The Battle of Fives: How the NE555 and LM555 are Different

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A customer recently asked us some questions about the Three Fives discrete 555 timer kit. One in particular really got my attention.

What is the difference between the National Semiconductor LM555 and the Signetics NE555 timer ICs? Well, the Signetics part certainly came first and the National part was a second source, but the customer noted that The 555 Timer Applications Sourcebook, on page 5-31, states

…Table S-2 points out that the threshold overrides the trigger for the type LM555H (National), but the threshold is overridden by the trigger for the type NE555V (Signetics).

Let’s compare the two. First, here’s the NE555 schematic (click for larger versions).
LM555sch
And here’s the LM555 schematic. I’ve kept the component numbering consistent with the NE555 datasheet rather than National’s datasheet to make comparisons easy.
NE555sch
The LM555 makes three minor changes to the timer design:

  • The trigger comparator now has a current mirror active load (Q26 and Q27) instead of resistor load R6.
  • The threshold comparator gets a current mirror active load (Q28 and Q29) and an emitter follower buffer Q30.
  • R10 is now 7.5K instead of 15K, but I suspect there is a typo. Imagine several generations of photocopies. The 1 starts to look like a 7, and a decimal point appears.

The most interesting changes are the first two. How do these changes reverse the priority of the two comparator inputs?

The original NE555 gives priority to the trigger signal because transistor Q15 can always overpower the current coming from Q19A and Q6.

For the LM555 in the normal case where the trigger signal is active, Q15 is on, Q16 is off, and Q17 is on hard since its base is pulled to VCC through Q18, R10, and the current mirror Q19.
However, if both the trigger and threshold inputs are active, then both Q15 and Q30 are on. This leads to an interesting situation where the collector of Q18 is pulled to ground through Q15 and Q30. At that point, there’s nothing to provide current to the base of Q17, and any residual charge will probably drain away through the reverse leakage current of Q18. Q17 then turns off, and the output does the opposite of the NE555! Leaving the gate of Q17 hanging like that seems really odd, so I bet this was unintended behavior. I ran some LTSpice simulations so you can see what is going on. First up is the NE555:
NE555 Flop
And here is the LM555:
LM555 Flop
If you look carefully at the V(comp) trace right before 12ms, it actually goes negative due to Q18 behaving like a diode clamp.
This behavior doesn’t seem to get in the way of normal operation, but it is something a circuit designer would need to take into account. This is why designers and purchasing people should always be wary of “drop in” replacements, especially when the manufacturer claims “improved performance!”
National Semiconductor made the changes to improve the performance of the comparators, specifically their performance over temperature. I ran some more simulations so you can see the difference. Here is the NE555 set up in a simple astable circuit, with superimposed waveforms at 0C, 35C, and 70C:
 NE555drift
And the LM555 at the same temperature ranges.
LM555drift
Note that I put the temperature coefficients only on resistors inside the 555 timers, not on any of the external oscillator components.
If you want to play with the LTSpice circuits, click the links below to download them.
A quick side note about the names: The LM in the part number stands for Linear Monolithic, which National Semiconductor used to describe many of their analog ICs. The NE probably stands for Network Electronics (the sources are anecdotal). Apparently the Signetics name came from Signal Network Electronics.

CRT Phosphor Video

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Inspired by commenter Katemonster, I’ve put together a short clip with a couple of CRTs from my collection, demonstrating various types of phosphors. There are charts out there that talk about persistence using vague terms like “medium” (compared to what?), so it’s nice to see a real video showing what such a CRT actually looks like.

For the video I’ve used my “orbiter” demo that uses Newton’s law of gravity and Newton’s 2nd law of motion (F=MA) to generate simulated planets that orbit around a sun. It’s a nice way to demonstrate persistence (the way the phosphor fades as the electron beam moves away).

P1 Phosphor
This is the basic green phosphor. At 525nm primary color wavelength, it looks slightly more blue than common super-bright green LEDs. The chart linked above lists the persistence time as 20ms which seems reasonable. The formulation for this phosphor varies between manufacturers so some tubes might be slower than others. It’s very common in early oscilloscopes and oscillographs, and apparently some radar systems as well.

P2 Phosphor
The P2 phosphor color has even more blue in it than the P1–it’s very close to “stoplight green”. The persistence is much longer as you can see in the video (30 seconds or more, depending on the ambient light levels). The charts and reference documents I have list the primary applications as oscillography and radar.

