the C373 that makes the circuit work.

There are ways to make the other CMOS chips work, but you have to diddle with RC delays on the inputs and clocks.

[wokeupfalling]

oops