Pulse Generate on Toggle switch state change - Page 1
Forum section: electronics / projects

author: ZoNiE
author_rank: Newbie
timestamp: December 20, 2014, 02:27:37 am
content: Hi folks, I'm new here. I took electronics in High School and College, but have been working largely outside of design for 25 years. That said, I'm really rusty. Like 1986 rusty.Here's my project: I want to control a momentary switch with a toggle switch. I am basically replacing a DPDT switch with a SPST wireless relay and the relay will latch on the push of the remote button, and unlatch on pushing it again. I want to make it work the same way with a toggle switch. I was thinking a flip flop that issues a pulse every time the state of the input changes and is wired in place of the pushbutton on the wireless remote. The pushbutton pulls the ckt to gnd.Operation:Relay starts out unlatched when power is applied to ckt (Normal condition).flip toggle switch to "ON" and circuit pulls a line to GND for a few mS then goes high/float.Nothing happens so long as the toggle is left "on".When toggle switch is flipped "off" the circuit pulls the same line to GND again for a few mS, then goes high/float.On the other end of the wireless system, the relay turns on and off as if the switch is hardwired to the coil.Yes, I could just push a momentary button to toggle the thing on and off, but I want to use the other pole on the toggle to light up an "enable" LED. (This is for a remote rocket launch pad) and I could put a latch ckt to toggle the LED on and off, but the main thing is to still have the toggle switch work like a toggle switch.That way, if the switch is up (on) the pad is armed and the LED on the console is On, and if it is down (off) the pad is safe and the LED on the console is off. It needs to be idiot proof for users who don't understand anything other than on off switches.I'm digging through my old notes, and my usual pile of Forrest Mimms books, but cannot find exactly what I'm looking for. I was thinking some sort of one-shot, but they all seem to work on the single input pulse only. Perhaps some sort of RS flip flop...I would just use toggles in place of the momentary buttons, but if you hold one down, the others are disabled/locked out. You cannot energize multiple relays (there are 4 on this unit) at the same time. Kinda put a kink in my original design.Any suggestions?

author: SL4P
author_rank: Super Contributor
timestamp: December 20, 2014, 03:08:14 am
content: Look for 555 or 74LS121 / 74LS221 as a non-retriggerable one-shot.Minimal parts - reliable.

author: ZoNiE
author_rank: Newbie
timestamp: December 20, 2014, 05:14:48 am
content: Ahh! "Non-retriggerable" that's probably what I need to search for.Thanks!

author: ZoNiE
author_rank: Newbie
timestamp: December 20, 2014, 06:49:14 am
content: So far, I can only find what works Backwards. I built one that was non triggerable, but the output stays high as long as the switch stays on, or 5 sec, if I turn the switch off right away.This one:http://en.f-alpha.net/electronics/integrated-circuits/timer-555/go-on/experiment-12-non-retriggerable-monoflop/I need a single output pulse when the input goes high and another pulse when the input goes low. This pulse will send a radio signal to the relay on the receiver which will latch on. when I turn the switch off, another pulse will trigger the relay to switch off.I'm trying to load all the electronics in the local controller, so I only have the receiver and power relays in the remote location. I guess if I cannot figure out one without going Arduino or PIC, then I could just use some more relays. This thing needs to be able to be troubleshot and repaired in the field by people other than myself, and relays are easier to tshoot and replace than transistors or 555 timers...

author: Richard Crowley
author_rank: Super Contributor
timestamp: December 20, 2014, 07:33:18 am
content: Using a conventional toggle switch to generate generic "on/off" pulses is just asking for trouble. You have no way of ensuring that your switch will stay IN SYNC with the on/off toggle. And made even worse by a local indicator that indicates whether your local switch is "ON" or "OFF" but in no way actually indicates whether the far-end circuit is enabled or not.This is especially problematic for something as dangerous as your proposed application. It is not beyond the realm of possibility that it could maim or even kill someone. I would highly discourage you from pursuing this scheme. This is just asking for disaster.

author: mikerj
author_rank: Super Contributor
timestamp: December 20, 2014, 10:11:37 am
content: Quote from: Richard Crowley on December 20, 2014, 07:33:18 amI 100% agree with this. A one way wireless system that simply toggles a relay state is completely inappropriate for this application.

author: ZoNiE
author_rank: Newbie
timestamp: December 20, 2014, 06:12:25 pm
content: I have a latching relay on the other end with light and sound that will tell me if the business end is energized, and considering using another unit on the other end to energize indicators on the control end. A lot of people are using the wireless remotes for this application now, and we have NAR guidelines we follow, so no, not dangerous, and not likely to kill or maim someone. I also plan to use it to continuity check from safe distance as many people are checking at the launch pad, which is not something I like, and is allowed, strangely. We're talking modrocs here, not high power rocketry.

author: Richard Crowley
author_rank: Super Contributor
timestamp: December 20, 2014, 06:37:26 pm
content: From the view of user interface design. Stick with a momentary switch and NO local indicator.The momentary switch will force the user to look at the primary site (the far end) to see what the status is.Having a local status indicator without some positive method of synchronization is unreliable at best.

author: ZoNiE
author_rank: Newbie
timestamp: December 20, 2014, 07:41:04 pm
content: Yeah, I figured as much. Unless I go Arduino, this the way to go. Less wiring as well.