P7 Phosphor
P7 is a very interesting phosphor. It is a cascade phosphor, meaning that it has two layers of material. The electron beam strikes the first (outer) layer which emits a bright blue light with some light near ultraviolet. This high energy light excites the second layer (inner, in contact with the glass) which is a much slower material that emits a yellowish-green light with a very long persistence (around a minute). In the video I move the “orbit” trace off to the side so you can see that original afterimage persists.

It was used mostly for radar and sometimes in oscilloscopes to capture one-time events before storage tubes were invented.

So why use a cascade phosphor? One source states that it was originally designed to be used in intensity-modulated displays (varying brightness levels), but it turns out it also helped prevent radar jamming. Since the jamming signal was not synced to the radar pulses, a long persistence phosphor could average out the jamming signal and allow the operator to see the true signal as viewed on an A-scope (time-based pulse waveform monitor). [Cathode Ray Tube Displays, MIT Radiation Laboratory Series, pg. 626]

P12 Phosphor
This one is my favorite. It’s an orange medium-persistence (a few seconds) phosphor that was apparently used for radar indicators. I don’t know of any that were used in oscilloscopes.

P31 Phosphor
The P31 phosphor was invented as an improved P1 phosphor. It’s much brighter (P1 is 32% as bright) and has short persistence (<1ms). The color has a bit more blue in it–in fact, very close to the P2 phosphor’s color. I would say most analog oscilloscopes from the 70s to today use CRTs with the P31 phosphor.

In many cases these CRTs would be installed behind a colored piece of plastic acting as a color filter. For example, P7 CRTs were often installed with an orange plastic filter in front to make the blue/white phosphor look more similar to the secondary yellow phosphor. P31 CRTs usually have a blue or green plastic filter.

For further reading:

CRT Driver Boards, Now With Altium Sources

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Take a look at my crt-driver GitHub repository. I tidied things up a bit and more importantly, released the Altium project files, schematics, boards, and even the output job file. It’s all licensed under the Creative Commons Attribution-ShareAlike 3.0 license. Read the Creative Commons page for the full terms, but basically you can share or adapt any of it as long as you give me credit (a link to this blog would be appreciated) and make sure that you keep the same license so that others can do the same.

If you don’t have Altium (expensive, closed source), you can at least open and edit the schematics with CircuitMaker (free, closed source, limited). Sadly, CircuitMaker will not let you edit the Altium PCB layout.

Deuterium Arc Lamp

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On Saturday I found a deuterium arc lamp at a local surplus store. It was used, and most likely pulled from an ultraviolet spectroscopy machine. I could not find data on the specific lamp model, but I found a similar lamp. On the chance any of you might know what it is, the lamp is marked

D 805 K

56066349

West Germany

H9

Before running any tests with the lamp, I wiped it down with isopropyl alchohol to remove any fingerprint oils. When heated, they can cause the glass envelope to bubble and even melt, destroying it.

To run this lamp, which is a gas-discharge type, you first have to heat up the cathode. There is a very thick double-spiral tungsten filament inside that uses 2V at 4.5A (or 9 watts!). Once it’s warmed up for a minute or two, you apply the high voltage to the anode. I connected it to a current-limited electrophoresis power supply set to 50mA. The lamp started at 350V and settled to an operating voltage of about 84V. Incidentally, the heat generated by this helps keep the cathode hot, and the filament current can be reduced to improve its lifetime.

Here’s a quick video showing what it looks like.

Deuterium is an isotope of hydrogen: hydrogen has one electron and one proton, and deuterium takes that and adds a neutron. It is not a radioactive isotope, unlike tritium, which has two additional neutrons. According to Wikipedia, Deuterium is used in these lamps because it emits more UV with a wavelength less than 400nm.

If you’ve got one of these lamps and you plan to light it up, you’ll need eye protection. I ran it at a very low beam current (most likely it was designed for 300mA!) and the light was not so intense, but you might want more than just a pair of sunglasses if you’re going to full power…

Inside a TTL Logic IC

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Since 2014 is generally considered the 50th anniversary of TTL logic, I thought I’d take a TTL logic chip apart and do a little analysis.

So I started with a DM7438N, lot code M:P9006Y. Looking at National Semiconductor’s device marking convention document, I take this to mean that it was manufactured in week 6 of 1990 at a subcontractor’s fab in the United States and assembled in Malaysia.

The 7438 is a quad 2-input NAND buffer with open-collector outputs. That means the die should look symmetrical to a degree.

To take it apart, I used a rotary tool to carve out the encapsulation material on the top and the bottom, and then picked at it with side cutters until the chip fell out. Sadly I cracked off a corner of the die including one bond pad, but it’s still possible to figure out how it works.

