12V Signal On/Off Delay - Page 1
Forum section: electronics / projects

author: Vakito
author_rank: Newbie
timestamp: September 03, 2020, 05:57:12 pm
content: 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 thanksPeter

author: Ian.M
author_rank: Super Contributor
timestamp: September 03, 2020, 08:04:03 pm
content: 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.

author: ledtester
author_rank: Super Contributor
timestamp: September 03, 2020, 10:30:15 pm
content: Found this athttps://www.electro-tech-online.com/threads/need-advice-on-a-rising-and-falling-edge-sensor-circuit.92431/which may help:And ideas for monostable multivibrators using CMOS logic gates:https://www.electronics-tutorials.ws/sequential/seq_3.html

author: glentek
author_rank: Regular Contributor
timestamp: September 03, 2020, 10:30:46 pm
content: 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.

author: eblc1388
author_rank: Frequent Contributor
timestamp: September 04, 2020, 06:00:38 am
content: 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.

author: Ian.M
author_rank: Super Contributor
timestamp: September 04, 2020, 06:21:52 am
content: 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 .

author: eblc1388
author_rank: Frequent Contributor
timestamp: September 04, 2020, 07:02:20 am
content: 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.

author: jfiresto
author_rank: Frequent Contributor
timestamp: September 04, 2020, 08:36:50 am
content: 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.

author: Vakito
author_rank: Newbie
timestamp: September 04, 2020, 10:59:51 am
content: 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

author: Ian.M
author_rank: Super Contributor
timestamp: September 04, 2020, 11:57:16 am
content: 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.