Ed Nisley's Blog: Shop notes, electronics, firmware, machinery, 3D printing, laser cuttery, and curiosities. Contents: 100% human thinking, 0% AI slop.
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:
NE555 pulse generator
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.
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:
AC Line Cord Plug – clamp nut
But something seemed odd, so I unscrewed the finger-tight clamping nut:
AC Line Cord Plug – clamp fingers
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…
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.
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:
HP10525T – original caps
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:
HP10525T – replacement caps
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:
HP10525T Logic Probe – glowing
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.
Suddenly a resonant thwup-thwup-thwup-thwup fills the house, but no helicoptersfill 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:
HK Powered Speakers – wall wart
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:
HK Powered Speakers – cable grommet
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:
HK Powered Speakers – PCB foil side
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.
The Smell of Molten Projects in the Morning 