Having like many of us a few Led diodes of all kinds and sources, I set out to build an instrument that would allow me to better identify them.
My most faithful review Elektor has proposed in its issue of July 2010 ( n ° 385/386 page 107 ) a very interesting little article on this subject.
The diagram uses two operational amplifiers as well as two conventional NPN transistors, each of the two groups thus designed forming two voltage-current converters.
Why will you tell me?
Simply because on the one hand the PDO used here is in a double box, and it can also be useful sometimes to compare 2 LEDs while supplying them the same current, this of course to better visualize their brightness and to compare them.
The circuit here uses an operational amplifier in a follower assembly, its gain is then unity. The output of the AOP directly drives the base of the NPN transistor, its emitter serving as regulation on the inverting input of the LM324.
Over the entire adjustment range with P1, the connected Led will be supplied with almost constant voltage ( except at the bottom of the scale, see "strap" paragraph ), only the current supplied by the transistor will vary. The circuit is a simple Voltage / Current converter, also called a current sink.
These formed by an AOP and their associated transistor are controlled in voltage simultaneously thanks to a simple potentiometer, and it is of course possible to connect a digital multimeter ( or even analog ...) in order to measure the current delivered to the LEDs by the assembly.
To do this, it will be necessary to place the Led to be identified in the support provided for this purpose, and to connect the Voltmeter in the DC position on the 2 sockets of 4 mm.
This measurement is rather practical since the scale is 100 mV / mA crossing the Led.
I slightly modified some component values, in particular to use those I had in my drawers, here is the list:
- IC1 = LM324N
- T1 / T2 = BC548B
- D3 = BZX85C / 5.6V
- P1 = 100KΩ log. ( easier adjustment at the bottom of the scale ... )
- D4 = strap
Zener diodes D1 and D2 guarantee that the voltage on the 2 LEDs to be analyzed will never exceed 4.7V, which excludes their deterioration in the event of reverse polarity.
With the transistors, the two AOPs form voltage-controlled current sources using the potentiometer P1. The current is measured through the 100Ω resistor R1 (1%) thus allowing an almost direct reading by simply applying Ohm's law (U = RxI). Thus for a reading of a voltage of 2V we obtain a current of 20 mA.
Due to its design, the assembly allows the "hot" connection and disconnection of the LEDs to be inventoried without risk of destruction whatsoever.
About the strap ...
On my copy, I voluntarily replaced the diode D4 with a strap because at the very start of the scale ( at least potentiometer ) the transistor remained on. The diode indeed introduces an offset of 0.7V ( junction voltage ) on the positive input of the AOP, the latter then driving the base of the transistor with a voltage of + 1.44V ( offset voltage + voltage V BE). In fact the weak conduction of the transistor always lets appear a voltage of + 700mV at the terminals of the resistor R1, the Led remaining supplied with a current of 7mA. By strapping diode D4, the offset voltage becomes zero and the current flowing through resistor R1 also. The Led is now completely off. This minor modification does not change the overall operation of the machine, the performance remains strictly the same. Only the behavior at the very beginning of the range is thus modified. You are free to keep the original diagram or to make the modification.
The operating range therefore extends according to this last condition from + 550mV to + 6.39V at the output of the AOP, hence a zero voltage on the emitter of the transistor at the minimum setting since the latter is blocked. By deducting the voltage V BE of the transistor ( 0.7V in the present case ) we obtain a voltage of 5.65V (max) on the emitter. This corresponds to a current in the Led between 0 and 57 mA (max ).
About the simulation ...
I was able to observe under ISIS v7.9 voltages a few times much higher than those I actually measured. If the operation of the model works well in simulation, the fact remains that the voltage values must be taken with caution. The strap mentioned above appears here as a switch mounted in parallel on D4, which makes it possible to observe the behavior of the system in both situations.
Realization and boxing
This step does not pose any particular problem, the longest undoubtedly being the wiring on a test tablet ...
I did not draw a printed circuit for this small device, having chosen to use a prototyping board Elektor Labs ( but any type of test board will be suitable ... ), all mounted in a Multicomp MCRH3135 box, with a silkscreen printed on 3M adhesive aluminum foil bought a few years ago from Selectronic, a company which has unfortunately disappeared today. So far I have not been able to find at 3M the reference of this really nice product ...
A support is used for the LM324 but is not really necessary, it's just in case for a possible maintenance, I don't really like soldering "permanent" integrated circuits ( except of course the SMDs ) ...
The machine is supplied simply with a DC12V adapter through a DC-DC Step- converter. Down equipped with an LM2596Sthat I have already described in one of my pages and adjusted here to + 9V, but you can very well use instead a 9V battery like the author of this project, the total consumption being only about 80mA. There is no M / A switch, not really useful for my use.
The support receiving the LEDs to be studied comes from a simple female snap-off strip ( usually used to connect 2x16 LCD displays ) that I fixed with cyanoacrylate glue ( super-glue, what ... ).
Copy of the article by Herbert Musser published in Elektor of July 2010 ( n ° 385/386 page 107 )
with also the drawing of the front face that I created with Front Designer, the datasheet of the Multicomp MCRH3135 box and the file of simulation drawn under ISIS