Universal Bulb/Dimmer Test Tool

A German version of this Instructable is available for download in PDF format. To do so, click the link in Step 9. - Eine deutsche Version dieses Instructables ist zum Download verfügbar, Link in 'Step 9'.

As Shakespeare said 'Though this be madness, yet there is method in’t...'

First of all, please let me explain why I had the idea of building such a crazy, ugly gadget.

One evening per week I assist in the repair workshop of a community center in Zürich. It's similar to a Repair Café, except that we don't serve coffee. From time to time we get - among other things of every possible and impossible flavour - luminaires with symptom descriptions such as 'no light', 'flickers', 'flashes', or 'slack joint'.

Since a few years now, almost all incandescent bulbs are banned from the market, and there are LED replacement bulbs available now with massively increased efficiency (which is good, of course). In my opinion, good LED replacement bulbs are also better than compact fluorescent lamps (CFL), since LED bulbs have their full brightness right away, compared to the slow-starting CFLs, and they do not contain mercury. But CFLs seem to have disappeared from the shops as well, anyway.

It happens, however, that someone replaces an incandescent bulb by an LED bulb that is mechanically, but not electrically, compatible. Some LED bulbs can be dimmed (more or less nicely), and some of them cannot - pictograms on the packing and on the bulbs themselves (see above) inform about that, but sometimes are overlooked or ignored by an innocent customer. Some dimmers do cooperate with dimmable LED bulbs, but alas, some of them don't. In addition, customers often are confused by the fact that the power consumption (in W/Watt) of an LED bulb is much lower than the one of an earlier, incandescent bulb at the same brightness. For the regular end user, the Lumen (lm) specification indicating the brightness is too new in order to be memorized and cannot, therefor, be compared easily. In the end, we not only try repairing luminaires but also do some consulting: What bulb to use for the specific application, and where it can be bought for a reasonable price.

In order to know more than the customer about these facts I often compared two different makes of bulbs or tested a dimmer. To do so, I had to cobble together a more or less electrically safe (in clear: a more or less dangerous) contraption - a procedure that was, in the long run, time-consuming and unsatisfying.

This was reason enough for me to design my Universal Bulb/Dimmer Test Tool. The goal was to use it for testing any 230 V AC bulb, as well as comparing the brightness of two similar (or perhaps not so similar) bulbs of (more or less) identical dimensions and socket type. Therefor, there is a pair of every type of bulb socket, like in Noah's ark. In addition, the tool allows checking whether the bulb cooperates with the built-in standard light dimmer. An external dimmer may be connected as well, in order to find out whether that dimmer is compatible with the different makes of bulbs. A (Swiss) standard 230 V socket is provided for checking a complete luminaire instead of only a single bulb.

As always, I tried to re-use as much material as possible from my junk box; there might be some prettier sockets available in the shops, but I only need them functioning ok.

It's understood that a similar gadget can be made for use in 115 V countries. If so, all the components need to be specified for 115 V operation. I'm sure that most, if not all, lamp sockets are somewhat different from the ones here in Europe, but I'm afraid I don't know exactly in what they differ.

In a second step, 12 V AC/DC and 24 V DC sections are planned. They will be used for testing/comparing, e.g., the smaller, low-voltage halogen and LED bulbs such as the ones used for cable light systems, or for bulbs intended for use in vehicles. The wiring for these sections is not implemented yet.

Disclaimer:

Since this project deals with a mains voltage of 230 V (or 115 V, depending on your country), you absolutely need to know what you're doing. This is NOT a beginner's project, neither while constructing nor while using it. Some of the lamp sockets are not foolproof, allowing an inadvertent finger to be inserted into themselves, causing an electrical shock that is extremely unpleasant at best, but can kill you in the worst case! Sometimes, learning by doing might be too slow.

Apart from that, it is understood that not every fault in every luminaire can be found with this tool.

You need a box with sufficient space for everything you wish to install on its top and inside. Such a box is preferably made from an insulating material, such as wood (plywood, chipboard, MDF, even solid wood, you name it) or plastic. A metal box can be thought of, too, but it is more difficult to be machined, and it is mandatory to connect it to the PE (protective earth) terminal of the mains inlet for safety reasons.

I had a box made from 10 mm thick MDF (medium density fiberboard) hanging around that I had made for a different, earlier project that didn't work out. It was a bit too large for the current project, so I cut it down and re-glued it.

