I proposes the construction of a leds and diode tester, simple and inexpensive using a USB port for power.
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Step 1: Required Components:
copper strips board
red or green led
resistors (normal or SMD)
diode (normal or SMD)
Step 2: Scheme
LEDs have a operating voltage minor than 4 V, and the maximum reverse voltage is about 5V, so we can spoil them if you connect them to one higher voltage. Therefore, we can use a USB port to feed a tester of leds, which will also serve for normal diodes and continuity tester.
USB ports can have an output voltage, especially with small loads, of more than 5 V. In order to avoid damaging components I have added a silicon diode ( the most current) in series with the circuit. As these diodes have a fall of voltage of 0'7 V, on load, the voltage applied to the led to test is less of 5 V, but enough for the correct operation.
Step 3: Use
The positive voltage, with diode D1 in series and through resistance R1, feeds a led D2 that will lights when the circuit is onn. In addition, the voltage is applied to test terminals with a R2 in series. The value of these resistors is flexible, depends on what we want to make leds light, without going over the 20 mA to avoid damage of low power leds. R1 depend on used led; for one red or green can be between 220 to 820 ohms; An R2 of 330 ohms will be about 15 mA in short-circuit, 10 mA with a red led and 6 mA with white one, sufficient for testing. If we connect the led terminals "upside down" it not lights. The test tells us both, the polarity and correct operation. If the led does not lights in any of the positions it is damaged or is not visible light.
To test diodes (or infrared led) connect them in parallel with the fix led; as they have a voltage lower than the fix led, this will turn off when the diode is connected well polarized and will remain lighting if the diode is connected upside down. If the led remains lighting in the two positions, the diode is defective. In addition, we can use these terminals to test continuity; If the led turns off there is direct contact, if not, the circuit is interrupted.
Step 4: Assembly
In the fix USB ports, contacts are at the top, so the connector must have tracks up. One solution is to use SMD components as shown in the last photo. I used a plug by removing one female. In this case the test is made by touching the tracks with the terminals (with error, the tracks + and - are marked inverted).
On the final circuit, I've pasted two 4 strips of copper boards by the smooth part. I use copper strips as connector. Measurements should be 12 mm wide x 1 mm thick (the board I've used is 0'5 mm thick). Cuts for resistors are indicated in the scheme and to isolate the two central contacts from the USB port, the X "filled" are cuts in both sides and the discontinuous X only in low side. Extreme connector contacts are the positive and negative of the port if we need them.
The first photo shows the final assembly with D1, R1 and R2 SMD and a fix led normal, and in this case you must solder bridges between the two sides. On the second the bottom side. Led or diode to be tested are entered into a dual connector, which allows you to connect wires to measure voltage or test components without dismantling. Tracks that stand out from the connector may be used to test SMDs by placing them over the tracks.
Mounting with discrete components, connectors will be weld to the tracks up and down (third photo).
Step 5: Label and "box"
It is important to identify the contacts by placing a tag on the connector. Here the final appearance, I have covered part of the circuit with retractable sheath for aesthetics without making a box.
After, the test of an SMD led and the working voltage. Contacts + and - are used to feed the digital voltmeter. If you test a SMD diode, interconnect the led contacts and place the diode between the two central tracks. For convenience in the photos I have used an USB cable male-female. Your installation may be an adaptation based on available materials.