Closing two circuits with a single switch

Thread Starter

Zurn

Joined Mar 4, 2019
117
Here is a typical application where the output of a 555-timer circuit is used to switch on an NPN transistor.
+VLOAD can be different from +VCC. The load can be an LED, opto-coupler, relay, etc.
View attachment 194383
Oh I see, I didn't tie the grounds together... rookie mistake!

Next up is determining what is happening at SW1, so we know which end is being switched in which direction. Same two questions as in #17 for SW1.
SW1 is much weirder:
- When the TTL level is high (the system is running), open terminal a has 4.92V and terminal b has -0.07V
- When the TTL level is low (the system is shut off), open terminal a has -11.08V (!?) and terminal b has -16.06V (which would change with whatever potential the batteries are storing at the time).
- When closed with the TTL level high (turning system power ON), both have -0.68V
- When closed with the TTL level low (turning system power OFF, both have -15.65V

I hope that doesn't confuse things.

Also, what power is available to run the timer circuit?
I can take power directly off the system batteries (which are controlled by uC1), which is 12-16V depending on the charge state. I imagine I could wire it in separate from the MOSFET switch so that it's always on.
 

Thread Starter

Zurn

Joined Mar 4, 2019
117
Hi everyone,

I'm trying to wire a 555 timer to be monostable with a 3 second delay when triggered. While I'm closing and holding the switch, I want the timer to wait three seconds before setting the output high. If the switch is release at any point I want it to stop what it's doing and return to the stable off state. Here's what I'm working with: the delay works, but the output stays on indefinitely and the switch need only be pressed once, rather than held...

555-On-Delay.png

The reason for this is that I'm designing a power control circuit for two microcontrollers (as outlined in this this thread - I figured this was so particular to 555s it was worth starting a new thread, please correct me if I'm wrong). Both devices use simple switches to turn them on/off and I want to trigger them with only one switch, which I'll call SW1 (further detail: it needs to be a reed switch, and double poled reed switches are scarce hence the 555s). The catch is that one of the uC switches needs to be delayed by 3 seconds or so to give the first uC time to go to sleep before its power is cut by the second uC. So basically: I close and hold a switch, which in turn closes and holds the switch for the first uC, it goes to sleep, then 3 seconds later the the switch for the second uC is closed and held (while still holding the first uC's switch closed). When the power is cut I release the switch.

I've been trying to wrap my head out this design but can't quite figure it out. Any input would be greatly appreciated.
 

MrChips

Joined Oct 2, 2009
30,706
The problem is not about the 555-timer circuit. The problem has to do with the sequence of events required to perform the function that you desire. This is why I asked for a timing diagram.

I will merge the new thread back into this one.
 

MrChips

Joined Oct 2, 2009
30,706
If you want to use 555-timer ICs here is what needs to happen.

(1) Condition the push-button output to give a clean signal with no bounce. You could us a 555-timer for this.
(2) Trigger a 555-timer with a 3-second pulse width duration.
(3) On the trailing edge of signal (1), check the output of signal (2).
(4) If the result of (3) is affirmative, trigger another 555-timer with x-ms duration.
 

Thread Starter

Zurn

Joined Mar 4, 2019
117
If you want to use 555-timer ICs here is what needs to happen.

(1) Condition the push-button output to give a clean signal with no bounce. You could us a 555-timer for this.
(2) Trigger a 555-timer with a 3-second pulse width duration.
(3) On the trailing edge of signal (1), check the output of signal (2).
(4) If the result of (3) is affirmative, trigger another 555-timer with x-ms duration.

Hmm... I don't quite grasp what you're asking me to do here. Let me see if I can ask some clarifying questions for each of the numbered suggestions.

(1) This is IC1, which is just wired to give a high output when SW1 is closed, correct?
(2) This is IC2, wired as a one shot with a 3 second pulse width, correct?
(3) When IC1's output goes low again... check the output of IC2? You mean with an oscilloscope?
(4) If the output of IC2 goes low... trigger IC3? Three 555s?

My apologies for the thick-headedness. I am also not bound to 555s, it's just what was suggested to me.


The problem is not about the 555-timer circuit. The problem has to do with the sequence of events required to perform the function that you desire. This is why I asked for a timing diagram.
I still don't actually grasp why the problem isn't with the 555 timer circuit!
 

MrChips

Joined Oct 2, 2009
30,706
Let me try to put it differently.

(1) The switch SW1 has to be held down for at least 3 seconds (IC1). If shorter than 3 seconds, no action is taken.

(2) We use IC2 to time 3 seconds.

(3) Here we have two options. (I have not analyzed which is the better option.)

Option A - When the button is released (1), use the trailing edge to register the status of IC2. There are different ways to do this, We can use the trailing edge IC2 to generate a short positive pulse which is ANDed with NOT IC2. Or you can use a D-type flip-flop that registers the status of IC2.

Option B - Interchange IC1 and IC2 and do something similar. That is, take the trailing edge of IC2, generate a short pulse and AND it with IC1. Or use a D-type flip-flop as above.

No, we are not using an oscilloscope here. We are implementing everything in hardware.

Everything would be clearer if I were to draw a circuit diagram but I am too lazy (read busy) to do it right now.
 

Thread Starter

Zurn

Joined Mar 4, 2019
117
I see... so what I'm trying to do technically isn't possible the way I'm imagining it (i.e. triggering two 555s with same switch and having one wait a few seconds to turn on). I have to throw in the towel for the day but I'll check back in tomorrow.

Thanks for the input!
 

MrChips

Joined Oct 2, 2009
30,706
Here I use a dual monostable multivibrator MC14528 or MC14538 instead of 555-timers.
The monostable offer both positive and negative polarity outputs making the design easier.
(I happened to have MC14538 in stock.)

Switch S1 is placed on the high side. If you need the switch on the low side I can change the design.

When S1 is not pressed or is released, the second half of the 4538 is in a RESET state.
The rising edge on pin-4 triggers the first half of the monostable to produced a 3-second pulse.
At the trailing edge of the 3-second delay, a rising edge on pin-7 triggers the second monostable (as long as S1 is still pressed) to produce a 1.4ms pulse output. You can choose this pulse width by modifying C2 and R3 to suit.

The power supply voltage +V can range from 3V to 18V. The IC is powered with GND on pin-8 and +V on pin-16. Always put a 100nF ceramic capacitor across GND and +V.


Button held for 3 seconds.jpg
 

Thread Starter

Zurn

Joined Mar 4, 2019
117
Hmm... amidst the holiday scramble I must have forgot to reply with an earnest "thank you" few weeks back!

Back to it now... the circuit looks very interesting. Ordered a couple of MC14538s to give it a go, though unfortunately they only seem to be produced in surface mount packages these days. I think I can still work with that, but in the meantime I'm still going to try and substitute the 555. Of course, any pointers on that would be greatly appreciated.
 

Thread Starter

Zurn

Joined Mar 4, 2019
117
Alright: massive update/breakthrough...

I couldn't get the MC14538 circuit to work... probably because it's so small and I was having a lot of trouble wiring it. HOWEVER...

A new, much simpler design has emerged: switching both uCs from the changing TTL level! Check out this schematic. J17 on uC2 is what SW2 was closing.

Basic-Power-Schematic.png

So my question now is regarding IC1. All I need is something who's output will match whatever state the input is but with a delay of about 20 seconds on the falling edge (delay can be on both rising and falling, but just falling is ideal so you don't have to wait for it to power ON).

Here's my ideal timing diagram:

Power-circuit-timing-diagram.png

I feel like this is a simple digital thing but I'm having trouble figuring out what its called. Any suggestions would be greatly appreciated!
 
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