I am new to 555 Timers and Proteus and I need help to fix a circuit I have built. The intended function of the circuit is as follows.

  • The logic is controlled by an external circuit but it is represented by logic toggles within this circuit to purely test the 555 Timer functionality. -The logic toggles will be user controlled in this circuit and they control which 555 timer must be turned on using logic 1.
  • Only one 555 timer must function at any time as each 555 timer output has been configured with the use of resistors and capacitors to roughly drive the motor (depicted as a lamp) at respective speeds. - The speeds configured by U1, U2 and U5 should roughly be 80V, 150V and 220V. For example, if U1 is toggled on by logic 1, the motor must be driven at 80V.
  • I calculated the resistances and capacitance to produced the desired speeds and rounded them up to the nearest whole number for ease of building. I am however unsure if I calculated them correctly.

enter image description here

I have tried simulating in Proteus but I got the following errors which I cannot solve.

enter image description here

I would appreciate if anyone can kindly assist in determining the problem and perhaps provide a solution.

  • \$\begingroup\$ I don't use Proteus and never did much 555 work but I suspect that the simulation may have difficulty with U2 and U3 having 0 V supply. Try turning all the 555s on and see if the error goes away. \$\endgroup\$
    – Transistor
    Commented Apr 22, 2022 at 16:54
  • 1
    \$\begingroup\$ are you sure that you want to use three 555s, and two external logic gates, six capacitor and as many resistors for a job that the cheapest microcontroller could do together with a single decoupling (read: much smaller) cap and a current limiting resistor (which you forgot to add in series to your optocoupler's LED anyway)? \$\endgroup\$ Commented Apr 22, 2022 at 20:11

1 Answer 1


Logic toggle is supposed to be a logic input and therefore cannot be used as a power source.

For any 555 models inside Proteus, if you connect GND properly but leave the VCC pin unconnected (i.e. unpowered), Proteus will assume that the 555 is powered from the POWER terminal, which is a 5V source located under TERMINALS. So, actually, all 555 subcircuits in your simulation are running regardless of the toggle states (You can verify this by simulating one of your 555 circuits with VCC unconnected, in a separate simulation file).

If you want to simulate user activation of individual 555 oscillators then you should use the SPDT switch. Press P to open the Pick Devices window and search for SW-SPDT. There are momentary and latched versions, so pick one suits your needs and connect like following (POWER can be a POWER terminal from TERMINALS window, or a DC voltage source):


simulate this circuit – Schematic created using CircuitLab

One another problem with your circuit is that there is no current limiting resistor for the optocoupler's LED. Place a few hundreds of ohms or even a few kiloohms. And adjust according to your needs. Plus, adding a pulldown resistor to SCR's gate (between gate and low-side anode) and a series resistor to optotriac's output would help. A few hundreds of ohms should work.

Final note: Proteus sometimes needs ground on the isolated side as well.

  • \$\begingroup\$ Thank you for your suggestion and timely response. I have amended the circuit accordingly but I am still getting the error. [SPICE] Gmin step [14 of 120] failed: GMIN=8.91251e-005 [SPICE] Gmin stepping failed [SPICE] Source step [0 of 120] failed: source factor = 0.0000 [SPICE] Too many iterations without convergence. Do you by any chance know what else could be wrong? \$\endgroup\$ Commented Apr 22, 2022 at 18:31
  • \$\begingroup\$ @EdwardFrancis see the update. \$\endgroup\$ Commented Apr 22, 2022 at 19:56

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

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