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I'm designing a circuit based on the 555 timer. I'm using EasyEda as simulator.

The problem I'm having is that, with a voltage supply of 5V, I'm getting 3.3V in the output, while I was expecting 4.4V minimum.

I'm expecting the mentioned value because the VOH stated for 5V supply in the LMC555 datasheet (see 6.5) is that. However, since the voltage I'm getting is lower, I suspect that my confusion comes from the 555 my simulator is using. Let me explain:

As far as I understand, the 555 timer is kind of a standard part. Many manufacturers make it, but they all should match some pre-established specification. However, there might be different families of 555 timers depending on the technology they use (CMOS, TTL...). Is this correct?

If so, my guess is that the simulator I'm using is simulating a TTL 555 timer, while the datasheet I was reading (the one above) speaks about a CMOS 555 timer. That would explain the voltage drop I'm seeing (it makes sense that BJTs drop more voltage than CMOS, right?). Is this correct?

To verify this I have done the following, I'm not sure if I have understood it correctly though:

I have noticed that the 555 timer that is available in the simulator is called 555_BJT_EE. That BJT part makes me thing that it is simulating a 555 based on BJTs, that is, TTL technology. Next, I have searched a datasheet that corresponds to a TTL 555 timer and I have found this: LM555 timer datasheet.

In the part 6.5 I can read in the table that the output voltage drop (high) for 5V is between 2.75V and 3.3V.

I think that output voltage drop means the voltage you need to substract to your Vcc to get Vout. Is this correct?

If it is, is my above reasoning correct? (namely, I was looking at the wrong datasheet because it was describing a CMOS 555 timer, while I'm using a TTL (BJT) one. Thus, I need to stick to a datasheet which corresponds to a TTL 555, which happens to be the one my simulator has)

EDIT

I have no load for now, just an oscilloscope to measure the output frequency.

Here is the schematics (555 in astable mode to generate a sqare wave signal at 2 Hz):

555

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  • \$\begingroup\$ You haven't posted a schematic or described what load is on that output pin. \$\endgroup\$
    – user16324
    Commented Jan 22, 2021 at 15:48
  • \$\begingroup\$ C3 is rather large. \$\endgroup\$ Commented Jan 22, 2021 at 19:36
  • \$\begingroup\$ @StainlessSteelRat C3's size could make it chirp a bit on start-up but other than that is should run fine. \$\endgroup\$ Commented Jan 22, 2021 at 22:34
  • \$\begingroup\$ Do you, by any chance, have your scope input set to 50 ohms? (I know some real scopes have this feature, I don't know for simulated scopes) \$\endgroup\$ Commented Jan 22, 2021 at 22:39

2 Answers 2

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As far as I understand, the 555 timer is kind of a standard part.

Yes, definitely. That used to be called "multi-sourcable".

Many manufacturers make it, but they all should match some pre-established specification.

Hopefully. They only do if there's actually an agreement between these manufacturers, which often isn't the case. It's not the case here: The original IC was just so simple and laxly specified (by modern standards) that it's easy to be "compatible" with the original specs, and that's kind of the common denominator. You still should check the datasheet of the exact 555 you're using if you're using it at high frequencies, high currents, low latencies...

However, there might be different families of 555 timers depending on the technology they use (CMOS, TTL...). Is this correct?

yes.

If so, my guess is that the simulator I'm using is simulating a TTL 555 timer, while the datasheet I was reading (the one above) speaks about a CMOS 555 timer.

That might well be the case.

That would explain the voltage drop I'm seeing (it makes sense that BJTs drop more voltage than CMOS, right?). Is this correct?

It would be a possible explanation.

Is that output voltage drop the drop you must expect from your Vcc? In other words, If my voltage supply is 5V I should expect 2.25 maximum, correct?

No, you must really read the datasheet of your chip to get that data.

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  • \$\begingroup\$ In the EasyEda web I have found a description of the 555 model I'm using. This description says that its parameters are based on the following datasheet: ti.com/lit/ds/symlink/ne555.pdf. I have examined this datasheet and it sates the same for the output voltage drop (high). My question is rather if this parameter is what I think it is, namely, the voltage drop to be applied to Vcc to get Vout. \$\endgroup\$
    – Martel
    Commented Jan 22, 2021 at 15:44
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    \$\begingroup\$ yes, but under specified load conditions! \$\endgroup\$ Commented Jan 22, 2021 at 15:48
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You have linked the LMC555, not the LM555. Assuming you meant to link this datasheet, your guess does not seem to be correct, however I think the datasheet is written (or copied over) very badly, which isn't really your fault.

enter image description here

It is not the voltage drop but the actual voltage output, and so the "min" is deceptive, and they don't even tell you the test current for the +5V Vcc case!

Since it's such an old part and real data is available elsewhere, I guess nobody complained enough for them to fix it.

The original manufacturer of the bipolar 555 back in the dark ages was Signetics, who named it the NE555. With that bit of ancient history, we can find another TI datasheet that is a bit more informative, and shows us this:

enter image description here

So the 2.75V is the actual minimum output voltage, 3.3V is typical with it sourcing 100mA and there is no maximum guaranteed. That's at room temperature. No guarantee over temperature, but we can see from the SE555 that it might be as bad as 2V, or even a bit worse, at temperature extremes. Kind of the opposite of what the other datasheet seems to say.

Of course under a lighter load, the output voltage will be closer to Vcc and you can refer to Figure 4 for typical values. At 1mA the drop (now it actually is drop) is typically around 1.4V so we would expect an output voltage of about 3.6V on a 5V supply at 25°C. Down to about 3.3V typically at very low temperatures. Typical numbers are not necessarily representative of any given sample of the part, of course, but they usually are close.

Now, why are there two similar TI datasheets for the (very, very similar) bipolar NE/LM555? TI bought National so their LM555 was almost identical to the NE555 that TI made under their own brand.


Your actual quandary is with the CMOS parts. They vary more than the bipolar versions, which are pretty much the same from one manufacturer to another. So an LMC555 (former National, now TI) is going to differ in material ways most likely from a TLC555 (TI original), not to mention the ICM7555 or the ultra-low-voltage TLC551. So get the datasheet for the part you have or intend to use and refer to that. If you need to simulate, try to get a model from that manufacturer and install it (and maybe do some sanity checks to make sure it is behaving sensibly and yielding numbers close to typical in the datasheet under reference conditions).

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