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I built the circuit below on my breadboard. I've checked every connection and it doesn't work. The thing I didn't know how to do was connecting the output (a LED) and the input. (a button) I did something pretty random and, no surprise, it doesn't work. My question is how do I connect the input and output and can I do it with a button and a LED?

enter image description here

I will use this as a SR Flip-Flop, I found it can be used this way here.

Also, I've used 3.3k resistors instead of 3.9k resistors. Is that bad?

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  • \$\begingroup\$ Try simulating your circuit first to understand the values you are getting. What is obvious to me is that the output won't be able to drive an LED high, only pull it low. \$\endgroup\$ – Eugene Sh. Aug 13 '15 at 20:07
  • \$\begingroup\$ What do you have available for a trigger input? You can put an LED in series with 2K in parallel with one of the 3K9 resistors. \$\endgroup\$ – Spehro Pefhany Aug 13 '15 at 20:40
  • \$\begingroup\$ @SpehroPefhany Why 2K? \$\endgroup\$ – Eugene Sh. Aug 13 '15 at 20:51
  • \$\begingroup\$ Since the drawing shows a low-going pulse for the trigger input, connect a resistor from the trigger input to +Vcc, and your switch from the trigger input to ground. \$\endgroup\$ – Peter Bennett Aug 13 '15 at 20:54
  • \$\begingroup\$ @EugeneSh. Why not? \$\endgroup\$ – Spehro Pefhany Aug 13 '15 at 21:03
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enter image description here

The LEDs should go in the collector circuits for each transistor, as shown above. The 3.9K resistors should be reduced (470 Ω shown here) to provide sufficient drive for the LEDs.

Other than that, I would leave the rest of your circuit as is.

This circuit also has separate switches for the set and reset functions.

Other ways of triggering are as follows:

enter image description here

The circuit on the left uses a pushbutton, and connects up to the Trigger input of your original schematic.

The one on the right uses a SPDT switch; you could also use a On-Off-On center off switch.

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  • \$\begingroup\$ The LED works great but I can't toggle it on or off. Also, could my circuit use a button to do both set and reset? Where and how would I connect it? (in my circuit) \$\endgroup\$ – Nicolas Aug 14 '15 at 0:47
  • \$\begingroup\$ @Atom I have updated my answer with a couple of suggestions. \$\endgroup\$ – tcrosley Aug 14 '15 at 1:01
  • \$\begingroup\$ What resistor and capacitor should I use for the pushbutton circuit? Also can I use 6 volts instead of 5? \$\endgroup\$ – Nicolas Aug 14 '15 at 10:27
  • \$\begingroup\$ @Atom Try 20K and 0.1 µF. If that doesn't debounce the switch adequately (flip-flop toggles more than once per button switch), increase the resistor value. The positive rail should be connected to the same voltage as Vcc in your original schematic. \$\endgroup\$ – tcrosley Aug 14 '15 at 11:29
  • \$\begingroup\$ Nah, the pushbutton circuit doesn't work for me, I guess something's wrong in my circuit. Anyone have a suggestion? \$\endgroup\$ – Nicolas Aug 14 '15 at 22:03

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