Author Topic: 12V Signal On/Off Delay  (Read 1035 times)

0 Members and 1 Guest are viewing this topic.

Offline VakitoTopic starter

  • Newbie
  • Posts: 5
  • Country: gb
12V Signal On/Off Delay
« on: September 03, 2020, 05:57:12 pm »
Hi all, I'm in need of a simple 12v circuit designing which triggers an output going to a MOSFET for an adjustable amount of seconds.
When the input is turned on, the output should be on for a few seconds (example, 5 seconds) and the same should happen when the input signal is turned off.
Hopefully the timing diagram attached will make things clearer.

Many thanks
Peter
 

Online Ian.M

  • Super Contributor
  • ***
  • Posts: 13218
Re: 12V Signal On/Off Delay
« Reply #1 on: September 03, 2020, 08:04:03 pm »
Well the obvious lineup is a Schmidt trigger to guarantee consistent fast input transitions, an XOR edge detector and a retriggerable monostable for the output pulse, with a variable timing resistor, all in 4000 series CMOS logic.  Even if you cheat and use a quad XOR with two of its gates strapped as non-inverting buffers with positive feedback to make them Schmidt triggers for the input and delay path, that's still two ICs.

I think you can do it with a single 4106 hex Schmitt inverter used creatively.  One gate to square up the input, then AC couple it with a short time constant to get a bi-polarity pulse from both edges, eliminate the negative going half, and square it up with another gate, leaving several gates to implement the monostable.  It *MAY* need a transistor to discharge the timing cap faster than a 4000 CMOS output can manage.
 

Offline ledtester

  • Super Contributor
  • ***
  • Posts: 3286
  • Country: us
Re: 12V Signal On/Off Delay
« Reply #2 on: September 03, 2020, 10:30:15 pm »
« Last Edit: September 03, 2020, 10:34:48 pm by ledtester »
 

Offline glentek

  • Regular Contributor
  • *
  • Posts: 56
Re: 12V Signal On/Off Delay
« Reply #3 on: September 03, 2020, 10:30:46 pm »
Here's part of a circuit that I designed to control an actuator to open/close a vent. You could combine the outputs with diodes to do what you want.

 

Offline eblc1388

  • Frequent Contributor
  • **
  • Posts: 400
  • Country: gb
Re: 12V Signal On/Off Delay
« Reply #4 on: September 04, 2020, 06:00:38 am »
Your problem can be solved using a 555 timer set up as a monostable with a 5-second duration. It only needs to be triggered on both edge of input trigger signal to get the desired output.

 
 
The following users thanked this post: Ian.M

Online Ian.M

  • Super Contributor
  • ***
  • Posts: 13218
Re: 12V Signal On/Off Delay
« Reply #5 on: September 04, 2020, 06:21:52 am »
That's neat but wont work on a slowly falling input signal as the rate of charge transfer through C1 must develop enough voltage across R1 to turn Q1 on hard enough.   I calculate it as needing min 0.022 V/us slew rate which comes out to slightly over 0.45 ms fall time. OTOH if you use a 556,  with the other half's Threshold and Trigger strapped together as a Schmidt inverting buffer with 33% Vcc hysteresis, to drive it it will work on even very slowly ramping signals, if you can tolerate the lag caused by the wide hysteresis ban .
 

Offline eblc1388

  • Frequent Contributor
  • **
  • Posts: 400
  • Country: gb
Re: 12V Signal On/Off Delay
« Reply #6 on: September 04, 2020, 07:02:20 am »
Yes, its nice to know there is a fallback solution should the current simple implementation doesn't work out satisfactorily. It will still be a single chip solution, albeit a 556 instead.
 

Offline jfiresto

  • Frequent Contributor
  • **
  • Posts: 896
  • Country: de
Re: 12V Signal On/Off Delay
« Reply #7 on: September 04, 2020, 08:36:50 am »
Look at TI's CD14538 dual one-shot. It has a 3-18V supply range and negative and positive edge, schmitt trigger, trigger inputs. With two RC pairs as edge detectors, you might be looking at the IC and perhaps seven added discretes, six of which are cheap. Or you could use both halves and fire one on positive edges and the other on negative, with independent leading and trailing edge pulse times.
« Last Edit: September 04, 2020, 08:39:16 am by jfiresto »
-John
 

Offline VakitoTopic starter

  • Newbie
  • Posts: 5
  • Country: gb
Re: 12V Signal On/Off Delay
« Reply #8 on: September 04, 2020, 10:59:51 am »
Thanks everyone for the responses so far, I think my best bet would be to use a 4077 XNOR as the rising and falling edge detector followed by a 555 monostable for the timing.

What size capacitor would you recommend between the Vdd and Vss of the chips if any?

Thanks
 

Online Ian.M

  • Super Contributor
  • ***
  • Posts: 13218
Re: 12V Signal On/Off Delay
« Reply #9 on: September 04, 2020, 11:57:16 am »
Depends.  Traditional bipolar 555s (e.g. NE555) tend to 'crowbar' the supply with shoot-through as they switch.  The only hope of taming them so they are good neighbors is 10uF to 100uF of low ESR electrolytic decoupling as close to the supply pins as you can get it.  CMOS '555's are much better behaved.

CMOS chips however generally only need 10nF to 100nF ceramics for adequate decoupling.  If you are using 4000 series at low voltage, their slow transitions mean you can even get away without decoupling every chip if your layout is fairly tight.
 


Share me

Digg  Facebook  SlashDot  Delicious  Technorati  Twitter  Google  Yahoo
Smf