USB chargeable flashlight & circuit tester with charge indicator LED

This is another supercap (super capacitor) application. I like supercaps. Although it can hold much smaller charge than rechargeable batteries and a little bit expensive, it can be charged extremely quick (about 30 seconds through USB for 2x 2.5V, 10F), light, and has no memory effect. If the load is not very heavy, you can use it for many applications as a power source.

I used the small blue box that I have introduced for the Wii nunchuck mod and the JeonLab mini v1.0.  The inside of the box is just big enough for 2 supercaps and other parts. What I’d like to make with this box and the supercap is handy, portable, and quick USB chargeable LED flashlight AND a simple circuit tester (beeps when two probe touch a closed circuit like the one of the multi-meter functions). I also wanted to include a charge-complete indicator LEDs.

Not very critical but exciting part was the charge-complete indicator. At first, I thought I could use an NPN transistor, two resistors for a voltage divider to turn the transistor on and off (by the level of charge), and an LED (connected to the  collector and the emitter).  But I couldn’t find proper resistor combination from what I had. I didn’t want to add a variable potentiometer in such a small box. So I searched on internet to find better and simpler voltage indicator and found this post from a forum called ModRetro, written by Daftmike, and found his circuit for low voltage indicator is very similar to my design except for another LED to indicate the power which is connected to the emitter of the transistor and ground. I quickly added an LED and found a good resistor combination (2.2k and 3.3k for my case) with this additional LED. Adding an LED simply solved my problem.  I’d like to thank him for sharing the circuit idea.

Anyway, here is the whole circuit diagram.

USB light n tester

I have added a schottky diode (1N5818) to prevent discharge of the supercaps when they are not charging. There is about 0.5V of voltage drop across the diode, but it is fine for this kind of circuit to light an LED or beep a piezo buzzer. It is not necessary to be exactly 5V.

Here is a picture of parts before assembling.

supercap light & tester parts

Part list:

  • supercap: 2.7V(2.5V is fine as well), 10F x2
  • LEDs:
    • white: high intensity, 3V, 5mm
    • red: 3mm, 10mA, 2V
    • green: 3mm, 10mA, 2.1V
  • transistor: 2N3904
  • schottky diode: 1N5818
  • resistors:
    • 2.2k, 1/4W
    • 3.3k, 1/4W
    • 100, 1/8W x2
  • piezo buzzer: 2.73kHz, 5V, 9.6mm
  • slide switch: small
  • mini-B USB female connector
  • 2-pin socket (any)
  • blue box (Hammond 1551F, ABS)

Here are some pictures showing my tests to see what voltages for each stage.

1. Discharged, unplugged from USB (both red and green LEDs are off)
supercap light prototype_unplug n discharged

2. Charging, plugged (both LEDs are on)
supercap light prototype_charging

3. Charge indicating LED (red) is almost off at 4.6V
supercap light prototype_charging RED off

4. about 30 seconds after plugged in, it is 4.75V (red LED is completely off)
supercap light prototype_full charge

With USB, after it reaches about 4.8V, it takes long time (more than 10-15 minutes) to approach to 5V. Maybe impossible.  I just didn’t have enough patience to wait hours to see if the USB can charge this up to 5V. 😉  But if you plug it in to 5V, 1A wall adapter, it is much faster (less than 5 seconds) to reach this point and even charged to over 5V.

Here is a picture showing the inside of the fully assembled ….. hmm what should I call it… JeonLight maybe? 🙂

IMG_0014 (Large)

As for the circuit tester, you can connect any solid wire (about 20-18 AWG) or long header pins.

Some pictures with lights on.
IMG_0018 (Large)

IMG_0025 (Large)

IMG_0024 (Large)

There are some more detail pictures on my Flickr photo library.

Should you have any questions or comments, please leave a comment below or send me an email.




2 thoughts on “USB chargeable flashlight & circuit tester with charge indicator LED

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