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This is a circuit suitable for powering LEDs to visually assess their brightness, colour, and pattern. It is also able to display the forward voltage of the LED at the various currents the circuit provides, using a digital multimeter.
As the output of the die (chip) moves from red to amber / orange to yellow to green to the modern emerald, cyan, blue, violet and UV the forward voltage for a specific current increases. White, better yellows, some reds, pinks, some purples, and rare ones like raspberry and peach all use blue dies and a phosphor. Ditto some plant growth LED. There are also lime and mint LEDs.
Infrared (IR) LEDs, once called IREDs, are used in remote controls, some mice (optical, and encoder wheels inside ball ones), and IR illuminators for cheap security video cameras. They drop about 1.6 volts. Most camera phones lack the filter needed to remove the IR content, so IR LEDs they appear in them, unlike digital SLRs.
Unlike a filament lamp, which in generally supplied with a fixed voltage, LEDs require a limited current. This can be a circuit such as we are using here, or it an be a voltage source with a resistor in series. For standard LEDs, a current of 10 to 20 mA is typical.
For 1 watt or 3 watt "star" LEDs, the current are around 300 mA or 1 amp, although they will light at the currents this circuit provides. There are also intermediate LEDs rated at 50 or 100 mA. Star type LEDs are almost always supplied by an electronic current source.
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| The circuit. Resistors should be 1% or better, if possible. |
I used a standard alkaline PP3 9 volt battery. This should last pretty much its shelf life. A non-alkaline is OK too.
This uses the standard LM317T current source circuit. This is the TO-220 version with a tab. The LM117 or LM317 with no or a different letter is fine too if the mounting is suitable for your preferred building method (this includes surface mount), although the L or LZ suffix is a TO-92 with limited heat dissipation capacity. It supplies the LED via test clips, and places the voltage across it onto the leads which plug into a multimeter via 4 mm "banana" plugs. This indicates the forward voltage.
Note that the LM317 family is officially a variable voltage voltage regulator, a cousin of the LM7805 5 volt regulator. It maintains a voltage of 1.25 between its Output and Adjust pins. Normally a resistors are used to place the ADJ pin 1.25 volts below the desired output voltage, relative to the ground or negative rail. Here we use a resistor to make a small current flow between the Output and Adjust pins. This current, plus the current required to power the IC pass, into the Input, and out the ADJ pin to the LED under test.
I used a SPDT Centre-off switch with one side staying on when selected, the other spring-loaded or momentary; also called ON-OFF-(ON). While you could use other configurations, this one makes it obvious which position is the medium, and which is the high. It also makes it less likely the highest current will be used for an extended period.
Current flows from the battery, into the input of the '317 IC. A small amount runs the IC and flows out the Adjust pin. Most flows out the OUT pin via Rs, and any other resistors in circuit, and along with the IC supply current, into the Devices Under Test, and back into the battery negative. A fraction of a microamp is however diverted into the meter, which should be a digital meter with at least 10 MΩ input impedance. A FET input analogue or VTVM would be OK, but a cheap analogue could divert a milliamp, or close to it. This thus displays the voltage drop of the LED, etc.
Unless you are worried about drain due to plugs touching in a tool box, no power switch is required, as the return path to the negative of the battery is via the LED, and to a far lesser extent, the meter. Note that if left connected to a meter the battery will slowly discharge through it.
I built mine in a small clear plastic project box from Jaycar. Hammond in Canada also has suitable cases: 1591T Series. I mounted the regulator with the pins flat, but on the copper side of the stamp-sized piece of strip-board, also branded Vero board. The countersunk 3mm plastic screw through the TO-220 tab was all the mounting it needed, as there are only a few resistors mounted on it. Various sellers have cases which will hold a PP3 volt battery.
Hammond is great, with the owner coming from Canada each year to support Electronex, the next in Sydney (Rosehill) in June 2026, and likely MCEC in Melbourne in [May] 2027. They have an office in NE Adelaide.
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| Layout on stripboard. The component side is upwards. I did however bend the leads upwards, put them backwards through the boards, and back on themselves, then solder, so the board was clear of the lid it was mounted on. |
Rs sets the standard current, in parallel with any parallel ones to set or subsequently trim the value. They can be mounted vertically or short ones can be mounted horizontally and likely diagonally.
For the toggle switch I mounted the two resistors on the switch, allow additional current options. Mounting here, and joined, meant only 2 wires are needed between it and the PCB.
