Small-footprint, non-uC latching relay toggle?

[MrStab]

hi guys,

i still have much to learn about relays, and i plan to do a lot more reading on top of what i've already read (from the Geofex articles to various web sources), but whilst all the pieces are there, i can't seem to work out a strategy for what i want to do here.

basically: true bypass with a latching relay, toggle-operated by a momentary switch. simple enough - RG's driver circuit using a CMOS hex inverter seems perfect - but the IC & circuitry almost take up as much real-estate as the effect itself! so is there a way of doing this effectively with a smaller IC and/or less components, without venturing into PIC territory? (or SMD, ofc!)

as i say, i still have much reading to do, but i get the impression both single and dual-coil latching relays are useful here. however, as the Geofex circuit uses a 2-coil, i'm paranoidly biased towards using that, and all the other circuits & explanations i can find are about single-coil or non-latching relays. gah! is one more ideal than the other for bypassing, with regards to performance & current-consumption, or is the difference moot?

i know some people use the 555 chip for non-audio relay applications, but somewhere on AMZ, there's a very vague comment from someone who claims to routinely use it in his stompboxes. i couldn't find any more on that. i'm aware 555s are to be held at arm's length in most cases, due to potential ticking, so that's just one suggestion based on my limited knowledge.

as always, any help is greatly appreciated. cheers!

[R.G.]

You do realize that you're asking for either (1) a custom-purpose IC or (2) an 8-pin microprocessor, don't you?   

There are two-inverter tiny-CMOS circuits, but they're SMD. You could cobble up something out of discretes, but then it would be as big as the 14 or 16 pin DIP you don't want to use.

The 8-pin PIC is ideal for this. Costs about $1, does what you want, and is small. Yeah, there's that programming thing. It's always something. There is the capacitor-in-the-middle circuit, but I'm always suspicious of getting that to work first time, every time. 555's are in general an audio disaster, as you note.

You could do a discrete two-transistor flipflop like Boss and Ibanez use, then tie it to a dual-coil momentary and don't use it momentary.

[mth5044]

Now is your perfect chance to try out a PICAXE! It's seriously a uController for dummies. Very low low cost to start (probably $40 after shipping) then you can keep programming 3-8$ chips all day. You can go from zero programming knowledge to flipping a switch via the PICAXE in less than a day. You could what you want is an 8 pin PICAXE.

[MrStab]

You do realize that you're asking for either (1) a custom-purpose IC or (2) an 8-pin microprocessor, don't you?

i was kinda hoping someone would point me to some magic 74-series IC i was unaware of, but i guess i'm outta luck. well, i've come across a few multi-channel ICs for driving relays, but that'd still leave me with a fairly-large chip.

...then tie it to a dual-coil momentary and don't use it momentary.

could you elaborate? i'm quite the simpleton.

The 8-pin PIC is ideal for this.

Now is your perfect chance to try out a PICAXE!

damn! i was hoping i wouldn't be pushed over the edge, but it may have been some time coming... i've been interested in testing those waters for a while now, though i'm pretty stretched for cash at the moment and it'd get in the way of scoring other parts. am i right in thinking that i can program PIC/AXE chips with just a serial cable and a DIY circuit, instead of buying a kit? or does the software make up much of the price? i'm going off-topic a bit, but iirc i was confused about all that last time i had the uC urge.

thanks a lot for the input, guys

Edit: FWIW, here's the 555 post i mentioned: "But I have recently been using a plain old 555 timer with an optical isolator instead of a transistor." no further details, and i trust you guys more than a random internet one-liner anyway lol

[mth5044]

The PICAXE needs a special usb cable that's $25 or so if I remember. You can build the circuit for programming as it's about 5 parts - resistors and whatnot and 1/8" jack. The program is free. The big price is the cable, but I think you can actually use a normal old serial cable if you have that port, it's the usb ports that need the special cable.

That's all from memory from a bit ago. I can have a better look if you are interested.

[MrStab]

The PICAXE needs a special usb cable that's $25 or so if I remember.

iirc, i'd reached the assumption that i could actually use an el-cheapo USB-Serial adaptor, but something does seem fishy about it being that straightforward.

thanks for the info, i'll save this thread from going into this different topic altogether but you guys may well have set the ball rolling for PICAXE experiments.