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741 Contest

12/24/2017 2:17 PM

I have this circuit:

The 555 feeds the 741, and the pot controls the duty cycle of the square wave output. I'm using a 9v single power supply. So the square wave only goes down to about 2v, not 0 (we all know why). Here's the contest: what is the simplest way to make it go to 0? For extra points, how can I modify the circuit to adjust both the high and low levels of the square wave?

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#1

Re: 741 contest

12/24/2017 3:16 PM

what is the simplest way to make it go to 0?

Replace the op-amp with a cmos op-amp which swings essentially from rail to rail.

http://www.ti.com/lit/ds/symlink/lmc7111.pdf

how can I modify the circuit to adjust both the high and low levels of the square wave?

Add variable resistors between the +V and -V power pins of the op-amp.

Note: I wouldn't really design circuits this way. It's just the "simplest" way.

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#2
In reply to #1

Re: 741 contest

12/24/2017 4:07 PM

Thanks, Rixter. If I wanted the high level to be say, 1v, how would I do that? I don't think the op amp would function that low.

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#3
In reply to #2

Re: 741 contest

12/24/2017 7:44 PM

Here are a few things you need to know about op-amps. To a first approximation:

  • Always use negative feedback (output to inverting input)
  • Consider gain as infinite (with feedback, + and - inputs are at same voltage)
  • Consider input inpedance for both + and - inputs as infinite (no current flows into inputs)

In the circuit above, you can replace Rin and Rf with a potentiometer on both the inverting input V1 and the non-inverting input V2. Connect your square wave (555 timer pin 3) to V1. Adjust the pot on V1 to adjust the amplitude of the output square wave. Adjust the pot on V2 to set the center voltage of your output square wave.

http://www.electronics-tutorials.ws/opamp/opamp_8.html

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#5
In reply to #2

Re: 741 contest

12/25/2017 9:46 AM

You are doing it wrong.

You do not need an OPAMP, but a comparator. The ones that go from the minimum to the maximum values of the voltage supply are calles "rail to rail".

I think this should be enough to get you some effective results with a web search.

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#6
In reply to #5

Re: 741 contest

12/25/2017 8:25 PM

The CR4 community is such a fantastic resource. I had really no idea of comparators until Ivan pointed me in the right direction. Here's what I can do:

...I can power it with a single 9v supply, and use a pullup resistor to whatever voltage I'd like logic 1 to be. Looks like the perfect solution to my application!

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#7
In reply to #5

Re: 741 contest

12/26/2017 7:30 AM

The 555 timer pin 3 output is a square wave, so he doesn't really need a comparator. If he wants it perfectly square (pretty), he can use a CMOS timer.

He still wants to vary the limits, so he needs an op-amp to shift the level and vary the amplitude, preferably a CMOS variety that he can get close to the rails without those pesky junctions getting in the way.

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#8
In reply to #7

Re: 741 contest

12/26/2017 9:57 AM

I'm using pin 6 of the 555, the charge/discharge waveform, and the 741 was to vary the duty cycle of the final square wave. And the 555 has a pot to vary the frequency. Slick way to get a square wave with both frequency and duty cycle adjustable. But now I'll use a comparator to get rail to rail. Is OK, n'est pas?

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#4

Re: 741 Contest

12/25/2017 12:23 AM

I'm not even going to get into this. The Op-Amps are magic, man! Me and my friend Jeff had a schematic, and we made fuzz-boxes based on an ultra sensitive "electronic ear" circuit. It was purported to allow hearing of individual ant "steps" as they walked across the membrane of a microphone.

That's all I remember!

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#9

Re: 741 Contest

01/11/2018 5:20 PM

You do not specify a frequency of the 555.

For low frequencies a simple relay would do.

For higher frequencies I might use an SSR or opto isolator.

For another idea, sending the op output to fet transmission gates like the 4066 will allow you to place any voltages within the range of the 4066 on the common output bus.

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Ivanov327950 (1); LongintheTooth (1); Mikerho (1); Rixter (3); SSCpal (3)

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