Getting 20% Duty Cycle From a 555 Timer

I want to stress-test some LEDs for the long-stalled bike taillight project with a high current / low duty cycle drive. The usual specs give something like 100 mA at 10% duty cycle in a 100 μs period, but maybe they’ll withstand more abuse than that; I don’t have any specs whatsoever for these LEDs. The usual DC rating is 20 mA, so 100 mA at 20%, say 2 ms in a 10 ms period, should give the same average power as the DC spec. I plan to run them continuously until some failures to pop up or it’s obvious they’re doing just fine.

Although this would be a dandy Arduino project, a classic 555 timer IC makes more sense for something that must run continuously without changing anything. The usual 555 circuit restricts the duty cycle to more than 50% for high-active pulses, a bit over the 20% this task calls for. The simplest workaround is a Schottky diode across the discharge resistor to separate the two current paths: charge uses the upper resistor, discharge the lower, with the diode forward drop thrown in to complicate the calculations.

Rather than putz around with calculation, a few minutes iterating with Linear Technologies’ LTSpice IV produces a reasonable result:

In round numbers, a 1 μF timing capacitor, 2.7 kΩ charge resistor, and 13 kΩ discharge resistor do the trick. Given the usual capacitor tolerances, each resistor should include a twiddlepot of about half the nominal value: 1 kΩ and 5 kΩ, respectively.

I’m thinking of repurposing those Wouxun KG-UV3D batteries for this task and found a 7.5 V 3.5 A wall wart in the heap that will be close enough for the test rig. The 555 output should drive a logic-level MOSFET just fine, although even an ordinary FET would probably be OK for the relatively low current required for LED toasting.

Commercial Line Cord: Details Matter

Driven by forces beyond my control, I had to rent a carpet cleaner from a local Big Box home repair store. The rugged line cord plug had an unusual (to me, anyway) strain relief fitting on the back, consisting of a circumferential clamp around the cord and a large diameter, deeply recessed opening on the nut to prevent the cord from flexing sharply:

But something seemed odd, so I unscrewed the finger-tight clamping nut:

Whoever installed the cord cut the insulation back far too much, as those fingers should close on the insulation jacket, not the conductors.

I fought down my instinctive response, took a deep breath, clicked my heels together three times, repeated “This is not my problem”, and suddenly it wasn’t my problem any more. I tried reporting it to the harried clerk at the Big Box Store, but she instantly fluttered off to help somebody else after noting my return in the Big Book of Rental Contracts.

Having been unable to find a single listing of all the ARRL Hands-On Radio columns(*) by Ward Silver, N0AX, in QST magazine, I scraped their lists, did some cleanup, and roughly categorized each column’s topic. If you want to bootstrap yourself (or someone you know) from zero to pretty good, he can get you there!

[Update: (*) You must be an ARRL member to access the collection, but you need not hold an amateur radio license…]