For the leads to the test clips and to the meter I used some flexible red and black wire recovered from an older appliance power cable, but you can buy a metre each of red and black wire, or perhaps colour-coded figure-8 or "zip" wire. This might be 0.75 or 1.0 mm², or 16 or 14 AWG. You can also use specific multimeter test lead wire.
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| The '317 mounted on the lid via a plastic screw. The copper side (red) is nearest the box lid, shown in pink. Note that the board and panel (lid) continues beyond that shown, and should have at least 10 holes in use. |
If you want to put more resistors on the board, you can cut the strips near the ends. This can be with a twist drill in the hole, or cut either at a hole, or between them, using an art knife, carefully.
Depending on your work-bench layout, etc, run about 30 cm of each wire to 4 mm "banana" plugs, assuming this is what your meter uses. Perhaps use longer for a bench meter on a shelf. For the other end you can use small Alligator clips, or E-Z Hooks / IC clips. Mine has ended up Alligator clip on the negative, and an IC clip on the positive.
The red wires go to the ADJ pin of the regulator, while the black ones are connected only to the battery negative wire. Thus the device only draws power if there is an LED in circuit.
I used a fine file (aka needle file) to make small notches in the side of the box, near the lid, for the wires to pass out, while providing some clamping.
Plug the red banana plug into the + (Positive) or V input to the meter; and the black into the - (Negative), perhaps called COM for Common. In voltage mode you may see around 8 volts on the display. Assuming you are testing a 5mm or 3mm LED, connect to the black clip to the shorter wire, and the red to the longer one. For a normal red LED the voltage should drop to around 1.8 volts. For yellowish-green or yellow it will be 2.2 volts; and for emerald green, blue, violet, or white, around 3 or so volts. Note that there are red phosphor LEDs, driven by blue dies.
If you flick the switch to medium and high current modes, you should see both an increase in brightness of the LED, and small increases in the voltage across it.
Assuming your meter has a milliamp range, it is a good idea to at least check the current output to the LED. With no LED in place, unplug the positive lead at the meter, set the it to an appropriate mA range, and insert the lead again. Flick the switch to see the current on the other settings.
Mine got 9.64 mA, 19.9 mA, an 29.7 mA. This achieves the goal of putting several current levels into a LED, but you may want it to be a closer value.
If the current is too high you will have to replace the resistor with a higher value one, or insert a small series resistance. If it is too low, you could replace it, or place an resistor in parallel with it. The formula to determine the resistor is R = 1.25 / I, noting that 1 milliamp is 0.001 amps. For 1 mA the value would be the nearest available value to 1250 ohms, or 1.25 kΩ.
Should you wish to tune the current ideally you would measure the voltage between the centre and left pins, and substitute that value for 1.25 volts below:
In my case the deficit in current would be addressed thus: R = 1.25 / (0.010-0.00964) = 1.25 / 0.00036 = 3472.22222 ohms. If available, the ideal value is 3.48 kΩ, from specialists, but with a 1 week wait in Oz. Retail options are 3.6 kΩ then 3.3 k&Omega. You make be able to get E192 values as one of the more human scaled surface mount sized (1206 or 805) on the copper side.
One way to obtain a specific resistance is to series connect two resistors, or to parallel connect two (or three).
If you put the desired value in ohms in the A1 and repeat it down, you can put a range or commercial values greater than the desired value, to about twice it into the B column, in ohms. At C2 you can place =1/((1/A2)-(1/B2)), and copy it down. Ditto at D2 =C2/1000, then copy down. This is the value in kilohms, which you can format to a few decimal places for ease of reading.
A better workflow would be to put just the 130 ohm or a similar value in the Rs position, then measure the current, and address the deficit, as above. They you can add the needed values for the extra 10 mA and extra 20 mA for the toggle switch positions.
E3 to E192 resistor series. Vishay E-series chart (PDF) These values are repeated in each decade, so say 5.6 Ω, 56 Ω, 560 Ω, 5600 Ω 5.6 kΩ, (5600 Ω), 56 kΩ 560 kΩ 5.6 MΩ. Some series may include lower and higher values. Likewise, 1.27 Ω, 12.7 Ω, 127 Ω, 1.27 kΩ, and so on, is another example.
Adjusting the standing current up to 10 mA meant it was high, so I inserted a 1.2 ohm 5% retail resistor in series with the 120 ohm, resistor at RL. This fixed the higher current, so I left it as 62 ohms.
A final check is to short the test-clips and to select the millivolt range. The few millivolts is due to resistance in the leads to the LED. This will increase in the two higher current positions. It is usually maybe 10 mV or less. You can add it if the value is critical, or upgrade to 4 wire (below).
