The Groaner
Pointless Electronics PE-002 · Pick it up and it groans · A tilt switch, a recording module, an amp and a metal box · By Onion Madder · Mess O' Pedals
It sits on the desk doing nothing. Pick it up and it groans at you. Put it down and it groans again. That is the entire behavior, and there is no control on the outside of the box to change it, stop it, or apologize for it. It is the second Pointless Electronics device, and it has a product page.
There is no circuit board in this build. I designed one, drew it, drilled it and got most of the way through soldering it before working out I did not need it - more on that below, because the mistake is more useful than the build. What actually goes in the box is a recording module, a tilt switch, a little amplifier board, three cells and a speaker, and the whole thing is a handful of solder joints.
It takes about an hour, most of which is the grille.
Before you start
tap to expand / collapse
Record first, swap the resistor second. This is the only order that works, and getting it backwards is the one mistake in the build that looks like a fault. The reason is in The 470K Trick below - read that section before you touch the iron.
Drill before you solder. Aluminum swarf is conductive and it goes everywhere. Grille, switch hole and speaker mounting all get done first, then blow the box out, then start on the electronics.
The box is a conductor. A bare module rattling around inside a metal enclosure is a short waiting to happen. Tape it, sleeve it, or glue it down before the lid goes on.
What you'll need
tap to expand / collapse
- 1× ISD1820 recording module - spot it: small blue board with a black blob or DIP chip, a microphone can, and three tactile buttons marked REC, P-E and P-L. Records about ten seconds through its own mic. The ISD1820 guide has the whole board mapped if you want it
- 1× tilt switch (SW1) - spot it: two legs, and it makes and stays made when you tip it past an angle. If it only clicks briefly when you tap the bench and then goes quiet, that is a vibration switch - a different part with different manners. Both work here, see Things To Try
- 1× 470K resistor (R) - spot it: yellow, violet, yellow, then gold. This replaces the ~100K already on the module and it is the whole sound of the thing
- 1× LM386 mini amplifier board - spot it: a small board with an 8-pin LM386 on it, a volume trimmer or pot, power and input pads at one end and speaker pads at the other. Any of the cheap ones. It runs at full volume in this box
- 1× 8 Ω speaker, 0.5W - spot it: two terminals, no polarity worth worrying about at this level. 36-45mm suits the medium grille
- 3× AA and a holder - spot it: three cells, not two. 4.5V sits comfortably inside the module's 3-5V window and stays there as they drain. Two cells start at 3.0V and fall off the bottom within a week
- 1× power switch (SW2) - spot it: any small SPST toggle. Without one, the box groans in a drawer, at nothing, forever
- 1× 125B enclosure - spot it: 122 × 66 × 40mm cast aluminum with a screw-on backplate. The grille goes in the backplate
The whole circuit
There is no more of it than this. The module records and plays; the tilt switch is a button gravity presses; the amp makes it loud enough to be rude.
Steps 1–5 Record the groan
Do this on the bench, before anything is mounted and before the resistor is changed. Keep it short - the resistor swap is going to stretch it several times longer, and a groan that runs on for fifteen seconds stops being funny about eleven seconds in. The module can drive the speaker on its own for this part; the amp comes later.
| Step | Do |
|---|---|
| 1 | Connect the battery holder to the module's VCC and GND pads. |
| 2 | Connect the speaker to SP+ and SP−. |
| 3 | Hold the REC button down and groan into the microphone. Two seconds is plenty. Let go. |
| 4 | Press P-E. It should play your groan back at normal speed and normal pitch. |
| 5 | Groan again until you like one. Each recording overwrites the last. |
Steps 6–9 Drop it down
| Step | Do |
|---|---|
| 6 | Find the oscillator resistor on the module - the ~100K one, not the LED's. It sets the sample rate. |
| 7 | Desolder it and fit the 470K in its place. |
| 8 | Press P-E. Your groan is now several times slower, several octaves down, and has lost all its top end. |
| 9 | Write “REC 100K / PLAY 470K” on a scrap of tape and stick it inside the box. Future you will need this. |
Steps 10–13 Wire the tilt switch
| Step | Do |
|---|---|
| 10 | Find the two pads of the P-E button on the module. |
| 11 | Solder one leg of the tilt switch to one of them. |
| 12 | Solder the other leg to the other one. The switch is now wired in parallel with the button, and no polarity is involved. |
| 13 | Tip the module by hand. It should groan once per tip, all the way through, with no stutter. |
Steps 14–24 Box it
The grille and the switch hole come first, then the amp goes in between the module and the speaker, then everything gets sealed, wrapped and screwed down.