 Exp Title DC Audio Digital Power RF Theory 1 The Common-Emitter Amplifier x x x x 2 The Emitter-Follower Amplifier x x x x 3 Basic Operational Amplifiers x x x 4 Active Filters x x 5 The Integrated Timer x 6 Rectifiers and Zener References x x 7 Voltage Multipliers x x 8 The Linear Regulator x x 9 Designing Drivers x x x x 10 Using SCRs x x 11 Comparators x x x x 12 Field Effect Transistors x x x x x x 13 Attenuators x x x 14 Optocouplers x x x 15 Switchmode Regulators, Part 1 x x 16 Switchmode Regulators, Part 2 x x 17 The Phase-Shift Oscillator x x x 18 Frequency Response x x x 19 Current Sources x x x 20 The Differential Amplifier x x 21 The L-Network x x 22 Stubs x x 23 Open House in the N0AX Lab 24 Heat Management x x 25 Totem Pole Outputs x x x x 26 Solid-State RF Switches x 27 Scope Tricks x x x x x x 28 The Common Base Amplifier x x x x 29 Kirchhoff’s Laws x x x 30 The Charge Pump x x x x 31 The Multivibrator x x x 32 Thevenin Equivalents x 33 The Transformer x x x x 34 Technical References x 35 Power Supply Analysis x x x 36 The Up-Down Counter x 37 Decoding for Display x 38 Battery Charger x x 39 Battery Charger, Part 2 x x 40 VOX x 41 Damping Factor x x x 42 Notch Filters x x x 43 RF Oscillators, Part 1 x x 44 RF Oscillators, Part 2 x x 45 RF Amplifiers, Part 1 x x x 46 Two Cs: Crystal and Class x x 47 Toroids x x 48 Baluns x x 49 Reading and Drawing Schematics x 50 Filter Design 1 x x x 51 Filter Design 2 x x x 52 SWR Meters x 53 RF Peak Detector x x x 54 Precision Rectifiers x x 55 Current/Voltage Converters x x x x 56 Design Sensitivities x 57 Double Stubs x 58 Double Stubs II x 59 Smith Chart Fun I x x 60 Smith Chart Fun 2 x x 61 Smith Chart Fun 3 x x 62 About Resistors x x x x 63 About Capacitors x x x x 64 Waveforms and Harmonics x x x x x 65 Spectrum Modification x x x 66 Mixer Basics x x x x 67 The Return of the Kit 68 Phase Locked Loops, the Basics x x x x 69 Phase Locked Loops, Applications x x x 70 Three-Terminal Regulators x x x 71 Circuit Layout x x x x x x 72 Return Loss and S-Parameters x x 73 Choosing an Op Amp x x x 74 Resonant Circuits x x x 75 Series to Parallel Conversion x x 76 Diode Junctions x x x 77 Load Lines x x x x 78 Bridge Circuits x x x 79 Pi and T Networks x x x 80 Battery Capacity x x x 81 Synchronous Transformers x x 82 Antenna Height x x 83 Circuit Simulation, Part One x x x x x x 83 Circuit Simulation, Build and Test x x x x x x 85 Circuit Simulation, Complex Parts x x x x x x 86 Viewing Waveforms in LTspice x x x x x 87 Elsie Filter Design, Part 1 x x 88 Elsie Filter Design, Part 2 x x 89 Overvoltage Protection x x x x 90 Construction Techniques x x x x 91 Common Mode Choke x x x 92 The 468 Factor x x 93 An LED AM Modulator x 94 SWR and Transmission Line Loss x x 95 Watt’s In a Waveform? x x x x x 96 Open Wire Transmission Lines x 97 Programmable Frequency Reference x x x 98 Linear Supply Design x x x 99 Cascode Amplifier x x x x 100 Hands-On Hundred 101 Rotary Encoders x 102 Detecting RF, Part 1 x x x x 103 Detecting RF, Part 2 x x x x 104 Words to Watch For x 105 Gain-Bandwidth Product x x x x 106 Effects of Gain-Bandwidth Product x x x 107 PCB Layout, Part 1 x x x x x x 108 PCB Layout, Part 2 x x x x x x 109 PCB Layout, Part 3 x x x x x x 110 PCB Layout, Part 4 x x x x x x 111 Coiled-Coax Chokes x 112 RFI Hunt x x 113 Radiation Patterns x x 114 Recording Signals x x 115 All About Tapers x x 116 The Quarter-Three-Quarter Wave Balun x 117 Laying Down the Laws x 118 The Laws at Work x 119 The Q3Q Balun Redux x 120 Power Polarity Protection x x

Corrections, amendations, commentary? Let me know…

Large UV LED Self-Fluorescence

Just got an ultraviolet LED in a 10 mm epoxy package that’s water-clear in visible light and slightly fluorescent in its own UV:

The epoxy usually has some fluorescence, but this seems more dramatic than usual. In any event, the die’s wide beam angle shows clearly; the beam along the axis out in front is actually pretty tight.

It’s sitting on the back of a white ceramic tile and the colors came out surprisingly close to real life.

Adding this to an Arduino would follow the same logic as, say, the pager motor: power the LED + resistor + MOSFET from a +5 V external regulator that won’t heat the Arduino board, then define an unused bit in the shift register as, say UV_LED.

It runs at 20 mA and drops around 3.3 V.

HP10525T Logic Probe: New Timing Capacitors

While putting together the PIR sensor, I had occasion to haul out the old HP10525T Logic Probe (a bookend for the Tek logic probe) to figure out why the shift register wasn’t updating; that was easier than hauling the breadboard to the oscilloscope. While it showed the problem (wire tucked into wrong hole, hidden behind a cluster of other wires), it didn’t seem to be blinking quite right. The HP10525T Logic Probe Operating and Service Manual says it should blink at about 10 Hz for any pulse train from about 10 Hz up through 50 MHz (yes, 50 megahertz), with a minimum pulse width of 10 ns (yes, 10 nanoseconds), but it didn’t do that for the PWM going to the RGB LED strip or the shift register clock.

