The Button
Pointless Electronics PE-001, on stripboard · A button that turns on a light, on four strips · By Onion Madder · Mess O' Pedals
The first Pointless Electronics device, and the smallest layout on this shelf. It is the whole idea in a box: press the button, the light comes on. There is no mode, no second function, no secret long-press. The board is four strips of four holes with one resistor standing on it, and the resistor is only there because the LED would otherwise not last the week.
It takes about half an hour, and most of that is drilling aluminum. There are no track cuts, no wire links, and exactly one thing that can go in backwards: the LED. If you have never built anything on stripboard, start here - this is the build where you learn to cut a board, stand a resistor, and solder a wire to copper without anything important riding on it.
I put the LED above the button rather than beside it, because a momentary switch means the light only exists while your thumb is down, and it would be a shame to spend an afternoon building an object whose one output you then cover with your own hand.
⚠ Before you start tap to expand / collapse
Drill before you solder. Aluminum swarf is conductive, it gets everywhere, and it will find the one place you didn't want it. Do all the metalwork first, then blow the box out, then start on the electronics.
The box is a conductor. The board's copper side is going to be loose inside a metal enclosure. Wrap it, tape it, or glue it down before you close the lid. A rattling board against an aluminum wall is a short waiting to happen.
What you'll need
tap to expand / collapse
- 1× 4k7 resistor, 1/4W (R1) - spot it: four color bands, yellow, violet, red, then gold. A blue body means metal film, a beige body means carbon. Either is fine here
- 1× stripboard, a scrap of it - spot it: copper stripes on one side running the length of the board, plain brown on the other. You need a piece four strips tall and four holes wide
Off the board
- 1× 5mm superbright red LED (D1) - spot it: water-clear body, not red plastic. One leg longer than the other, and a flat spot on the rim beside the short one
- 1× 16mm round momentary pushbutton (SW1) - spot it: two solder lugs, and the cap springs back when you let go. If it clicks and stays down, that is a latching one - wrong part for this build, see Things to try
- 1× 5mm LED bezel - spot it: small black collar with a lip. The LED presses into it from behind and the lip sits proud of the panel
- 1× 9V battery and snap - spot it: red lead is positive, black is negative, and the snap only goes on one way round
- Hookup wire, four short pieces - spot it: anything stranded and thin. About 100mm each. Red and black for the LED pair if you have them, anything for the switch pair
- 1× an aluminum box - spot it: mine is a finned extrusion with screw-on end plates. Anything that takes a 16mm and an 8mm hole will do
The board
Four strips, four holes, no cuts. R1 stands upright between the second and third strips in the third column. Everything else leaving the board is a wire going somewhere on the panel. That is the entire layout, and it is the smallest useful board I have ever drawn.
The layout file is the guide
Same as the rest of my stripboard builds: the file is the document. Import it into Copper Bottom and the editor draws the board, walks the placement, runs the electrical checks and prints a build sheet: ↓ Download PE001_The_Button.json
The netlist
Straight out of the layout file. Every connection on the copper, written as nets; the
@ names are the off-board pads.
Steps 1–8 Metalwork
Pick your face. I used the finned side, which looks like something that belongs in a rack. Check the button body clears whatever is behind it before you commit - these round ones are deeper than they look. There is a 1:1 drill template at the end of the page.
| Step | Do |
|---|---|
| 1 | Mark the button hole and the bezel hole on the face, with the bezel above the button. |
| 2 | Center-punch both marks. |
| 3 | Pilot-drill both marks at 3mm. |
| 4 | Open the button hole to 16mm. Measure your button's thread first - the common round ones are 16mm, but they are not all 16mm. |
| 5 | Open the bezel hole to 8mm. |
| 6 | Deburr both holes. |
| 7 | Blow every last chip of swarf out of the box. |
| 8 | Test-fit the button and the bezel, then take them both back out. |
Steps 9–15 The board
One component, and it has to stand up. At 4k7 the resistor is a 6.3mm body being asked to cross 2.5mm of holes, which it will not do lying down - so it goes upright, both legs out of the same end. Columns are 1 to 4 from the left, strips a to d from the top, so c2 is the third column on the second strip.
| Step | Do |
|---|---|
| 9 | Cut a piece of stripboard four strips tall and four holes wide. |
| 10 | Sand the cut edges smooth and wipe the copper clean. |
| 11 | Lay it copper side down with the strips running left to right. |
| 12 | Make no track cuts. There are none in this build - if you have cut one, you have cut it by mistake. |
| 13 | Bend R1's top lead back down alongside its own body so both legs leave the same end, about 2.5mm apart. |
| 14 | Push R1's two legs into column 3, strips b and c, body standing upright. |
| 15 | Solder both legs and trim them flush. |
Steps 16–21 Wires off the board
Six wires leave the board, and four of them are cut to length now. The other two are the battery snap's own leads. Solder all six at the board end first - it is far easier than reaching into a box later.
| Step | Do |
|---|---|
| 16 | Solder the battery snap's red lead into a1 (+9V). |
| 17 | Solder a wire into a4 (SW_A). This is switch wire A. |
| 18 | Solder a wire into b1 (SW_B). This is switch wire B. |
| 19 | Solder a red wire into c4 (LED_A). This is the LED anode wire. |
| 20 | Solder a black wire into d4 (LED_K). This is the LED cathode wire. |
| 21 | Solder the battery snap's black lead into d1 (GND). |
Steps 22–27 The LED and the button
The LED is the only part in this build that can be wrong, and it is wrong in exactly one way. Long leg is the anode; short leg is beside the flat on the rim.