| Step | Do |
|---|---|
| 14 | Drill the speaker grille in the backplate - see Boxing It Up. |
| 15 | Drill the hole for the power switch in a side wall. |
| 16 | Blow every last chip of swarf out of the box. |
| 17 | Wire the power switch into the battery's positive lead, between the holder and the module's VCC. |
| 18 | Take the speaker off the module and wire the module's SP+ and SP− to the amp board's input instead. |
| 19 | Wire the amp board's power to the battery, positive through the same switch, and its output to the speaker. Turn its volume all the way up. |
| 20 | Seal the speaker to the inside of the backplate over the grille, with putty or glue all the way round the rim. |
| 21 | Tape or sleeve the module and the amp board so no bare copper can touch the case. |
| 22 | Mount the tilt switch so it sits just inside its threshold with the box flat on a desk - putty first, glue once you like the angle. |
| 23 | Fit the batteries, put everything in, screw the backplate down. |
| 24 | Pick it up. |
The off-board wiring
| Pad | Goes to |
|---|---|
| VCC | Battery +, through the power switch. 4.5V from three cells. |
| GND | Battery −. |
| P-E | One leg of the tilt switch. The other leg goes to VCC. In other words, straight across the P-E button's own two pads. |
| SP+ / SP− | The amp board's input. On the bench, for recording, the speaker can sit here directly. |
| Amp VCC / GND | The same battery, positive through the same switch. |
| Amp OUT | The speaker. Sealed to the back of the grille. Volume all the way up. |
| REC | Nothing. The onboard button is the only thing that ever touches it, and only before the lid goes on. |
If it doesn't work
| Symptom | Check |
|---|---|
| Nothing at all. | Power switch. Then the cells. Then that you actually recorded something. |
| The onboard buttons work but tilting does nothing. | Meter across the tilt switch's two legs and tip it. If it never makes, the part is dead - and if it appears to have one leg, the other one snapped flush at the seal and it was dead before you bought it. |
| It groans at normal pitch. | You re-recorded with the 470K fitted. Put the 100K back, record, refit the 470K. |
| The groan stutters or restarts. | Your switch is a vibration switch, not a tilt switch. Either live with it or put the one-shot board back in. |
| It groans constantly, or whenever anyone walks past. | The tilt switch is mounted right on its threshold. Rotate its mount a few degrees so it sits clear of the trip point at rest. |
| Thin, papery, no weight to it. | The speaker isn't sealed to the plate. Air is going round the back of the cone and canceling the low end. Putty all the way round the rim. |
| Quiet, or the groan clips into a buzz. | The amp's volume. All the way up is the setting; if it buzzes at that, the cells are tired. |
| Worked on the bench, dead once boxed. | The module or the amp is shorting against the case. Take it out, wrap it, put it back. |
How it works, which is barely at all
The module's P-E button does exactly one thing: it connects the P-E pin to the positive rail. The chip sees that pin go high and plays the whole recording. That is the entire trigger mechanism.
So a tilt switch wired across that button is also a button. It's just one that gravity presses. Tip the box, the contact closes, P-E sees the positive rail, the groan starts. Tip it back, it closes again on the other edge, and it groans again.
The 470K trick
This is the whole sound of the box and it is one resistor.
