Need a simple pulse activated circuit. - Page 1
Forum section: electronics / beginners

author: illusive
author_rank: Regular Contributor
timestamp: July 21, 2015, 11:08:05 am
content: Hello everyone!I need a simple pulse activated circuit. I have set up an op amp comparator which output goes high just for a split second. I need a circuit that will hold that high output level for some time (minute or more). I thing something with a 555 timer will do the job, but i'm not very familiar with that IC so i can't choose the right circuit.How about this circuit ?

author: Stray Electron
author_rank: Super Contributor
timestamp: July 23, 2015, 11:33:04 pm
content: As long as your output impedance is very high, you can use a blocking diode and let that charge a capacitor. I saw a circuit like this once that was used to control a Mosfet gate and the cap would hold it on for several minutes and I think they were using a .1 uF cap or something in that range. The most important part is that the diode have averylow reverse current flow.

author: Kappes Buur
author_rank: Regular Contributor
timestamp: July 24, 2015, 04:56:48 am
content: http://home.cogeco.ca/~rpaisley4/LM555.htmlMaybe 4. Basic Circuits For The LM555 Timer, circuit 3

author: mikerj
author_rank: Super Contributor
timestamp: July 24, 2015, 07:07:28 am
content: Quote from: Stray Electron on July 23, 2015, 11:33:04 pmYou want a LOW output impedance to do this. If you had high output impedance, a short pulse may not be able to charge a cap of reasonable value in time.A monostable such as a 555, 4047, 4098 etc. would be a better idea.

author: tautech
author_rank: Super Contributor
timestamp: July 24, 2015, 07:28:49 am
content: Quote from: mikerj on July 24, 2015, 07:07:28 am+1If you don't know the 555 you need to.Use it.If low power consumption is required use the CMOS versionPlenty of calculators online for appropriate values to suit your needs.http://www.csgnetwork.com/ne555timer1calc.html

author: Stray Electron
author_rank: Super Contributor
timestamp: July 24, 2015, 07:50:39 pm
content: Quote from: mikerj on July 24, 2015, 07:07:28 amYou need Low input impedance to charge the cap but high output impedance to prevent draining it too soon.I agree about the 555 probably being a better choice but the OP wanted asimplecircuit.And the other poster was right about getting to know the 555. It's one of the most useful and versatile ICs ever made! 'The IC Timer Cookbook' is a good place to start. I don't know if it's still in print or not but itshouldbe.

author: michaeliv
author_rank: Frequent Contributor
timestamp: July 27, 2015, 07:59:08 pm
content: If you need the output to remain high for at least a period of time (you don't care if it turns off after a certain period) then you can just use a latching circuit.If you do need the output to go back low after a period of time, using a 555 might not be ideal for your scenario as it does not play very well with long time periods.You can use the op-amp to turn on a MOSFET that powers a circuit like this :https://www.elprocus.com/wp-content/uploads/2013/04/CD-4060-TIMER.pngAnd you can have output Q12 reset the power to the timing circuit ( turn it off ).

author: mikerj
author_rank: Super Contributor
timestamp: July 31, 2015, 01:20:29 pm
content: Quote from: Stray Electron on July 24, 2015, 07:50:39 pmOther way around. The output is charging the cap, therefore a low impedance is needed. A buffer following the cap would require a high input impedance to prevent discharging the cap too quickly.