The 230 V AC section

1 x mains inlet (e.g. IEC-60320 C14)

1 x fuse holder with a T2A fuse (2 Ampère, slow blow)

1 x 230 V dimmer module (e.g. this one: https://www.conrad.ch/de/p/tru-components-drehzahl-und-leistungsregler-230-v-ac-15-a-200-w-1-st-1570778.html); the potentiometer has to be ordered extra

1 x potentiometer 470 kΩ lin. (or B-curve) (with plastic axle and rotary knob

1 x 5-pole DIN socket of the 240° variety (these seem to be specified for 250 V max., as opposed to the 180° versions). Of course you can use any other, compact 5-pole socket you have available.

3 x 5-pole DIN plugs of the 240° variety with insulated, plastic handle. Of course you can use any other, compact 5-pole plug you have available.

2 x E27 sockets

2 x E14 sockets

2 x R7s, 78 mm sockets (for 230 V/80 W halogen/LED line bulbs)

2 x R7s, 118 mm sockets (for 230 V halogen/LED line bulbs of 160+ W)

2 x G9 sockets (for 230 V plug-in halogen/LED bulbs)

2 x GU10/GUZ10 sockets (for 230 V plug-in halogen/LED bulbs)

1 x B15d bayonet socket (for incandescent bulbs used in e.g. sewing machines or ovens)

1 x 3-pole mains socket (in my case: a Swiss standard 'Type 13' socket)

1 x three-pole screw terminal

2 x 4 mm banana sockets (for current measurement in the neutral path)

1 x jumper for the above sockets (or a short jumper cable with 2 banana plugs)

1 x resistor (shunt) 1 Ω/5 W

Some screw terminal strips or other means for wiring/connecting the different sockets

Some insulated wire (solid and stranded) for the internal wiring

Some mounting hardware (screws/washers/nuts M3 and M4)

Some aluminium sheet (if using a wooden or MDF box)

The 12 V AC/DC section (optional - more about that later):

1 x transformer 230 V/12 V, 2.5 A (30 W)

1 x power supply unit (aka power brick), 12 VDC, 6 A (72 W)

1 x DPDT toggle switch (for AC/DC switchover)

2 x universal socket for different halogen bi-pin bulbs up to 30 W: G4, GX5.3, G6.35, MR11, MR16

2 x bayonet socket for BA15s car bulb (reverse light, indicator etc., 21 W)

2 x bayonet socket for BA15d car bulb (taillight 5 W, brake light 21 W)

1 x SPST toggle switch (for brake light in BA15d socket)

2 x bayonet socket for BA9s car bulb (5 W)

2 x plug-in socket for wedge base car bulb W5W (5 W)

2 x 4 mm banana sockets (for current measurement in the negative path)

1 x resistor (shunt) 0.1 Ω/3 W

Some screw terminals or other means for connecting the different sockets

Some insulated wire (solid and stranded) for the internal wiring

Some mounting hardware (screws/washers/nuts M3 and M4)

The 24 V DC section (extremely optional):

1 x wall wart 24 V, 290 mA (7 W) (Ikea exclusive, see step 4)

1 x socket GUZ5.3 (Ikea exclusive, see step 4)

Tools:

Power and/or table drill with different drill bits (some of them rather large)

Jigsaw

Rasp and/or file

Flat-nosed pliers

Wire cutter

Wire stripper

Phillips and flat-blade screwdrivers, different sizes

Label printer if available

Wood glue

Two-component epoxy adhesive

Hot glue gun

Multimeter with continuity check function (or a simple continuity tester)

(Very) dark, anti-dazzle glasses for comparing the brightness of bulbs (such as welding goggles)

I started this project by checking what bulb sockets I already had available in my junk box. I wanted to have some more, so these were purchased. Once I had everything together I started laying them out on top of my box. The distances between the different sockets, in some cases, depend on the dimensions of the bulbs rather than on the dimensions of the sockets! The two GU10 sockets from my first sketch, e.g., are much too close, one of them had to be shifted later - as shown in the pictures in step 7.

As already mentioned, the box was a bit large for the purpose, so I started my drawing software and sketched a layout that I thought might work out (picture above), then cut and (re-) glued the box accordingly.