| Step | Do |
|---|---|
| 22 | Slide a short piece of heat-shrink onto each of the two LED wires. |
| 23 | Solder the red wire from c4 to the LED's long leg. |
| 24 | Solder the black wire from d4 to the LED's short leg. |
| 25 | Slide the heat-shrink over both joints and shrink it. |
| 26 | Solder switch wire A to either lug on the button. |
| 27 | Solder switch wire B to the other lug. The switch has no polarity and does not care which way round. |
Steps 28–35 Test, then close
Test it flat on the bench, before anything is mounted and while everything is still reachable. If it works here it will work in the box.
| Step | Do |
|---|---|
| 28 | Clip the battery onto the snap. |
| 29 | Press the button. The LED lights. |
| 30 | Unclip the battery. |
| 31 | Wrap the board in heat-shrink or tape so no copper can touch the case. |
| 32 | Press the LED into the bezel from behind and fit the bezel into the 8mm hole. |
| 33 | Fit the button through the 16mm hole and tighten its nut from inside. |
| 34 | Clip the battery back on and tuck it and the board into the box. |
| 35 | Screw the end plate down and do not open it again. |
The off-board wiring
Six pads leave the board. Every one of them goes to exactly one thing.
| Pad | Goes to |
|---|---|
| +9V (a1) | Battery snap, red lead. Straight to the switch by way of the strip - strip a is the whole of the positive rail. |
| SW_A (a4) | One lug of the momentary button. Either lug. |
| SW_B (b1) | The other lug. This is the switched positive coming back to the resistor. |
| LED_A (c4) | LED anode, the long leg. This is the bottom of the resistor, so the current is already limited by the time it gets here. |
| LED_K (d4) | LED cathode, the short leg beside the flat. |
| GND (d1) | Battery snap, black lead. Strip d is the whole of the ground rail. |
Boxing it up
Two holes, and neither is fussy: 16mm for the button bushing and 8mm for the LED bezel. Keep both at least 12mm in from any edge, and check what is behind the face before you drill - on a finned extrusion the fins are on the outside, but the internal rails are not always where you expect.
Put the LED above the button. A momentary switch means the light only exists while you are holding the button down, so if the LED sits under your thumb the object cannot do the one thing it does.
And tape the board. It is a 10mm square of bare copper loose in a metal box, and there is nothing in this circuit interesting enough to be worth debugging a short for.
Print at 100% / “Actual size”, never “Fit to page” - check the bar measures exactly 4″ before you touch a drill. And measure your own button and bezel first; not every round button is 16mm.
How long it lasts
This is the only calculation in the project and it took longer than the design did.
A superbright red drops about 2.0V, so the 4k7 sees 7.0V, which is 1.49mA. A 9V alkaline is roughly 550mAh and at that trickle it gives you close to all of it. 550 divided by 1.49 is 369 hours, which is 1,329,000 seconds - and since the switch is momentary, a second of light costs a second of battery.
Approx. 1,300,000 presses, or five years, whichever comes first.
That is the conservative figure, taken at the fresh-battery current. Let it run all the way down to about 5.4V, dimmer the whole way but still clearly lit, and the average current falls to about 1.06mA and you get closer to 1,870,000. The truth is somewhere in between and nobody is going to check.
The five years is the more honest half of the spec. At a hundred presses a day the battery lasts 51 years, which is well past the shelf life of the chemistry - so unless you are pressing it something like a thousand times a day, the alkaline dies of old age long before the button wears it out. The box will outlive its own power supply doing nothing at all.
If it doesn't work
| Symptom | Check |
|---|---|
| The light does not come on. | Press the button. |
| You are pressing the button and the light does not come on. | The LED is in backwards. Swap its two wires. This is the only mistake the build can make. |
| The light stays on when you let go. | You bought a latching switch. Not a fault, but not this project - see below. |
| It worked on the bench and went dead once boxed. | The board is shorting against the case. Take it out, wrap it, put it back. |
| It is lit but very dim. | Check the resistor bands. Yellow-violet-red is 4k7; yellow-violet-orange is 47k and will give you a tenth of the light. |
Things to try
- Change the brightness. R1 sets it and nothing else does. 2k2 roughly doubles the current and halves the life; 10k dims it and buys you twice as long. Recalculate the number on the back when you do.
- Build the latching version. Swap the momentary for a latching switch and the object changes meaning: now it can be left on, sealed inside a box you cannot easily open, quietly eating its own battery in a drawer. Same board, same wiring, no other change.
- Label the panel. One word under the button. I would use BUTTON.
- Engrave the spec on the back. The press count is the funniest part of the object and it is invisible unless you put it somewhere.
- Use a bigger cell. Anything with more mAh multiplies the press count directly - do the division yourself, it is the whole engineering content of the project.
A resistor and an LED across a switch is the first circuit anybody draws and it belongs to nobody. The board, the file, the arithmetic and the words are Onion Madder's. Build it, bend it, make it yours.