What does all this do? See the image below. I’ve cropped all but one gate and highlighted the various semiconducting regions in different colors. I’ve also given designators to all the components.

Red represents the N-type collector epitaxial diffusion. Cyan represents the P-type base diffusion, and purple represents the N+ emitter region.

The schematic looks like this:

That dual-emitter transistor (Q1) sure looks strange!

How does it work? Well, if both A and B inputs are a logic high, then Q1 is off, but some current flows from R1 (4K ohm) through the base collector junction (since it is, after all, a PN junction) and feeds the base of Q2. Q2 turns on, and its emitter current feeds R3 (1K ohm) and Q3. Q3 turns on as well, and the output Y gets driven low. The non-inverted version of the output signal is available at the collector of Q3 (biased through R2, a 1.6K ohm resistor), but this particular chip doesn’t use it.

If either A or B goes low, then Q1 gets turned on. Current flows through the base emitter junction and the base gets pulled to about 0.6V above ground. No current flows through the base of Q2 because the voltage on the collector of Q1 is just too low for any current to flow. Q3 therefore stays off, and the output Y goes high impedance. By the way, this is what open collector means–the collector of the output stage transistor is left “open” with no corresponding transistor above it to pull it high.

Diodes D1 and D2 are just for input protection.

There are a couple of unused components. There is a resistor right below R1, and another resistor below R2. There are two extra transistors with a shared collector to the left of Q3. A different top metal mask could connect these extra components into the circuit and change the function of the device.

Can you think of some other gates that could be built by changing the top metal mask? Remember that there is only one metal layer which limits where you can route the traces.

XL741 – Discrete Op-Amp

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We’ve done it again! My friends at Evil Mad Scientist Laboratories have a new kit for sale. Following on the success of our Three Fives discrete 555 timer kit, we’ve had a lot of requests for a discrete 741 op-amp.

The XL741 is based on the datasheet schematic of the original uA741 op-amp IC from 1968. You can wire it up in a classic op-amp circuit and probe nodes inside the IC so you can see how the chip works. Play with differential pairs, modify the compensation, and change bias currents to your heart’s content!

Flea Market Find–Dual Gun CRT

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At the electronics flea market on Saturday, I found a 3ABP7 dual gun CRT. This one was built by DuMont, most likely intended for the 3″ version of their 5″ Type 279 Dual Beam Oscilloscope.

So of course I had to fire it up. There are two sets of deflection coils, so I drove them with one deflection board and cross-wired the deflection coils to flip the image around on the second gun.

The guns themselves have a common cathode connection and separate grids, which forces me to drive them both in parallel since my deflection board video amplifier keeps the grid at a constant potential and drives the video onto the cathode.
Dual gun 3ABP2 CRT

Here’s a closer look at the guns. The filaments are connected in parallel so this tube uses twice the normal current.
Dual gun 3ABP2 CRT

CRT Board BOM Updates

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I have corrected some mistakes in the bills of materials for the CRT driver boards (the files in GitHub have already been updated):

  • ScopePower U2 part number from TS271CN (DIP package) to TS271IDT (SOIC).
  • ScopePower R3 part number from VR37000003305JR500 (33 meg) to VR37000001005JR500 (10 meg).

If you already ordered parts, I apologize. The TS271CN is a great little op amp and the DIP package version will work fine in a breadboard. That’s what I will do with the ones I accidentally ordered. The 33 meg resistor is useful as a bleeder resistor if you build the post deflection acceleration module (to be posted soon).

P10 Dark Trace CRT – The Skiatron

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I have a little story to tell. Years ago, I met someone who had a very large collection of CRTs. He had everything from common 3BP1s all the way to rare little gems like the 1EP1 and some vintage prototype CRTs. He showed me an item in his collection which was a rectangular CRT with a P10 phosphor. Finding a CRT with the P10 phosphor is like finding a unicorn. P10 is not really a phosphor; it designates a screen coated with some sort of alkali-halide (potassium chloride) that darkens when hit with an electron beam–a scotophor. The darkening effect lasts until the coating is heated, and typical P10 CRTs have a built-in heater that erases whatever was recorded on the screen.

Anyway, a few years go by and I lose contact with the guy. Rumors are flying around, and it turns out that he has decided to sell his entire collection. Bits and pieces of it start showing up at auction houses and flea markets. Another friend of mine mentions that he picked up a lot of CRTs at an auction house and asked if I wanted to pick through it. While sorting through it, I recognized the rare beast and bought it on the spot.

