ECN Forum Forums Technical Discussion Electrical Theory and Applications Need help with a circuit

Hi all,
I need help with a circuit I'm trying to design, and I tried this on my own before posting here. I'm trying to find a circuit that generates a quick logic pulse when voltage is applied... similar to a switch debouncer, but will make a short output pulse no matter how long the input pulse lasts. I tried google, but I'm having a hard time sorting things out. If anyone has a link or can give me a hint, it would be much appreciated.
thanks,
Josh

You could use something like a 555 timer with a capacitive coupling on the trigger input. Here's an example:
http://www.doctronics.co.uk/555.htm#triggering

They are call one-shots or monostable circuits. Depending on you precision needs and output voltage requirements, you can use a 555 timer chip.

How about some more details like what is the input, what is the output pulse time, what is the output voltage, etc.

LarryC

You could save up a few hundred bucks, buy a PLC & some I/O to go with it, and learn to program it, and there you go. Simple.

Always trying to help,
Radar

I love building things around LM or TLC555s but sometimes they're overkill. Many times you're better off just using a suitably rated transistor with a pullup or down to your logic level. Just choose your base resistance to saturate the heck out of the transistor. Pick your series base capacitance for the pulse width that you want. The limitation here is that the input pulse has to be at least as long as the output pulse. You also have to discharge the capacitor between pulses. This might involve throwing in a diode and a discharge resistor.

Anything that I build around ICs usually involves firing up the CADD and running a sheet of the Dyna-Arts paper through the printer. It looks good after I have it etched and tin plated but at least half a day is gone.

If I were to build a trigger ckt like I mentioned above, it would take less than an hour. I would just take a scrap of copper clad that I always have lying around. I just use a utility knife or Dremel to relief cut the foil pattern. Then, I usually use a perf board with .1" centers to guide my Dremel drilling. With TO-92 or TO-220 transistors, the cutting to isolate the pads isn't as difficult to pull off as it is with DIP packages. I never claim that the result is pretty but the electrons don't mind if we aren't playing with RF.
Joe

Or you could save money and time and buy a one shot relay.

This is a case where more info is needed. When someone tells me "quick logic pulse", I tend to think nS, uS, or possibly low mS durations. Obviously, Briselec is considering slower,"quick logic pulses" that a certain type of time delay relay provides. Finally, Radar offers the high priced governmental approach, or at least one of someone spending other people's money.

Anyone of these might be the best sollution, depending on what Josh needs. I think Radar was joshing but if Josh is already dealing with a PLC, maybe an input can be programmed as a 1-shot. Briselec's suggestion would offer the best electrical isolation. LM555s offer good current source and sink capability. My transistor solution is nothing fancy but might be all that you need.

There are several CMOS 1-shots like the old 4528 that might meet the needs. Are there any left over gates in the design, like XORs, that you can feed one input directly and the other with an RC network for a 1-shot? Here, you probably need to add a diode to create a pulse on only the leading or trailing edge.

Joe

Josh,
What sort of output are you looking for?
Are we talking a small flat-top square wave, with no negative component or are we talking a quick voltage/current rise and a quick decay?.
Have you thought about a UJT (Uni-junction Transistor) circuit?.
What sort of frequency are we talking here?.

Trumpy,
I was looking for a quick square wave or even a sharp pulse... something to trigger a 4027 flip flop or a decade counter (4017? I forget the number.) What I wanted to do was have the circuit monitor the battery, using comparators. If the solar charger didn't juice it up enough, it would use grid power to charge it. If the battery charge got too low, it would shed un-needed loads. (This is a multipurpose circuit, thinking about a lawn-bot, trash-bot, or a Bender (Futurama, if you didn't get the reference.) I solved the problem using a window copmparator, where both are off when the voltage is not full, but not too low. Low voltage will cause a high out from the "low side" and "set" the 4027, turning on the charger. When the voltage is in between "too low" and "not high enough," it would do nothing, continuing to charge. When it got to "high enough," it would turn the "high side" on, "resetting" the 4027. If anyone wants to thee the schematic, you can email me. The solution hit me last night as I was cursing my Windows XP... go figure.

Anyways, my original idea was to use separate comparators, all "on" at certain levels. the "one shot" idea I was questioning was too messy to even describe.

Joe - as far as boards go, when I need a finished product, I usually dish out $59 for expresspcb.com. I've had bad luck making my own, mainly 2 sided boards with the tops and bottoms not lining up. The other problem I have is I use dry transfer for the tracks, and sometimes in the Ferric (or ferrous?) chloride, they come off the board. I order the boards, the cheap option - no solder masks or silk screening. I do that on my own with usually satisfactory results.

My device was originally for a Wind mill/solar system (Did I just hear some groans?) group project for college, but we ended up with just using a dump load, rather than keep the battery on float charge because we were running out of time.

Hi Josh,
The LM339 came to mind as soon as I started reading your app. I guess it might call for a 139, or 239, if you need to deal with harsh temperatures. You just have to remember to keep away from your positive rail because the diff. amp is Darlington PNPs working downward. You could put alot of hysteresis in to control your charger. I would like to see your circuit if you wish to send it.

I know where you're coming from on the circuit boards. Been there, done that! This includes using the transfers from Radio Shack, Sharpie markers, and even the cheapest, ugliest nail polish in the store as etch resist. It turns out that the company I knew as DynaArts Inc. is now Pulsar.(www.pulsar.gs) They still call the process the "Toner Transfer System" and the sheets are about half the price they were when I bought them. They show a new film to deal with toner porosity and another for component screening that I wasn't aware of.

I use a glass tray on a hot plate when I etch boards. The key is to get in and out fast, to avoid the undercutting problem I know you've dealt with. I use a scrap of old board material to move the board in order to agitate the etchant. Prepping for toner transfer or tin plating is cleanser and Scotch-Brite pads followed up with acetone.

As far as the double-sided boards go, I hate them to. They aren't so bad when I can just use large pads as the vias. The Pulsar website has some hints for registering DS boards.
Joe