Finally, screw the lid on, and play!
Often I use it without a meter, just to check brightness, or colour.
Three red, or two LEDs of other colours, can be used in series. You can also check the VF of regular diodes, and the reverse voltage of low voltage Zener diodes.
If you want to determine the resistor required for a LED, measure its forward voltage, subtract this from the supply voltage, and apply Ohm's Law to this value.
R = Vr / I
If you want to place several LEDs in series, simply add their voltage drops together.
The alternative is to use a current source of this style as the LED driver, or something like a CL220 series driver.
One option would be to provide more ranges. 5 mA and 2.5 mA would be sensible values, needing 250 and 500 ohm resistors. 250 ohm resistors are quite common, despite not being part of an E-series. There are also unusual values.
One way to better calibrate is to include a variable resistor, also called a "trim-pot", short for trimming potentiometer in parallel in each resistor group. These come in single turn and 10. 15, or 20 turn.
Maybe you can try reducing the minimum current. One idea I had was making it 7.5 mA and having a 2.5 mA (500 ohms or 2 × 250 ohms) resistors connected via a normally closed push-button (often black at places like Jaycar) so teh default is 10 mA, press for 7.5 mA.
The On-Off-(On) or "on-off-mon" toggle switch came from a Jaycar, but it it has either been discontinued, or they are not being clear as to the function; there may be a DPDT version. The Salecom T8014B-SEBQE-H switch is available at Altronics as S1340. from Aus Electronics Direct in SW Sydney, RMS Components in Brisbane, and Rapid in Colchester, UK, which is also a Hammond dealer. Schukat in Monheim am Rhein will sell you a packet of 10. Meanwhile Radio Parts in Naarm (Melbourne) has ST2048 MINI TOGGLE SWITCH SPDT-Change Over, MOM-OFF-ON: Code 41372048 at $2.37 inc GST. Element 14 has MulticompPRO part numbers 9473475 (silver) and 9473483 (gold).
Buy any push-buttons from a reliable source. This style relies on finger pressure for a low resistance connection, and the online ones I got were poor quality. Try Element 14, etc. I wonder of the ones with a small rectangular body have a snap action, like many toggle switches. An example is at Radio Parts: Code: 41162032
Radio Parts, Jaycar, and Altronics have E24 1% resistors. Ditto Element 14, etc, along with closer tolerance items. The LM317T is very common, sold at all of these too.
Strip board / Vero board can be scored with an art knife on both sides, then snapped, or cut with a fine saw.
If you increase the supply voltage you can test more LEDs in series. Two 9 volt batteries could be used; or a 12 volt A23 battery, which can be fitted to an N-cell holder, although its output may drop at high currents. You could also use a surplus or other wall-wart power supply, or a variable lab supply, adjusting it according to the expected forward drop of the LED string. Remember that LEDs can be damaged by excessive reverse voltages. A series string of AAA, or the tiny AAAA cells is also possible. (Some PP3 9 volt batteries consist of 6 AAAAs, others a stack of rectangular cells, allowing 7 Ni-Cd or Ni-MH cells for 8.4 volts, rather than the 7.2 which 6 provides.
I mentioned the small voltage reading upon shorting the test leads. While you can subtract it (you could even calculate it out in the spreadsheet) something similar to a Kelvin connection can be used. In this case, current from the source travels down two wires to the test clips. Two more wires, carrying almost zero current, then feed the voltage to the high impedance multimeter. Thus there is almost no voltage drip from the probes to the meter. The voltage drop from the source to the LED does not matter. Thin coax, or unbalanced / shielded audio lead could be used, the current in the insulated shield, the meter connected via the inner, and these joined in each clip. RG-316/U is ideal, RG-174/U OK too. RG-179/U can be removed from an old VGA cable, and is likely colour-code. Twin (figure-8) wire would also work, one pair for each test clip. This is probably only worthwhile if you added something like a 100 mA range.
This arrangement also allows low-ohms measurement, which do not measure accuratly on the 200 ohm range, the lowest on many meters. 1 ohm at 100 mA would put 100 mV into the meter.
Open Office, OfficeLibre, Excel, or Google Sheets can all be used to plot the voltage for each current, and colour-code it for different LEDs.
You can use this to test the forward voltage of other diodes junctions at low currents. Likewise the Zener voltage of the lowest voltage Zener diodes can be read, but be careful not to pass excessive current if you use a higher supply voltage.
You can find links to lots more on the Learning Material page.
Written by Julian Sortland, VK2YJS & AG6LE, March 2026.
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