Noisemaker Volume Switch Repair

The Equipment:

Homedics SS-2000 Deep Sleep Sound Spa noisemaker – on loan from my dear mother

The Problem:

Potentiometers rely on moving contacts that do a lot of rubbing back and forth over their lifetime. With enough wear, this can create dead spots where continuity is diminished or even interrupted between the moving contact (known as the “wiper”) and the resistor track.

We sometimes leave the noisemaker on overnight to perpetuate the illusion in our son’s mind that things are still going on while he sleeps, something he grew fond of while in the hospital. And because of the wear in the volume adjuster, we had a hard time setting it to volume level that was not too quiet and not too loud and not in the middle of a dead spot. Worse still, sometimes while playing, the sound would briefly cut out entirely and junior would miss it.

I had enough and found exactly the same switch on Amazon for cheap.

The Plan:

Repair the volume adjuster knob on the noisemaker with a new power switch and potentiometer combo.

The Parts:

10 Pieces D Type Rotary Carbon Film Potentiometer 50K Ohm with Switch – $4.62 from Amazon

The Process:

Teardown was quick. Four screws out the bottom of the noisemaker and we’re in. Another four screws held the main PCB to the upper half of the clamshell case, and there was enough slack in the wires to gain easy access to the opposite side. The knob disconnected from the base-bone, and the base disconnected from the switch-bone (a little glue and a little screw).

Desoldering the five pins from the PCB was a little more intensive. I didn’t realize at first that the larger power pins were in fact riveted to the board, so my initial attempts to pry the pins away with the tip of my pencil iron were fruitless. But the nice thing about rivets is that they drill out easy, and once they were gone the switch began to loosen up.

Here I made another mistake. I pushed the switch all the way through without watching what was happening on the other side with the pins of the switch still partially soldered to the traces. So it came to pass that the solder pads delaminated from the the board. This blog’s namesake strikes again! Fortunately the traces did not break, so I could continue on to heat and free the pins.

The new switch soldered in without incident. I began measuring resistance on the potentiometer pins to check my work, and also happened to check across the pins of the switch when it was open. What I found was not OL on my meter like I expected. Instead I got continuity, but something like 5 MOhms for resistance. Practically an open switch for this application, I thought, but disturbing nonetheless. I removed the switch again, and tried a another one. Same result. Of course my new switches did not conduct in the open position apart from the board, but for some reason when I placed them in-circuit, they did. File under “But It Still Works, Right?”, I guess, because yes, it worked. I snapped the case back together and we’ve been enjoying white noise consistently at precisely the amplitude our little hearts desire ever since.

Review:

“If at first you don’t succeed, redefine “success” and celebrate your victory!” Comforting guidance for the little politician in all of us. ’nuff said.

Bouncy Seat Hack: Power from a Phone Charger

The Equipment:

Battery-powered automatic bouncy seat and music player / noisemaker – $35 from a Once Upon a Child store in Sioux Falls.

The Problem:

I love this bouncy seat. My son loved it almost as soon as we first put him in it, and it became his bed every night. The subtle bouncing and soft music (something more original than the standard “Twinkle Twinkle Little Star” in an endless loop) were a real boon during the first couple months at home with our boy. But the thing eats batteries for lunch. I’ve been changing out 3 x C batteries (in series – remember this) almost every week since we started using the bouncy seat. At about $1.25 per battery, that’s $15-ish per month. So cancel my paid subscription to this machinated baby-sitting service: we’re going hacking!

The Plan:

3 x 1.5V batteries in series yields 4.5V. A typical USB phone charger produces 800mA of 5V DC. First, power the bouncy seat from a wall outlet using a USB phone charger alone without batteries. Next, since the phone charger and batteries connect to the same points to power the bouncy seat, add a switch to isolate power sources so I can leave the batteries in and not end up charging very-not-rechargeable batteries.

The Parts:

DPDT 2-position .5A 50V slide switch $4.51 for 10 pcs.
I probably could have used just a single-pole double-throw switch for only the positive leads, but it seemed smarter to isolate negatives between power supplies as well. If you can educate me about why that is a good or a bad idea, please leave a comment!
3′ USB printer cable kinda like this that I had lying around. Free, I think.
About a foot each of 20-something gauge red (still too big, but it’s what I had) and black wire. Wire is cheap.

The Process:

This was such a fun, simple little job! To start, I made sure a phone charger could power everything by turning on the bounce and the music and measuring the current draw from the positive to the negative terminal in the battery bay. It measured something like 1.54mA, if I recall correctly. It was really tiny anyway, easily powered 500 times over by an 800mA charger.

Sidetrack: the first time I opened up the case for the bounce mechanism, I expected to see weights oscillated by a cam and motor or something like that. What I found was nothing so idiotic. The wizard who designed this rightly determined that a motor and cam was far too prone to wear out. The bounce motion is created by a weighted arm with a magnetic coil in the center that is energized in pulses to overcome the spring on the bottom in its attempt to wrap its magnetic arms around the steel pin in the center, and then release and bounce back up. Nothing touches! The only wear parts are the hinges! Genius!

