searching for a reliable circuit - Page 1
Forum section: electronics / beginners

author: LaserTazerPhaser
author_rank: Regular Contributor
timestamp: March 09, 2019, 08:31:42 am
content: What would be areliablecircuitwithoutthe need of programming a micro which would output a signal which is high when the input is high but goes low and stays low after the input goes low once and does not go high again even if the input goes high?The input signal would be many minutes to hours.These things can be done easily for short durations but long durations needs practical experience because since components can vary.

author: Dubbie
author_rank: Unknown
timestamp: March 09, 2019, 09:13:12 am
content: Could use an active low latch and hook up to the q bar output. Just one small component.

author: soldar
author_rank: Super Contributor
timestamp: March 09, 2019, 09:15:41 am
content: A simple flip-flop will do that.https://en.wikipedia.org/wiki/Flip-flop_(electronics)

author: LaserTazerPhaser
author_rank: Regular Contributor
timestamp: March 09, 2019, 08:41:31 pm
content: as in a 4013 latch?

author: IanB
author_rank: Super Contributor
timestamp: March 09, 2019, 08:51:26 pm
content: A single AND gate will do that, or a relay if you want a mechanical solution. A circuit like this is used all the time for the "push to start, push to stop" switching arrangement on machinery. (If you push the red button momentarily the machine will stop, and it won't start again until you push the green button.)

author: Dubbie
author_rank: Unknown
timestamp: March 09, 2019, 10:43:20 pm
content: How can you use a single and gate Ian? I can’t figure out how you would latch it so it can’t activate again.

author: IanB
author_rank: Super Contributor
timestamp: March 09, 2019, 11:44:46 pm
content: Quote from: Dubbie on March 09, 2019, 10:43:20 pmYou take a two input AND gate and feed the output back to the second input. Then as long as the first input is high, the output will be high. If the first input goes low the output will go low, taking the second input low. Thereafter, even if you raise the first input high again the output will remain low since the gate will only see one high input.Of course, the output will never be high in the first place unless you reset it externally. Therefore all such latching circuits need a reset switch to enable them. To reset the AND gate all that is needed is to momentarily set the output high and then it will stay there.

author: Dubbie
author_rank: Unknown
timestamp: March 09, 2019, 11:46:15 pm
content: Thanks.Logic is not my strong suit. Of course it seems trivially obvious when you spell it out!

author: LaserTazerPhaser
author_rank: Regular Contributor
timestamp: March 11, 2019, 03:16:14 am
content: Quote from: IanB on March 09, 2019, 11:44:46 pmThis?

author: Ian.M
author_rank: Super Contributor
timestamp: March 11, 2019, 05:10:48 am
content: Not quite. You still need to be able to drag pin 2 high to initially set the output if it happens to power up with its output low, so put a resistor in the feedback loop. and a momentary switch from pin 2 to Vcc.However if you want to automate setting the output at powerup it gets more complex . . .Also, there's a flaw in your original spec. It doesn't specify whether or not the output should be high if the input has been low since powerup.

author: hamster_nz
author_rank: Super Contributor
timestamp: March 11, 2019, 05:46:57 am
content: A dual pole relay can do this, and is often used this way in industrial machinery where safety is critical.

author: Dubbie
author_rank: Unknown
timestamp: March 11, 2019, 05:50:21 am
content: Lol blueskull. No half measures for you!

author: soldar
author_rank: Super Contributor
timestamp: March 11, 2019, 07:03:32 am
content: Quote from: IanB on March 09, 2019, 08:51:26 pmThis circuit will do that.

author: magic
author_rank: Super Contributor
timestamp: March 11, 2019, 07:46:43 am
content: I suppose a thyristor would do and wouldn't require a reset after power-up.