Given a manual printed in February 1975, I’m sure you know where this will end up…

Unlike contemporary gear, the manual tells you how to dismantle the probe, using the needle tip as a tool. Doing so reveals a tidy circuit board with gold plated PCB traces:

The two tiny black rectangular capacitors just to the left of the 8 pin DIP IC are C1 and C2, rated 10 μF at 2 V (yes, 2 volts). As you might expect, they had ESRs in the 3 to 5 Ω range, rather than around 0.2 Ω. The catch is that the case doesn’t have room for anything much taller, but I did contort some solid tantalum through-hole caps into the space available:

Buttoned it up again and … it works fine. There really isn’t that much else to go wrong, is there?

This picture shows the incandescent lamp glowing half-bright to indicate that the lethally sharp probe tip (on the left here, with its stud on the right in the other pictures) sees a floating input:

I love happy endings, although I’m sure the accompanying HP10526T Logic Pulser needs recapping, too. When that project comes around, I’ll probably use SMD ceramic caps, because the pulser’s circuit board packs even more parts into the same volume.

Speaking of unhappy endings, HP used to be run by real techies: The Fine Manual’s body starts with Page 0 verso, after the title and two pages of front matter. ‘Nuff said.

Ancient Harman-Kardon PC Speaker Re-Capping

Suddenly a resonant thwup-thwup-thwup-thwup fills the house, but no helicopters fill the skies; in fact, most of the noise seems to be inside the house and … it’s coming from the shop. We look at each other and dash toward the basement door, knowing perfectly well that this is the part of the movie where the audience chants “Don’t open the door! Don’t open the door!

Come to find out that it’s the pair of old Harman-Kardon powered speakers attached to the PC attached to the Thing-O-Matic; the PC is off, but I left the speakers turned on. Quick diagnostics: turning the volume down doesn’t reduce the motorboating, pulling the audio cable out of the PC doesn’t change anything, the only cure is to turn them off.

Under normal circumstances, they’re pretty good-sounding speakers, at least to my deflicted ears, although I have my doubts about the effectiveness of that reflex port. I plugged in a pair of unpowered speakers as subwoofers down near the floor, just because they were lying around; a pair of 75 mm drivers does not a subwoofer make, fer shure.

Pop quiz: what’s wrong?

Need a hint? Looky here:

Disassembly:

• The front cloth grille has four snap mount posts, two secured by hot-melt glue blobs: pry harder than you think necessary
• Two screws near the top of the bezel thus revealed hold it to the back
• The bottom two screws holding the driver frame in place also hold the bezel to the back
• Remove two screws from the grooves in the bottom of the back
• Amazingly, the driver has two different size quick-disconnect tabs; the neatly polarized wires slide right off

Cut the audio cable just behind the back panel, then push the two-piece cable clamp outward from the inside:

The bottom of the circuit board shows considerable attention to detail. Note the excellent single-point ground at the negative terminal of the big filter capacitor:

And, of course, that’s the problem: most of the electrolytic capacitors were dried out. My ESR tester reported the big filter cap (downstream of the bridge rectifiers) as `Open` and several of the smaller caps were around 10 Ω. Replacing them with similarly sized caps from the heap solved the problem.

It should be good for another decade or two…

Hall Effect Current Sensor: Doodles

Hanging a Hall effect sensor on an Arduino brings up the notion of building a DC current sensor that doesn’t depend on measuring the voltage across a resistor. This would be important for a battery-powered gizmo, where not dropping voltage in a sense resistor makes more voltage available for the load as the batteries discharge.

Pages 55-57 of that Honeywell booklet provides the outline: take a ferrite toroid with a cross-section larger than a linear Hall effect sensor’s package, cut a radial slit just barely big enough for the sensor’s thickness, wind N turns, and pass a current through the winding. Shazam! The sensor output varies linearly with the core flux, which varies linearly with the current, albeit subject to all the usual approximations.