P10 Dark Phosphor CRT

So I finally got the time to hook it up and try it out. There doesn’t seem to be any documentation. The part number is 06E024P10, made by Thomas Electronics. It works, but not particularly well. Since the pin connections are nonstandard and the electron gun has some extra elements, I’ve probably got it connected all wrong. Anyway, I was able to put some scribbles on the screen.

P10 Dark Phosphor Screen

Notice the dark purple areas. I am shining a lamp through an aperture in the top of the CRT.

P10 Dark Phosphor Screen

Looking through the aperture, you can see how the CRT has a standard green phosphor section on the top third. This might have been used to verify that the tube’s electron gun was in focus. It could also have been used for status information. Most likely this tube would have been used in an early form of storage oscilloscope for capturing single-shot high speed events, although most examples of P10 tubes were designed for radar displays.

CRTs with Magnetic Deflection

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Whew, Maker Faire was a lot of work, and a lot of fun!

Now that the Asteroids arcade machines are finished, I’m thinking about some suggestions that people gave me. A lot of people want a larger screen. Even with a precision 3″ CRT (3RP1A, for the curious), playing the game involves lots of squinting and hunching over.

In my collection I have a pile of 5″ CRTs, mostly electrostatic but a few magnetic. The electrostatic CRTs are quite long: 16 3/4″ is a pretty common length but some are even longer. The distance is necessary to maintain a reasonable deflection factor. 70-100 volts applied across a pair of deflection plates leads to 1″ of beam deflection. While I could certainly build a project with a very long case, this gives me a good excuse to experiment with a few of the magnetic CRTs I have.

Starting with cardboard harvested from old toilet paper rolls, I made a tube that can slip over the neck of a CRT. Next I cut 12 notches in both ends so I could hook magnet wire around them. Then I cut the whole arrangement into two halves to make it easier to wind the first set of coils on the inside of the tube.
Hand Wound Deflection Yoke
Then I wound 19 turns of wire on each half, starting with a small set of 3 turns spanning 2 notches, and then winding 7 turns across 3 notches, and finally 9 turns across 5 notches. After taping the two halves back together, I soldered the two sets of windings together in series. The polarity is critical because the magnetic fields need to add together, not cancel out. The second set of windings used the same winding pattern only this time I wound them on the outside of the cardboard tube and rotated 90 degrees.

This coil arrangement is called a semidistributed winding: look at (c) in the figure below.
Deflection Yoke Styles

After wrapping a layer of insulating tape over the windings, I wound a thin strip of soft steel around the whole thing. This provides a high permeability path for the part of the magnetic field outside of the CRT envelope. The idea behind the winding technique and all that is to create a uniform magnetic field in the path of the electron beam. The uniform magnetic field deflects the electron beam according to the Lorentz force law:
F=q(E + v x B)
E is the electric field. In this case, it’s the potential between the cathode and the anodes in the electron gun as well as the final anode. This accelerates the electrons forward towards the face of the CRT. The deflection force is the cross product of the velocity (v) and the magnetic (B) field. You can figure out the direction of force using the left hand rule. Since the electrons are moving towards the screen, a magnetic field in the up-and-down direction pushes the electron beam from side to side. This means that the horizontal deflection coils have to be positioned on the top and bottom of the CRT neck.
5AXP4 With Yoke

And it worked! I slid the yoke onto the neck of a 5AXP4 which is a CRT designed for electrostatic focus and magnetic deflection. It took nearly 1 amp to get a bit under an inch of deflection. To decrease the current I can add more windings. There’s a classic engineering tradeoff there between response speed (bandwidth) and current, since more turns have more inductance and parasitic capacitance. Incidentally, since the magnetic field strength is proportional to the current in the coils, I’ll have to drive them with a linear amplifier design that servos the current instead of the voltage.

The next step is to figure out how to handle magnetic focus. I have a 5FP14 which requires an external permanent magnet or electromagnet to focus the beam instead of the usual electrostatic lens. My friend Kent sold me a military radar display that uses a 5FP7A, and this display has a ring magnet connected with a screw and gear mechanism to adjust the focus.
5FP7A

A great resource for me has been the MIT Radiation Laboratory Series, Volume 22: Cathode Ray Displays, available here as a free PDF download. It’s full of details on how deflection and focus coils were manufactured.

I’ll leave you with this beautiful shot of the zero-first-anode-current electron gun assembly in the 5AXP4. The visible elements are (right to left): cathode, grid, accelerator, focus electrode,  and second anode (electrically tied to the accelerator).
5AXP4 Electron Gun

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