Electronics > Projects, Designs, and Technical Stuff

Need some help with a 555 timer circuit

<< < (4/12) > >>

Circlotron:
You could put a capacitor across the switch contacts. When they close the cap gets suddenly discharged and this current should wet the contacts. How big a cap? Others may care to comment.

Ian.M:
Energy conservation is not an issue*.  Simply use a 1K pullup (to +12V) for the switch to meet the 'rule of thumb' wetting current and voltage requirements.  However, if you care about your switch (or relay) contacts and EMI one should also be careful *NOT* to directly short any significant size capacitor without some resistance in the loop to limit the surge current and damp the capacitor ringing with the loop inductance to reduce EMI.

How big does the capacitor have to be before it matters?   Well that depends on the switch.  A massive wall mounted two-handed
knife switch with heavy solid silver alloy wiping contacts can withstand a lot more than a sub-miniature reed switch with only a thin flash of gold over the iron of the reeds for a contact surface. 

For boosting wetting current for small switches without the penalty of a high standing current when closed, 1nF to 10nF is in the ballpark. 

I start worrying about loop resistance and surge current if I'm shorting out caps of 100nF upwards (e.g. in a 'classic' MCU reset circuit).

Considering your circuit over at  StackExchange, remove the 100K resistor across the capacitor, add a 1K pullup to the junction of the capacitor and the switch, and add a small signal diode, cathode positive, across the resistor pulling Trigger high, to clamp it when the switch opens.

However I am *really* not happy with that circuit for a safety system.  Its got absolutely nothing to prevent activation by EMI pickup on the switch wiring or by ESD to the switch.  IMHO using a 555 trigger input almost directly exposed to the outside world is *CRAZY*.  Using another 555 to buffer, debounce and ESD and EMI filter the switch input would make me a lot happier.  I'll see about working up a sim for you so you can see what I mean. 
 

* Its feeding a fricking metal-smelting furnace, so an extra seventh of a watt in the pullup is truely negligable!

Pizzashape23:
WOW, I really appreciate the amount of detail you managed to include in that response. One question about your response, if I use a 1K pullup do I still need the capacitor across the contacts?

Correct me If I'm wrong, but your response suggests that the 555 circuit path is the correct way to go about what I am trying to achieve, or is there a better/more efficient way of switching the relays for a fixed pulse width?

Just note, the reason the "safety" is so rudimentary is becasause the circuit it is protecting is ridiculously basic, I could probably find a schematic if you would like. (It would be considered very *VERY* Basic, and is part of the reason why i opted for a safety circuit i the first place). However, if adding an additional 555 and a few more components is all it would take to drastically improve the safety, I whole hartedly agree that it would be the smartest thing to do. Be warned, I wouldn't know the first thing about going down that path and would probably need a lot of help, but I would definitely be up for learning.

Ian.M:
Ok, here's the 2x 555 timer version, as safe as I can reasonably make it.
R1 provides 12mA through the switch and 12v across it when open, so unless you use a particularly crummy switch you don't need to worry about wetting current.

Fs1 and R3 provide protection against shorts to ground in the switch wiring.  If Fs1 trips, IC2 is locked out via its Reset pin.  R4, C7 provide a power-up delay to ensure U2 doesn't pulse the relay at switch-on, and in conjunction with R3 also provide ESD protection to its Reset pin.

R2, C1 provide input filtering for debouncing and in conjunction with R1 provide ESD protection to U1, which is wired as a simple inverting Schmidt trigger with 1/3 Vcc hysteresis*.  Its Out pin is the inverted debounced switch state (i.e. low when pressed).

R5, C4, D3, R6 are the negative edge trigger pulse shaping network, that only triggers U2 on a new button press, once the current  timing period is complete. 

C5, R7 is the timing network and R8 limits the current into the Discharge pin.  R9, R10, Q1 is a 'bog-standard' relay drive circuit, though its imprudent to rely on the diode inside the relay coil to protect Q1.  If you are feeling moderately paranoid, adding a 24V Zener, cathode positive, from ground to Q1 collector, + a fuse in series with the feed to the relay coil wouldn't be a bad idea.

I've minimized the number of values used for BOM reduction. If that isn't an issue, R8 and R9 could benefit by being reduced, and the CV pins don't need so large decoupling capacitors.

LTspice sim with some nasty simulated contact bounce attached.   Zoom in on the edges of  V(in) to see it!  >:D  For the details of how the SIM only Switch Control works, see LTspice: Piecewise Linear Functions for Voltage & Current Sources and my comments on simultating switches [here].

H.T.H.

Ian.
Now everybody's got a chance to bash my ideas to death  :horse: while I chill  :popcorn: and hopefully we all learn something!

* A buffer following a RC filter for debouncing *MUST* have hysteresis.
You should probably (re)read Jack Ganssle's excellent article on debouncing: http://www.ganssle.com/debouncing.htm especially part 2 where he covers RC hardware debouncers.

Pizzashape23:
 :clap: :clap: :clap:
HOLY $**T

That has to be the best and most helpful response I have ever recieved. You nailed all of the design goals I set for myself AND managed to reduce the BOM.
In all seriousness, I cannot thank you enough.

I now have just two questions;
How long did this take you? (I probably would have taken a week)
Do you still have the shcematic file? (I wanted to get a custom PCB printed)

Navigation

[0] Message Index

[#] Next page

[*] Previous page

There was an error while thanking
Thanking...
Go to full version
Powered by SMFPacks Advanced Attachments Uploader Mod