Of course, in the following days I got one or three more ideas what might or should or had to be added, so my first sketch was updated several times. Since the box was already reduced in size it became, then, somewhat cramped. But I want to use it as a tool and not enter it in a beauty contest  :-)

Anyway, in hindsight I could have used less space for the 12 V AC/DC sockets in the right-hand side. Maybe next time. These, as well as one rather rare 24 V DC socket from an Ikea table lamp, are installed but not wired yet in the current project phase - the 230 V section has priority for me right now.

As is sensible for any electrical/electronical project, I designed a circuit diagram, see above. There are two versions - a simpler one, consisting of the 230 V section only, and a second one where the optional 12 V and 24 V sections are added in red colour.

The 230 V AC section mainly consists of 13 different bulb sockets, each of them except one provided twice for easy comparison of various makes of bulbs. These sockets are all connected in parallel. The mains supply connecs via an IEC cable to a wall socket.

The live wire goes from the mains inlet directly to a fuse holder, and from there to pin 1 of the 5-pole, 240° DIN socket. DIN connectors were used in European audio and video equipment when I was a teenager, and I think that Bang & Olufsen still uses them; the 5-pole 180° version is still used for MIDI links between different musical instruments, and the first PCs also had them for connecting the keyboard. In some data sheet I read that the 240° version is specified for 230 V operation, which I somewhat sceptically accept. I use it nevertheless because it is a cheap and, if I may say so, a rather elegant solution for the task at hand. It spares me the use of several switches and the required additional wiring (bad & good sockets and the matching plug shown in the 3rd and 4th picture).

The circuit diagram shows that also an internal light dimmer module is connected to this socket. There are 3 different dummy plugs provided. One of them has a wire bridge installed from pin 1 to pin 5; this plainly bypasses the internal dimmer module. The second has two wire bridges installed (one between pins 1 and 2, the second between pins 4 and 5); it connects the internal dimmer module to the bulb sockets; and the third one features a tree-pole screw terminal that allows connecting an external light dimmer, bypassing the internal dimmer module at the same time. Note that the pin numbers (they're quite small - you really have to look for them) are molded in the plug's and socket's bodies; they must correspond to the numbers given in the circuit diagram. The 5th picture above shows the pin assignment of the plug when viewing it from the side with the solder lugs).

The 'hot' wires going to the lamp sockets are connected to pin 5 of the DIN socket, refer to the diagram. The 'cold' wires of the lamp sockets, i.e. the return path, are connected via a 1 Ω/5 W shunt resistor to the neutral pin of the mains inlet. Two 4 mm banana sockets allow measuring the current through the bulb(s) under test; there you can connect a multimeter in AC voltage mode; it reads one Volt per Ampère of current (1 V/A).

The (optional) 12 V AC/DC section features both a 12 V transformer (for AC/alternating current) and a 12 V power supply unit (for DC/direct current). selectable with a DPDT toggle switch. Some LED bulbs are specified for only AC, some for only DC, and some for both AC and DC - and there are even ones the packing of which doesn't mention that at all. I want to verify if that really is correct, and if not, what is. In addition, there are BA15d car bulbs available equipped with two tungsten filaments used for both taillight (5 W) and brake light (21 W) within the same glass bulb. Normally only the 5 W filament is lit, and the 21 W filament can be added using a SPST switch. In the return path from the sockets there is a 0.1 Ω/3 W shunt resistor. Two 4 mm banana sockets allow measuring the current through the bulb(s) under test; there you can connect a multimeter in DC voltage mode; it reads 0.1 Volt per Ampère of current (0.1 V/A).

The (still more optional) 24 V DC section isn't strictly necessary, mainly because there exist very few LED bulbs for 24 V, and matching the crazy GUZ5.3 socket, at that. The only one I stumbled on is sold by Ikea, exclusively in their own lamps - and it is next to impossible to get a replacement if one of these should fail. I ranted about the impossibility to get a fitting replacement bulb, neither from Ikea nor any other source, in my earlier instructable "https://www.instructables.com/Saving-an-Ikea-Solbo-Lamp/". Since they had no spare bulb available, Ikea even recommended me to simply buy the same, complete lamp again, literally! After replacing socket and power supply by a standard socket and a 12 V DC wall wart, in order to install a standard 12 V LED bulb, the original socket and power supply were hanging around in my junk box, and in my tester here I found a more or less sensible, and perhaps the only, use for them. Unfortunately, the original bulb was already trashed when I got the lamp for repair. I totally forgot to find out where the + and the – poles were in the original configuration, so I don't know where they should be. But time will tell, hopefully. You best forget about that part  :-)

There is no picture of the drilled, unpopulated box cover, I apologise for that.

