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I am trying to build a circuit which toggles a lamp on and off. I am using a JK flip-flop for the toggling action.

I want a clock pulse of 1 Hz. I know there are many ways to generate a clock pulse, but I want to generate it using a Schmitt trigger using an op-amp.

Current pick of components is 74LS109 for a ("TTL-level") JK flipflop and LM741 as the Op amp, supplied with 15 V, -V connected to GND.

Please let me know if it's possible or not, and if possible, what the circuit parameters should be.

Any other way of generating a clock signal using an op-amp would also be appreciated.

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  • \$\begingroup\$ What's the maximum allowed rise/fall time for the flip flop's input? \$\endgroup\$
    – Andy aka
    Feb 5, 2023 at 10:47
  • \$\begingroup\$ The trick is to produce voltage levels suitable for the FF: what voltages is the power supply for the op-amp? \$\endgroup\$
    – greybeard
    Feb 5, 2023 at 10:54
  • \$\begingroup\$ The power supply for the op-amp is 15 V DC \$\endgroup\$
    – Jina
    Feb 5, 2023 at 10:56
  • \$\begingroup\$ IC 74LS109 JK flipflop IC, supply of Op amp is just 15 V the negative Vcc pin is grounded. LM741 is the Op amp IC. \$\endgroup\$
    – Jina
    Feb 5, 2023 at 11:08
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    \$\begingroup\$ Why do they even manufacture LM741 anymore? We managed to eradicate smallpox. \$\endgroup\$
    – pipe
    Feb 5, 2023 at 17:51

2 Answers 2

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When operated on a single supply, with its negative rail tied to GND, a 741 output stage cannot produce a TTL low signal level. IOW, the output cannot swing all the way down to +0.8 V (or less) above its negative rail. Except for this, the 741 is perfectly fine for what you want to do.

If you are stuck with a single-rail environment, consider the LM358. It can produce a TTL-compatible signal while running on a single 5 V supply. It is a dual opamp, but you can tie off the unused section. There are also many "rail-to-rail" opamps that will do what you want.

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The 741 will need a lot more components. You could do a classic op amp oscillator with an op amp that swings to the ground rail. Then using a ultra-low VF Schottky barrier diodes the cathode could be the input to the LS device. Reason the max voltage for LS is about 0.8V or less for it to see a low. The inputs when open are high. I would not trust this over temperature.

If it were me I would use something like a 74C14 (I have not done this with a 74LS device) then place about 10uf to ground on the input with maybe a 200K from output to input. These are just starting values. You could also add a trimmer in the feedback loop to adjust frequency.

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