# Can’t Get 555 Timer Capacitor to Ever Discharge or Charge to Expected 2/3 Vcc Value

I am trying to set up a circuit in which an NE555 timer's duty rate and frequency output is illustrated by it's output being connected an NPN transistor. When the out- put is high, LED1 will illuminate. When it is low, LED2 will be illuminated

I am setting up the circuit in LTSpice first and can't seem to get the desired effect of any actual oscillating signal being generated by the output. The capacitor, C1 only ever charges to VCC, which of course is not what I expect to see. What I would expect to see is C1 getting charged to 2/3VCC and discharing to 1/3VCC. I have set up the circuit such that TRIGGER pin is tied to the THRESHOLD pin and therefore can charge up through RA and RB which are in series with threshold. Likewise, RB is in series with the DISCHARGE pin so C1 should be able to discharge through RB when it reaches the 2/3VCC threshold.

Likewise,the OUTPUT pin waveform is one that stays at 0V. This makes sense as well since the internal flip-flop output will output Q = 0 and ~Q = 1 due to the fact that comparator tied to the RST input will be a logic 1 if V >= 2/3 VCC and the comparator tied to the SET input will be 0 if V >= 1/3VCC.

In the waveform in the screenshot, V(N006, N002) is the differential voltage of the OUTPUT pin and GND. V(N003, N002) is the differential voltage of the TRIGGER pin input and GND.

What is wrong with my circuit? The issue seems to stem from the capacitor only ever charging to VCC and never discharging nor discharging to the expected voltage level (2/3 VCC).

• I see a couple of problems with your schematic - does the DIS pin of the 555 connect to Rb? And if it does, does that mean that the bottom of Ra is connected to the neagtive side of the supply? Does the positive side of the supply connect to the Vcc terminal of the 555? Commented Jul 16, 2023 at 16:03

Your 555 is wired differently than the 555 circuit in the datasheet. Your transistor part of the circuit is not connected to ground. Simulations need all grounds connected together or with separate ground symbols.

In addition to the transistor and LEDs having no current return path,

the CV pin is short circuited to ground so the chip has no way of working properly - the levels will not be 1/3 and 2/3 of VCC if you ground CV. More like 0V and 1/2 of VCC.

Please look at any NE555 data sheet to double check connections for each pin.

• (1) The LTspice simulator usually needs a GND reference symbol, (2) the connection to Vcc of the 555 is unclear (connection merges with part outline), (3) it's best to use an actual part model rather than generic, and (4) please use meaningful labels for circuit nodes, and not defaults (n006, n002, etc.). Also DIS does not seem to be connected. Commented Jul 16, 2023 at 21:26
• The NE555 datasheet stated that the CV pin could be left open and that it would improve performance under specific conditions, which was vague. I got the schematic working without CV being capacitively coupled to ground, is that something I should still avoid? Commented Jul 21, 2023 at 2:36

Ask yourself "Where does the current in LED1 go?" "Where does the current in LED2 go?" Where does the current through the emitter of Q1 go?

If thinking about those questions does not solve your problem, let me know and I will give an expanded answer.

• The lack of a return path for the current sunk by these points means that a short circuit will be generated right? Commented Jul 21, 2023 at 2:35
• No............. Commented Jul 21, 2023 at 3:19
• Okay, I am not sure then what will happen to that current Commented Jul 21, 2023 at 14:18
• Without considering static electricity, current always flows in loops (i.e. circuits). If there is no loop for current to flow, then current won't flow. There must be two connections to a power supply for current to flow, so a section of a circuit that lacks an "exit path" for the current, will not have any current flow in it. Commented Jul 21, 2023 at 14:27

One of the problems you have is that you are not yet proficient with LTspice, so your schematic looks a mess and has errors that will be hard for you to spot. This is just a matter of practice and doesn't take long to learn.

The first thing you need to remember is that you will need a ground symbol in your schematic, LTspice won't like it if you don't have a ground node, and this is not just connecting things to the voltage source negative, you need the triangle ground symbol that denotes Node 0.

You will also want to try to minimize wires crossing over each other as much as possible. This can be achieved by judicious use of net labels. You should do this with your supply voltages at the very least, instead of running wires from the voltage source's positive and negative to every point they connect to, make the negative ground and label the positive something like Vcc and then use net labels to connect it to the other points as shown in this schematic.

This is the same basic 555 circuit that you have, but you can see it's much more compact and easier to follow. I rarely wire two ground points together, giving each it's own ground symbol instead.

Where two wires that don't connect cross each other there should be no dot. If you want to make it clearer that the wires don't connect you can draw it the way I drew the wire from the DIS pin, to make the diagonal wires hold down the Crtl key while drawing.

You should also bring a wire out a short distance from a component before attaching another component or a right-angle connection, this will help keep the connection from breaking apart when dragging components. You should also not have a dot on a component pin the way you see on LED1 in your schematic, a dot like that on a straight connection meas you have overlapped the wire with the component lead, to fix it delete the wire and redraw it.

You also have on place where you tried to make a connection but the wire doesn't actually connect to the component. This is where the DIS pin connects to Rb. If there was a connection at a right angle like that you should see a dot.

And the connection to Vcc on the 555 looks like it's been drawn so it doesn't come out from the chip at a right angle but rather at the top, which makes it hard to tell if there's a proper connection there (I believe there is, it just doesn't look good).

So do some practicing and learn the editing features of LTspice, when you can produce neat, concise schematics you will find they are a lot easier to work with and troubleshoot.

I'm not going to address the LED part of the circuit in this answer, that's a whole design problem on it's own. I'd say get the 555 timer part of the circuit cleaned up and working first, then maybe ask another question about how to design a driver circuit for the LEDs.

• Thank you so much! I'm a firmware engineer trying to get better at the circuit design/schematic analysis and EE side of stuff so this is a massive help. Can you briefly explain this concept: " You should also not have a dot on a component pin the way you see on LED1 in your schematic, a dot like that on a straight connection meas you have overlapped the wire with the component lead, to fix it delete the wire and redraw it." What's the difference between componnt lead and wire? Commented Jul 21, 2023 at 2:34
• @ElliottGoldstein in Ltspice components have places to connect wires to, let’s call them ‘pins’. Normally when you connect them there’s no dot. Sometimes it gets misaligned and shows a dot because the wire and ‘pin’ of the component overlap. You should fix this if you see it. Commented Jul 21, 2023 at 4:03