Anyway, I snipped the B end of the USB printer cable and stripped off the insulation, shield, and data wires. A USB cable has four wires, two for power (red and black), and two for data (green and white). Batteries were off the table during this stage, so I soldered the wires directly to the battery terminals for simplicity.

Once I got the switch, I traced its outline onto the case with Sharpie and drilled 1/16″ holes at the corners and then in a series along the edges until I could snip the rest out with a flush cutter. A little touch-up with my set of needle files yielded a fairly square hole. Luckily I had two of the tiniest screws possible from an old laptop case to secure the switch to the case. I desoldered the cord from the battery terminals and attached them to one side of the switch. The switch isolates the two sets of three terminals so it doesn’t short. I used new wire to connect the other side of the switch back to the battery terminals, and then came the slightly trickier soldering.

The negative lead that used to connect the PCB to the batteries was a very tiny stranded wire. It took a couple tries to get it stripped with enough strands left and then bend it to shake hands with the wire from the switch. The positive lead went straight to a capacitor, so I stripped a longer end and sidled it up to the cap and bridged them with solder. I covered both neg and pos joints with appropriately-colored heat-shrink tubing. I don’t have a heat gun, so I just held the tip of my 25W soldering iron near the tubing and let radiation do the work. Surgery complete. Close and confirm patient viability. Success.

Review:

I think one of the best things I could have done to make this project better would’ve been to use wire a couple gauges smaller. Soldering wires of unlike gauge and construction (solid vs. stranded) seldom ends well for me. I also would have liked to use install a female USB micro jack to avoid a dragging a power cord tail everywhere with an otherwise rather portable bouncy seat, but actually finding and procuring the right configuration of such a jack is not as easy as a trip down the stairs. Maybe someday.

The choice of switch worked out perfectly. There’s even enough overlap in the slide that if you move it fast enough, the bouncy seat doesn’t lose power, making transition between power sources seamless.

Ciao for now! Ideas, criticism, and education welcome in the comments!

Bob Parker Blue ESR Meter Build

I said I’d write when I had something else to break. Incorrect. I had something to build instead, and I’ll tell you all about it, but first let’s discuss . . .

The Theory:

ESR stands for equivalent series resistance. It’s a big term, but I’m going to try to make it simple. I don’t claim to fully understand it. Do your own research. Here goes. While every real capacitor, resistor, and inductor has its main property of capacitance, resistance, and inductance respectively, it also functions with slight values of the other properties additionally, though at far lower levels. For example, an inductor will mainly convert electrical current into a magnetic field, but it will also store a little energy by capacitance and waste a little energy by resistance as heat. An easy way to understand the way these “extra” properties of a real component behave in relation to the main effect is to think of them as being represented by other ideal components in series with the main one. Hence the term “equivalent series resistance”. ESR is the resistance in (for instance) a capacitor that is equivalent to a small ideal resistor used in series with an identical ideal capacitor. Got it? Let’s move on.

Then in case you haven’t noticed, we’re actually focusing on capacitors today. Electrolytic capacitors from the factory contain liquid electrolyte that helps store a charge. In brand-new condition, an electrolytic cap’s resistance (ESR) is measured with an AC charge usually at 100 kHz (somebody figured out that that works best for e-caps), and that’s the value it should keep for the cap’s lifespan. As the cap is used and abused, it degrades by means of the electrolytic fluid drying, and the ESR goes up accordingly, which screws with your circuit in ways I don’t yet understand. The kicker is that an electrolytic cap can dry out and go high-resistance while still retaining a measurable capacitance value that will appear to be within spec. So how can you tell if your cap has failed in this manner? Ya gotta have . . .

The Equipment:

Bob Parker Blue ESR Meter kit
The complete kit out of the box, plus a lovely set of 4mm parrot clips extra.

What we have here is a lovely product from Aussie electronics engineer Bob Parker. I chose this model because it’s the newer, handsomer version of the respected device in use by our man here.

This is basically a low-range Ohmmeter. It only displays .00 – 99 Ohms. Its important feature is its low-voltage, 100 kHz test signal for testing electrolytic capacitors in-circuit. The low voltage keeps most other components from activating and distorting the measurement on the cap or worse. The 100 kHz frequency matches most factory tests for easy comparison with a datasheet value. Beyond that, this thing is just really well-designed and assembled. The case is not only sturdy (it just feels good in your hand), but pretty. It only uses a single tact switch to power on and interact. The display is bright and easy to read. But I didn’t want to spend the extra $20 to have it assembled for me. I wanted to build it myself! So here is . . .

The Process:

Couldn’t be simpler! This PDF constitutes the manufacturer’s assembly instructions. I’m going to let the pictures do the talking here.

