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I can't believe how much trouble I've had figuring out how to do such a simple thing.

I have a variable frequency, variably duty cycle 555 configuration in astable mode. I'm using a set of pots to get a wide range of control. It works beautifully. The output is great, but I have a problem.

I am driving a couple mosfets directly from the output. But I want to have a third mosfet that turns on and off opposite to the timing of the first two. So if I'm running 25% duty cycle, the first two are ON for 25% of the cycle while the third one is OFF during that portion and then reverse this for the OFF period of the cycle.

I've seen some diagrams that talk about using one extra mosfet or one extra transistor along with a resistor to invert a signal, but they don't create hard/fast rise and fall times. The resistor causes slow rise times. I want a nearly perfect inversion of the square signal. Right at the moment that the first two mosfets flip off, the third mosfet needs to flip full on. It's critical that I get this to be as accurate as is feasible without getting into overly complex circuitry. Anything more than two or three 555s and/or two or three extra transistors (possibly needed to invert the signal) is more than I'm bargaining for.

Can anyone offer a simple solution to this?

I have both CMOS and non-CMOS 555's on-hand. I have a reasonable collection of other components as well. I can order what is needed.

PFET is not an option. I'm working with extremely high demands and my selection of NFETs is already highly limited. I wish it was as simple as a PFET.

I'm not posting this as an answer yet, because I'm waiting to hear a response from the other answerer. But apaprently you can run a 555 as a 200mA signal inverter. This is really simple for a person who already has 555s on hand, and the best part is that the 200mA current is WAY higher than a typical logic unit inverter (often topping out at least than 10 or 20mA). Here's a page with a circuit at the bottom explaining Inverter mode: http://electronicsclub.info/555timer.htm

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Without knowing what your power supply configuration is, I can offer a few suggestions with reasonable caveats.

  • If you need to ensure that there is no overlap between your current two transistors and the third, then your circuit will be more complicated than a few more 555's. You're then looking for something called a "dead time generator" and you probably would need to define your minimum dead time requirements.

  • If you are driving MOSFETs directly, and VDD for the 555 is the same as the load you are switching, you can use a PFET instead of an NFET, and that will work reasonably well. This will not work if your 555 is powered separately from your load. This is the simplest option and requires no additional parts. See schematic below:

schematic

simulate this circuit – Schematic created using CircuitLab

  • If you need to use all the same type of transistor (all NFETs or all PFETs) then the next best thing would probably be to use an inverter (or other integrated logic gate) that comes in a small package. This makes for a small circuit and easy to troubleshoot. If you're set on through-hole or breadboard-able parts, then there are a lot of chips with 6 inverters on them that will work great. Tons of options exist if you can live with some flavor of surface mount (you can have as few as 1 inverters in a package). This also has the advantage of working if your load is switched from a different VDD than your 555.

schematic

simulate this circuit

  • You could also use 555's to synthesize any logic function, such as an inverter.

schematic

simulate this circuit

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  • \$\begingroup\$ Thank you! I should have thought to mention that PFET is NOT an option! I will edit the post for clarity. It looks like an inverter is my only option. Can you make a suggestion on exactly what to look up on mouser? Do I specifically go to Logic chips and look up "inverter" ? Also, can you tell me what you mean by "overlap" in a circuit. I'm sure that's a newbie question but I don't know. Are you talking about something more than just an opto-isolator? \$\endgroup\$ – JamesHoux May 7 '13 at 3:24
  • \$\begingroup\$ The "inverter" section on Mouser will definitely get you started down the right direction. Make sure you pick an appropriate package and max VDD rating for the part. By putting an inverter in the path (or really by having any difference at all), you have the likely case where mutually-exclusive outputs are on for brief periods of time (when the 555 is switching). If this is a problem (e.g. shorts power and ground together) then you need to look at dead time generation. \$\endgroup\$ – W5VO May 7 '13 at 3:51
  • \$\begingroup\$ It is difficult to determine if you need dead time generation from the description you provided. I can think of scenarios where 1: It doesn't matter at all, 2: It causes undesirable but tolerable issues, 3: It interrupts normal operation, or 4: causes component failure. \$\endgroup\$ – W5VO May 7 '13 at 3:54
  • \$\begingroup\$ It's not an issue if the outputs have mutually exclusive periods or even slightly overlap. The mosfets are running independent loads that affect each other but not in a way that is crucial. If 97% of the cycle time is clean and 3% is mutually exlusive, that would be well within my parameters. Heck even a margin of 5% would be acceptable. 10% is pushing it. I don't understand this "negative going" and "positive going" voltage stuff in the inverter datasheet. My concept of logic was always that it is "zero" (low voltage) or high voltage. I need to do some reading... \$\endgroup\$ – JamesHoux May 7 '13 at 4:03
  • \$\begingroup\$ The inverter datasheets indicate that these inverters can't handle near as much current as I'm used to with a 555. Will I need a mosfet driver to beef up up the output of the inverter? \$\endgroup\$ – JamesHoux May 7 '13 at 4:14

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