Some variables:

• Ia = air gap (cm)
• Ic = mean length of core (cm)
• I = winding current (A)
• Bc = flux density in core (G)
• Ba = flux density in air gap (G)
• μc = relative permeability of core (dimensionless)
• N = wire turns around core (dimensionless)

Yes, they use capital-eye for both length and current. They probably know what they’re doing. I don’t have to like it.

Assuming a narrow gap with respect to the cross-section, Ba ≈ Bc. Assuming the core isn’t close to saturation, then Ba is proportional to current, thusly:

`Ba = (0.4 π · μc · NI)/(Ic + μc · Ia)`

I wondered how the numbers would work for a typical ferrite toroid…

An FT50-43 toroid looks to be both the smallest ferrite core that will surround the sensor and the largest lump you’d want in a gadget. Some specs (that collection will be helpful):

• 0.50 OD (inch) = 1.27 cm
• 0.281 ID (inch) = 0.714 cm
• 0.188 height (inch) = 0.478 cm
• 0.0206 area (inch2) = 0.1232 cm2
• 1.19 mean path length (inch) = 3.02 cm
• μ = 850 (that’s “initial” permeability, with 2000 peak)
• 2750 saturation flux (G) at 10 Oe
• AL = 523 in weird units: N=√(nH/AL)

More toroid info, including some background and inches-per-turn tables, lives there. A good guide to building the things, with more tables, is there.

The sensors on hand seem to be 0.060 inch thick = 0.15 cm, although cutting an exact gap may be a challenge; a diamond slitting wheel in the Sherline may be needed for this operation. They claim a maximum flux density anywhere from 400 to 1000 G, depending on which datasheet extract you believe and whether the parts match their descriptions.

Running the numbers for the higher flux density:

`1000 = (0.4 π · 850 · NI) / (3.02 + 850 · 0.15) = 8.2 · NI`

Note that the air gap dominates the denominator, which makes sense.

Rearranging to solve for NI:

`NI = 1000 / [(0.4 π · 850) / (3.02 + 850 · 0.15)] = 1000 / 8.2 = 122`

Which means in order to have 1 A produce 1000 G at the sensor, I must cram 122 turns through that little toroid.

The inner circumference of the toroid works out to 0.88 inch if you ignore the gap, which means a single layer requires 122/0.88 = 138 turn/inch. Consulting the enameled wire tables, that’s AWG 34 or 35. I doubt overlapping a few turns makes any difference and I’m certain I can’t wind that many perfect turns anyway, so that spool marked 32-33 AWG / 8.5 mil might actually get used.

The Specialty Wire Box has a nearly full spool of AWG 44-½ wire (that grosses nearly half a pound and might reach NYC), but that’s just crazy talk; the stuff is 1.88 mil in diameter, almost exactly 1 RCH. There’s also a small solenoid coil wound with 4.5 mil wire (about AWG 37), still deep in the realm of craziness for winding that many turns by hand.

Working backwards, NI varies linearly with flux density, so 400 G would require NI = 49 and only 60-ish turn/inch. That’s AWG 26 enameled wire and seems much more sensible.

The gotcha is wire resistance: all this should offer less resistance than a sense resistor on the order of 100 mΩ. AWG 26 wire is 42 Ω/1000 ft = 42 mΩ/ft and FT-50 cores have about 0.6 inch/turn, so a 60 turn winding would be 3 ft long = 126 mΩ. The finer wires would be much much worse, so this is not a clear win despite its overwhelming geekiness.

An op amp could boost the output by a factor of 10, reducing the winding to a dozen turns and the resistance to 13 mΩ, even if you didn’t use bigger wire. I like that a whole lot better, although the amp must remove the Hall sensor’s nominal Vcc/2 offset to get a sensible range & output for DC current, assuming unipolar current. We have control over the current, so we could turn it off, measure the op amp’s offset at 0 mA, then send the offset (as a filtered PWM output) to the op amp’s inverting input.

A gain of 100 would give full-scale sensor output for 100 mA current, although I’d be suspicious of the overall accuracy and stability. For pretty-close measurements, like for LED current control, it might be Good Enough.

Given the reduced number of turns, you could do a bifilar winding and then buck the main current with a sampling current. That has the benefit of reducing the core flux to zero during the measurement, so the sense amp can have huge gain and the sensor maintains a large dynamic range. At the cost of a calibrated current source, of course, but … maybe with more buck turns than sense turns?