[Updated with working schematic!!!]

I have a circuit which I tried to adapt from the 555 bistable configuration to have the 555 states controlled by input A and B; B always goes high before A.

I am trying to have the reset pin set high when B goes HIGH (1.5V) and since A would start HIGH, I expect that the output is HIGH.

When A finally goes HIGH (B will still be HIGH) the trigger pin voltage becomes 2V which is above 1/3Vcc, I expect the output to go LOW since the trigger pin has been pulled HIGHer than 1/3Vcc.

I tried the circuit below and an LED hooked to the output stays on through out, it doesn't change even if I try to pull the trigger pin to a significantly high voltage.

enter image description here

The overall truth table I am trying to get out of the 555 is included below:

| A | B | Output|
| - | - | ----- |
| 0 | 0 |   0   |
| 0 | 1 |   1   |
| 1 | 0 |   0   |
| 1 | 1 |   0   |

Can someone please tell me what is wrong with this configuration and/or suggest a workaround using the 555?

Working schematic:

enter image description here

Hope it helps:) (sorry for the inconsistency in the circuits.)

  • \$\begingroup\$ I'm not sure, but one key point you may be missing is that the trigger input is active-low. \$\endgroup\$
    – Dave Tweed
    May 17 at 2:21

2 Answers 2


To get the output to go low you need to take the reset pin high which removes the reset, then take the Trigger pin high (above Vcc/3) which removes the trigger signal, but the output will only then go low when you take the THR pin above (2/3)Vcc. This is the bistable operation.

The overiding signal is the RESET pin. The output will always be low when the RESET pin is low. If the RESET pin is held high then the output will always be high when the TRIGGER pin is below Vcc/3. But after the TRIGGER pin has been taken above Vcc/3, the output will remain high until THR is taken above (2/3)Vcc which will then force the output low.

If you tie the THR pin permanently to Vcc then, as long as RESET is held high, the output will be high when TRIGGER is below Vcc/3 and the output will be low when TRIGGER is above Vcc/3.

In short, tie THR to Vcc instead of GND.

  • 1
    \$\begingroup\$ Thank you very much. I took the THR pin to Vcc and I am getting the operation I needed. \$\endgroup\$
    – montso_m
    May 18 at 8:46

Have a look at the lock diagram of the 555 from Wikipedia:

enter image description here


Pin 4 is a logic input to the SR flip flop, so you’re not getting the comparison action you’re hoping for. Only pins 2,5 and 6 can be used to compare analog voltages - I won’t detail the solution to your specific problem here because you’re doing good work trying to build this oscillator.

Also without a way to discharge the cap (customarily pin 7 is used for this feedback) your circuit will not oscillate, though I’m not sure this is the intent of the demo achematic you provided.


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.