Electronics > Projects, Designs, and Technical Stuff
Need some help with a 555 timer circuit
Pizzashape23:
I haven't breadboarded Ian.M's circuit as I don't currently have access to the parts but I needed something to do, so I routed the pcb. I have done a few before but it never hurts to get someone else to check it off. The only differences are the 24v to 12v converter and the various connectors. I know the trace width is excessive for the relays but I had the space and would rather too thick than too thin.
Ian.M:
As your relays are driving a large solonoid with an expectation of high back-EMFs, I would keep as much as possible of the coil connection tracks out from under the relay footprints, so you can maintain the maximum possible creepage and clearance distances between the contact circuits and the coil circuits.
You could replace the Dc-Dc converter with a LM7812 regulator, which shouldn't even require any heatsinking if its only supplying two 555 chips and two 1K pulldowns. Ideally use 24V coil relays so the regulator doesn't have to handle their coil current.
Why's Q1 SMD?
Consider a ground plane under the whole control circuit area. Obviously you'll have to use parts of that side where tracks need to cross, but at the expense of a few extra vias to minimize the length of slots in the ground plane containing other tracks, youl'l gain considerable extra EMI immunity.
Pizzashape23:
I completely agree with needing to move the control tracks from underneath the relays, the low current one maybe not but definitely the high current relay.
The dc-dc converter is what ive got on hand, its a pretty good module that can supply 12v at 2A from between 18v and 36v. Keep in mind it will also be supplying the solenoid control circuitry which uses a pretty beefy mosfet, hence the 12v output.
From the short amount of time I spent searching mouser (usual supplier) i didnt see them in a through hole package, but i might have missed them. If you know of one let me know.
A ground plane is an interesting idea, but I've never done one before so I will probably do a bit of research tomorroe before implementing it.
I feel like I'm starting to repeat myself now, but thank you very much AGAIN. I am really appreciative and would probably have blown up my circuit a couple times without your help. Just curious but what is your background? You are clearly very knowledgeable.
Ian.M:
The transistor in the sim for Q1 was just a fairly random choice out of the possible 2N and 2S part numbers, avoiding legacy parts. Constraints were Ic_max, hfe_min and Vceo_max, and I picked what I did without knowing whether you were using on-board or off-board relays, to be able to tolerate a bit of abuse. You've got the data on the relays so can calculate its Ic, and I estimated its base current above, so you can determine the hfe_min required to get it far enough into saturation, if there isn't enough base current to run at a forced Beta of 10. I wouldn't go under 30V Vceo.
Zero999:
--- Quote from: Ian.M on August 11, 2020, 03:30:59 pm ---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.
--- End quote ---
Why use two 555 timers?
The standard 555 timer circuit is not retriggerable. As long as monostable delay is longer, than the bounce time, there's no need to for any debouncing.
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