# Help to build a circuit to trigger event only after input signal changes from "1" to "0"

I'd like to build a kind of logic as shown in the picture ,but I can't do it and I really appreciate if someone can help on that. Basically what I'd like to achieve is that when the input signal changes from 1 to 0 it must trigger a switch at last 2-3 seconds and after everything released. The input signal is not synchronous which means that it might last undefined time. Thanks !

Thanks to comments and based on the link HandyHowie provided at paragraph "8. More about triggering" as shown in the picture above it looks it might be the right solution ,but I'm not 100% sure. Please let me know if this might do the job.

Sounds like a job for a 555 timer configured as a monostable, see for example - http://www.doctronics.co.uk/555.htm#monostable

• I agree. That's all he/she needs here. +1 Commented Sep 4, 2015 at 11:37
• Thanks a lot for your quick help ! Really appreciated this and this forum looks fantastic ! I read on the webpage you provided ,on chapter "8. More about triggering" and it looks that could be the solution for my case. I'm not sure about what voltage values should be used as input and if the resistor R1 = 2MOhm + Capacitor C=1 microF will give an output signal to last 2 seconds based on the timing formula T=1.1(R x C). Please let me know if I'm right. Kind Regards ! Commented Sep 4, 2015 at 14:21
• I would go with 180K and 10uF which should give you 1.98 seconds. Commented Sep 4, 2015 at 14:44
• I am pretty certain that the circuit above will do what you want. Commented Sep 4, 2015 at 14:49
• Many, many thanks m8 ! I ran a successful simulation with the values you suggested for resistor and capacitor using as input 9V and supply of 3V for LM555CM timer. Wish you'll be my teacher ! Kind Regards ! Commented Sep 5, 2015 at 21:27

You can't do that with pure logic alone ! Detecting the negative slope on the input is easy, a JK flip-flop will do that. But you need a second even after 2-3 seconds to flip the output back. For this you would need a one-shot or a delay. Which are not pure logic circuits, they rely on (dis)charging something to "time" the time.

• It can be done with logic gates and a few discrete components (R and C). No need for a flip-flop. Commented Sep 4, 2015 at 11:34
• And a flip-flop is made of... logic gates. Commented Sep 4, 2015 at 13:08
• Thanks for you comments and suggestions ! Initially I thought to build using gates and building a true matrix ,but it was not so easy because every time you have to compare the input signal with a kind of 1 bit memory which register if the event(input signal) has been triggered. With 555 timer looks easy ,but I'm not sure if the input signal longs too much (even 1 hour maybe). Commented Sep 4, 2015 at 14:39

Any common microcontroller programmed with a "High to Low" transition interrupt trigger would do this. Which much less supporting parts.

• Thanks for your suggestion! This also interesting and I'll really appreciate if you can give a clue. Kind regards ! Commented Sep 7, 2015 at 15:30