When printing the sketch of the box (see picture in Step 1) in original size (select e.g. 'page scaling: none' from the print menu), it can be used as a jig for marking the centers of the holes. Simply fix it to the top panel and punch through the paper with an awl.

Drilling the large holes for e.g. the E27 sockets requires a 40 mm dia. drill. Do do so, I recommend a large drill press at low speed and corresponding auger bits. If not available, you might fall back on a jigsaw and a rasp/file.

Once the box was drilled, I installed all the sockets and the other components. The sockets, depending on the type, are inserted in their own holes, fixed with either M3/M4 or matching SPAX screws, or even fixed from the rear, simply using some hot glue. Most of the screws are of the vintage, slotted-head flavour - I used, a little reluctantly, what I had available.

Due to concept changes after drilling, some of the holes became redundant, and some even had to be filled up using two-component epoxy - which is, I admit, rather ugly but does not impede the function.

For mounting the fuse holder, the internal dimmer's potentiometer, the DIN socket and two toggle switches, the MDF was too thick, so I used two pieces of scrap aluminium as mounting base for these. 

First I tried a very cheap dimmer module from China (1st picture), the sibling of which operates nicely reducing the power (and, most of all, the noise) of my workshop vacuum cleaner; however its output cannot be completely turned down to zero, nothing, nada - which is no problem with the vaccum cleaner, but troublesome with light bulbs. So after a first test I replaced it by a dimmer module hardly more expensive (2nd picture) available from Conrad (see link in the 'Supplies' paragraph); this module only needs the addition of a potentiometer; however I recommend installing it on some small heat sink. I'm already using this module successfully in my DIY sewing machine 'gas pedal'. Its data sheet is attached below in PDF format. If, btw, you should be interested in this pedal, please let me know in the comment section at the end, I might provide a future instructable on this subject.

This doesn't mean, of course, that you cannot use different dimmer module, provided that it cooperates with LED bulbs.

The dummy plugs (3rd picture) don't need much explaining - two of them are fitted with wire bridges, the third with three wires and the screw terminal; the pin numbers correspond to the ones in the circuit diagram. However, I strongly recommend using DIN plugs with insulating plastic handles in order to reduce the risk of electric shock. I linked them mechanically using two lengths of stranded wire soldered to the internal screen (no electrical connection), simply for keeping them together.

This is really not rocket science but rather a puzzle instead that will be testing your patience. Just follow the circuit diagram and connect your lot of sockets in parallel, using, e.g., some insulated, stranded and/or solid wire and several screw terminal strips.

This process is tedious and time-consuming, and since I was in a sort of hurry it resulted in a very ugly, haywire setup that I feel ashamed about to show you. Kindly allow me to drape the cloak of silence over it.

As you can see from some of the pictures, I labeled all sockets and all the other elements (using my venerable Brother P-touch label printer), including the dummy plugs and the 3-pole screw terminal for the external dimmer. There, the label might look like "V" and "A", but this signifies arrows instead, indicating input to and output from the external dimmer unit. You will find the same V and A "arrows" in the circuit diagram.

By the way, my label printer has a keyboard layout in English language, and the user manual for it I downloaded from the internet was in German. The keyboard layout is not the same in these two languages. In particular the layout of most of the special characters is completely different, there even are different special characters available in these two versions. Therefor I first had to edit the user manual accordingly, and then, somewhat reluctantly, substitute the 'up' and 'down' arrows not available in my version by the 'A' and 'V' characters.

Update 2024-11:

I later realized that the two wires to and from the external dimmer unit may be interchanged without any negative effect since we're dealing with AC (alternating current) here. But if your external dimmer contains a switch, the dimmer and the connected bulb is electrically isolated from the mains upon switching off only if the input and output wires are connected as indicated above.

What I tell you here will apply for the 12 V AC/DC and the 24 V DC sections as well, once they are implemented. There are some minor exceptions because, e.g., no dimmer is available there. In addition, these two sections do not present any electric shock hazards to the user.