The ISD1820's oscillator resistor sets its sample rate, and the sample rate governs both recording and playback. Stock it's about 100K. Put 470K there instead and the chip runs several times slower.
The pitch drop lives in the mismatch, not in the resistor. Record at 100K, play back at 470K, and playback runs slow - the tape-speed trick, done in hardware. If you ever re-record with the 470K still fitted, it records slow and plays slow, the two cancel, and your groan comes back at normal pitch sounding like nothing happened. That is not a fault and nothing is broken. It is the resistor doing exactly what it says.
The reason it ends up sounding like a groan tube rather than like a slowed-down person is that sample rate takes bandwidth with it. Dropping the rate doesn't just lower the pitch, it throws away the top end. What comes out has no consonants and nothing above about a kilohertz - which is precisely what a groan tube is, acoustically: a spring, a membrane, and no treble.
470K is where I stopped. Lower values give you a shorter, higher, clearer complaint; higher ones go so slow they stop reading as a voice at all. It is worth clipping a few in before you commit, because this resistor is the only creative decision in the entire project.
The board I didn't need
I built a 555 monostable for this. A tilt switch chatters, I reasoned, and every bounce would restart playback and chop the groan into a stutter - so the switch would trigger a one-shot, the one-shot would hold its output high for a couple of seconds, and the groan would be guaranteed to play through.
Then I wired the switch straight to P-E to test it and it worked perfectly first time.
A decent tilt switch closes once and holds. It isn't a ball rattling in a tube. There was no chatter to clean up, so there was nothing for the board to do. I had designed a fix and then gone looking for the problem, which is the wrong way round and cost me an evening.
The board wasn't wasted, mind. What it actually is - a 555 monostable with an edge-triggered input - is “any filthy contact in, one clean pulse out”, and that's a permanently useful thing to have on a shelf. It debounces bend points, it turns a footswitch into a fixed-length gate, and it'll pulse a motor. It just has nothing to do with this box. The layout is here if you want it: ↓ Download PE002_One_Shot_Board.json - 9 by 18, a TLC555, drawn in Copper Bottom and never finished. The editor will report two pin-shorts on it; both are the 555's own tied pins, threshold to discharge and reset to supply, which is how a monostable is wired.
If you take one thing from this build, take that: wire the simple version up and see it fail before you design the thing that fixes it. It takes two minutes and it is occasionally humiliating.
Boxing it up
Two holes and a grille. The grille goes in the backplate, which keeps the outside of the box completely blank - no knob, no LED, no label, nothing to look at or operate. That's deliberate. A control implies an operator; a blank box implies a creature.
Print at 100% / “Actual size”, never “Fit to page” - check the bar measures exactly 100mm before you drill anything. Center-punch every dot before any drill touches the metal, and pilot every hole at 2mm.
The power switch goes in a side wall, not the face. Keep every hole at least 12mm in from an edge - the corner bosses and the lid lip live in there.
Things to try
- Use a vibration switch instead. Then it complains about things that happen near it - a door slamming, a book dropped on the desk, someone walking heavily past - rather than things you did to it. More paranoid, and a genuinely different object. You'll want the one-shot board for this one, because those really do chatter.
- Change the resistor. It's the only voice control the box has. 220K for a shorter, clearer moan; 1M if you want something that barely resolves as a sound.
- Record something that isn't a groan. A sigh. A single tut. Your own name, said with disappointment. The mechanism doesn't care and the object changes completely.
- Put a pot on the oscillator resistor and bring it out to the panel - 100K pot with 20K in series. I left it off on purpose, because a knob turns a creature back into an appliance, but it's your box.
- Weight it. A slug of something in the bottom. People trust heavy objects, and a heavy object that groans when you lift it is doing more work than a light one.
Credit where it isn't due: the ISD1820 module is a commodity part and the trick of pulling its oscillator resistor about is well-trodden ground among people who bend toys. The box, the arrangement, the 470K, the template and the words are mine. Build it, bend it, make it yours.