Killer helping hands tool
Killer helping hands tool
A Few Resistors In
Just a few resistors in and the fun is only beginning.
Resistors done, added caps and diodes. Note the tan-colored crystal in the lower left area.
Resistors done, added caps and diodes. Note the tan-colored 3.58 MHz crystal in the lower left area. It gets reduced to 100kHz somewhere in there.
Everything together with sockets for the ICs. I didn't feel up to soldering the ICs directly.
Everything together with sockets for the ICs. I didn’t feel up to soldering the ICs directly.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

The Outcome:

Success!
Success!

It worked first try! Pressing the top button once powers the unit on. Clipping the leads together and pressing the button again zeroes the meter to the resistance in the leads, and the meter powers off automatically. The manual details a calibration procedure using the the included 82 Ohm 1% tolerance carbon film resistor. With the unit out of the case, variable resistor 2 (the upper of the two dials with the phillips relief) can be adjusted until the display literally reads 82. Unfortunately, when I last tried it, adjusting the potentiometer had no effect. Not sure what’s up with that. But it seems to work and read fairly accurately. Someday I’ll take the time to learn.

While researching this article I came across a quote that was just too, too true.
“Three weeks in the lab will save you a day in the library every time” – R. Stanley Williams

My First Hack – Gateway FHX2300 Monitor Audio Out

The Equipment:

Gateway FHX2300 LCD monitor with internal speakers (Free, came with my bride), Amazon Fire TV stick ($39, Amazon.com last week), Logitech X-530 5.1 surround speakers (Free, came with a Dell 16″ monitor and partially gutted eMachine I got for a couple digits from a nice lady in another apartment last summer)

The Problem:

The Fire TV stick worked great once I learned how to boot it up right: plug in the USB power first, let it boot up, then connect to the HDMI on the screen. I realized after exploring the stick for a while that the 3.5mm audio jack in the back of the monitor was not the output I thought it was, but an input for the two internal speakers I didn’t know it had.

So I had a screen and a media acquisition and display device but no way to get sound out but a couple dollar-store speakers with all the resonant quality of a plastic pail.

After an hour of research, I found there really are no readily available or cheap solutions to say, extract the audio from the HDMI to a 3.5mm output. In desperation I settled on . . .

The Plan:

Tap into the audio circuit that handled input from the HDMI and 3.5mm TRS jack, and route the signal to a 1′-2′ cord with a female TRS jack on the end to plug my speakers into.

The Process:

I didn’t like tearing open the case one bit. I used my SOG pocket knife to pry into it, and every twist of the blade produced a new nick in the formerly flawless glossy bezel. The front now has a crack where there aint been no crack before.

IMG_1710Once it came apart, it took me a couple nights staring at the PCB and trying things out live to reverse engineer the main board and the audio circuit in it.  The 2×8-pin header in the lower left corner of the board connects what I believe is the audio-handling and amplifier circuit from a small board on the other side. Traces run out from that header up to a 4-pin jack for the speakers (driven at 2.5V) in the upper left corner of the board. Having learned that the typical output from a consumer-grade TRS jack runs at about .5V maximum, I was hoping that I would be able to steal an un-amplified signal right off that header at a safe amplitude. That didn’t work out. There was no such output on the header. Instead I opted to simply clip off the internal speakers and splice my output cable into their old arteries. I thought I would try to protect my speakers’ input by adding a simple voltage-divider attenuator circuit into the mix. This proved unnecessary. Once I made the splice on the terminal strip, I tested the output with a pair of headphones I had lying in a drawer. Nothing exploded! This was clearly a blessing on my progress so far that demanded a proper finish.

Repaired button board

In the middle of trying and testing on the desk, I made a cardinal mistake. I plugged the main board into power before plugging in the button board that contained the Power On switch. I accidentally dropped the board directly onto the main board, just above where the leads from the AC input jack were soldered. When I moved to pick it back up, it shifted just enough to short the 120V and pop went the weasel. Sure enough, the back of the button board was damaged and would not power up the monitor. Where the red wire on the left is soldered to the board used to be two pads for what I believe was a resistor for the LED to the right. I simply soldered across it (it exploded) with the wire and terminated the other end where the blown trace used to end. Notice the pale line where I removed the broken copper.

IMG_1722Once the button board was back in service, I finished up by supergluing the audio out cable to the side of the terminal strip and screwing the terminal strip to the top of the steel case. I laid tape across the wires to secure them, and reassembled the screen and all that other stuff. The new 2′ whip of audio out cable popped neatly out through the hole for the built-in audio input jack.
IMG_1729
OK to ship! A feature-enriched computer monitor made functionally indistinguishable from any TV screen.
The Lessons:
  1. Never buy what you can build (or hack).
  2. Always research your project heavily. Go crazy and don’t fear breaking it.
  3. Always observe lesson #2 only in that order.
  4. Always take the fox and the lettuce across the river before you bring the rabbit. See The Process, Paragraph 3.
  5. Never set your heart on material things. They go hard at disappointing when they break.

Ciao! I’ll write again when I have something else to break.