After a thorough double (even better: triple) visual check of all the wiring, you can check all points that should be connected according the diagram for continuity. Only if these checks are ok, you can plug the mains cable to the mains inlet and a wall outlet. Insert the "Bypass" dummy plug into the dimmer socket, connect the "1 V/A" test sockets with a matching jumper or a jumper cable, and insert a light bulb (either one of the old incandescent bulbs, or a dimmable LED bulb) into one of the matching sockets. It should light up. If not, check whether there is a T2A fuse (2 Ampère, slow-blow) inserted in the fuse holder. If still no light, check the bulb in a different luminaire. If still no light, unplug the mains cable and check your wiring again. Repeat until the bulb lights up  :-)

As a matter of fact, in order to thoroughly check every socket you need to insert bulbs known as being ok into each socket, one after the other.

Once everything is ok, remove the bulb used for checking, connect a luminaire known as being ok to the mains socket, and switch it on. It should light up as well.

Disconnect the luminaire and re-insert a dimmable bulb in its matching socket. Replace the "Bypass" dummy plug by the "Dimmer Int." dummy plug, and check whether the bulb can be dimmed using the dimmer rotary control. If not, unplug the mains cable and check the dimmer wiring again. The two pictures above show a dimmable LED bulb (left) next to a standard 100 W incandescent bulb, almost fully dimmed first, then 'full throttle' - you can see that not every make of LED bulb can be dimmed over the full range you're used to from incandescent times...

If this is ok as well, disconnect the mains cable, connect an external dimmer unit known as being ok to the "Dimmer Ext." dummy plug. The mains input to the dimmer is connected to the yellow "V" terminal, the lamp output of the dimmer is connected to the red "A" terminal. The blue "neutral" terminal normally needs not to be connected since standard dimmers only need an input and an output line (also refer to the 'Update' paragraph in the previous step). Plug the "Dimmer Ext." dummy plug into the dimmer socket. Now you can reconnect the mains cable. It should be possible now to dim the bulb by the external dimmer unit.

Disconnect the mains cable. Reconnect the "Bypass" dummy plug. Remove the jumper (or the jumper cable) from the 1 V/A banana sockets. Set your multimeter to one of the higher AC voltage ranges and connect it to the two test sockets. Connect the mains cable. You can reduce the voltage range once the bulb is switched on in order to get a more accurate reading. The meter reads a voltage that is proportional to the bulb's current consumption. Since we're dealing with alternating current here, no polarity needs to be considered.

Examples:

1. A 300 W linear halogen bulb inserted in the R7s, 118 mm socket will draw a current of about (300 W / 230 V = 1.3 A), resulting in a meter reading of 1.3 V AC.

Caution: Halogen bulbs, even while cold, MUST NEVER be touched with bare hands - always use some clean, soft, paper tissue. Be aware that during operation they get VERY HOT rather quickly!

1. A 7.5 W LED bulb inserted in the E27 socket should draw a current of about (7.5 W / 230 V = 0.032 A), resulting in a meter reading of 0.032 V AC = 32 mV AC.

Please note: In order to prevent damaging your multimeter, always start with the highest voltage range available. When switching on incandescent bulbs in particular they draw an inrush current that is a multiple of the operating current!

One more note: Depending on the design of your multimeter, the AC current through a dimmed bulb - regardless of whether using the internal or an external dimmer - may not be measured correctly, since standard multimeters are calibrated for sinusoidal AC voltages/currents only.

As a matter of fact, the operation is easy enough and kind of described in the "Testing" step above. I consider it unnecessary to repeat all this here.

In normal operation no more than one (or two, for comparison) sockets are populated. It makes no sense to install more than two bulbs at a time. If doing so anyway, it might happen that either the fuse blows, or that the shunt resistor used for current measurement gets overloaded and burns out.

As you can see in the video clip from the link given below, finding out whether an LED bulb is fit for dimming or not is easy  :-)

Famous last words:

Regarding the 230 V AC (or 115 V AC) section: Make sure that neither you nor anybody else puts a finger into any of the different bulb sockets while the tester is powered up. In each of the sockets there is live mains voltage present that can kill you! 

Therefor I strongly recommend using this contraption only together with an RCCB (residual-current circuit breaker) in order to improve the operational safety.

EYE PROTECTION: Staring into a brightly illuminated bulb can damage your eyesight. When comparing the brightness of different lamps, I strongly recommend using eye protection. I tried very dark tinted glasses for use when welding, but they are somewhat greenishly coloured, and therefor might negatively impact the result of the comparison. I also once tried glasses used for watching solar eclipses but had to realize that they are too dark to be helpful.