Positive & Negative pulse extender - Page 1
Forum section: electronics / beginners

author: tanlccc
author_rank: Newbie
timestamp: June 15, 2017, 08:15:25 am
content: Need help...I have an electrical signal that can give either a positive +10V square pulse or a negative -10V square pulse. The duration for each square pulse is 100ms.I would like to prolong the pulse width to 300ms while maintaining its polarity.What can I do?

author: Ian.M
author_rank: Super Contributor
timestamp: June 15, 2017, 08:43:14 am
content: What's the signal level between pulses?

author: tanlccc
author_rank: Newbie
timestamp: June 15, 2017, 09:11:24 am
content: Quote from: Ian.M on June 15, 2017, 08:43:14 amWhen there is no pulse, the voltage level is 0V.When signal comes, it will be either one of the following:0V -> +10V ~~~ 100ms ~~~ +10V -> 0V0V -> -10V ~~~ 100ms ~~~ -10V -> 0V

author: keethrax
author_rank: Contributor
timestamp: June 15, 2017, 01:21:05 pm
content: Some questions that occur to me:Is the input always 100ms? If it varies are you always looking for 300ms, or for 3x the original?How accurate does the 300ms timing need to be?Does the 300ms pulse start as soon as the input goes to 10V or -10V or does it wait until after the 100ms pulse is complete?How much delay is acceptable between receiving (or ending depending on the answer to the above question) the 100ms pulse and starting the 300ms pulse?Can a second pulse be received 300 ms?If a second pulse during the 300ms pulse is possible, what is the desired behavior? I can think of at least three possibilities.

author: tszaboo
author_rank: Super Contributor
timestamp: June 15, 2017, 01:25:41 pm
content: Do you have positive and negative power supply, or just this signal coming from a pair of wires?

author: tanlccc
author_rank: Newbie
timestamp: June 16, 2017, 12:38:18 am
content: Hi keethrax,Below are my replies:Quote from: keethrax on June 15, 2017, 01:21:05 pmThe input signal varies between 90ms to 110ms. The output signal will need to be at least 300ms, preferably fixed at 300ms. A 3x the original solution will also be fine.Quote from: keethrax on June 15, 2017, 01:21:05 pmReplied above.Quote from: keethrax on June 15, 2017, 01:21:05 pmNo need for the 100ms pulse to complete. As soon as it is triggered, the 300ms pulse can start.Quote from: keethrax on June 15, 2017, 01:21:05 pmAny delay less than 100ms is acceptable.Quote from: keethrax on June 15, 2017, 01:21:05 pmIf the second pulse received at 300ms, the output can be 300ms.Quote from: keethrax on June 15, 2017, 01:21:05 pmThe output can be the same.

author: tanlccc
author_rank: Newbie
timestamp: June 16, 2017, 12:41:36 am
content: Quote from: NANDBlog on June 15, 2017, 01:25:41 pmActually the output comes from a particular device (single wire) that gives positive and negative output. I need to alter this output to have longer pulse width (100ms to 300ms) that's all.

author: BrianHG
author_rank: Super Contributor
timestamp: June 16, 2017, 03:18:31 am
content: Quote from: tanlccc on June 16, 2017, 12:41:36 amYes, but if you don't have a +10v and -10v supply to drive your output 300ms signal, where would the power come from?Is there a relative GND reference. Example, is the 1 source wire going +10v with the other wire to GND, and when you have -10v, is the +10v wire going to GND and the other wire going to +10v making the signal at the wire look like it's going from -10 to +10, then both wires at GND for 0v?Or, is 1 wire a true fixed GND and the other having a 20v PP swing?When you have 0v, is this an open signal, or, is it shorted at GND?Also, how much current will you need to drive?Could an RS232 driver with a +/-10v supply give your enough current? And for 0v, just disabling the output drive leaving the output at 0v sufficient?

author: eblc1388
author_rank: Frequent Contributor
timestamp: June 16, 2017, 10:35:49 am
content: Based on your requirement, I have come up with the following circuit. It uses only two ICs, namely CD14538B Dual Monostable and Dual Opamp like LM358. The circuit requires +/- 12V supply to work. You can use a supply of +/-10V if you use Rail-to-Rail output Opamp.The input(IN) signal will be fed to both monostables trigger, with one triggering on the positive going +10V edge and the other -10V edge. The monostable output pulse width is fixed by R1/C1 and R2/C2 to 300mS. Addition circuitry is added to take care of the condition where another pulse arrives before the 300mS timing period has completed. The two separate output of the monostable are recombined into one using the dual opamp to obtain a single output(OUT).A simulation was done to confirm the operation of the circuit when normal and abnormal pulses are presented and the output is correct. Notice that the simulation is done in microsecond time scale rather than millisecond as it would take ages to simulate otherwise. The only difference is the value of the monostable timing component R1/C1 and R2/C2.Obviously this is just one way to tackle your problem and I'm sure there will be other ideas from different members soon.Correction: typo, CD4538B should be CD14538B

author: Ian.M
author_rank: Super Contributor
timestamp: June 16, 2017, 10:48:35 am
content: I threw a LTspice sim together based on rectifying the input pulse, thresholding it with a comparator and delaying and stretching it to gate a sample and hold circuit - however it was quite ugly with a mess of six MOSFETS to do the sampling, due to the lack of a 'real world' analog switch with enough voltage range and a LTspice model. The voltage range required is too high for 4000 seroes CMOS analog switches. Ideally, I'd use something like a Toshiba photo-MOSFET in the sample & hold section, but I couldn't find any SPICE models for them.

author: tszaboo
author_rank: Super Contributor
timestamp: June 16, 2017, 10:55:22 am
content: Quote from: tanlccc on June 16, 2017, 12:41:36 amYou need power to do that.

author: tatus1969
author_rank: Super Contributor
timestamp: June 16, 2017, 05:31:34 pm
content: After some tinkering I came up with this circuit that should do the job and does not need an additional power supply. The signal slope of the output is not very fast, and it cannot drive a low impedance very well. (C2 is providing the output level). Timing is adjusted by C1 / R3, and it depends on the voltages of D7+Q5 and D8+Q6 as well, so there is some part and temperature dependency. EDIT: the input also presents a large capacitive load, 22uF.

author: tanlccc
author_rank: Newbie
timestamp: June 21, 2017, 09:57:25 am
content: Quote from: eblc1388 on June 16, 2017, 10:35:49 amWow!! Thanks alot!!Appreciate taking your time to come out with the circuit. Will try it out.

author: tanlccc
author_rank: Newbie
timestamp: June 22, 2017, 12:05:59 am
content: Quote from: eblc1388 on June 16, 2017, 10:35:49 amThank you for providing the circuit solution. Will try it out.

author: tatus1969
author_rank: Super Contributor
timestamp: June 24, 2017, 09:36:13 am
content: any idea if my proposal could be useful?