My goal is to build a box to test LEDs instantly. It will be self-contained, battery powered, have a display, and will work with high voltages, low voltages, and variable currents such that it can power up to a ~1W led from ~1V to 150V. I want to be able to use it for small indicators, flashlight lamps, and series strings. It should not blow up an LED under test with normal use. It also must be made completely of cheap, easy to source parts. I will log my progress here.
How it will work
There will multiple sections
- Battery charging and power supply
- Logic and display
- Low voltage sense
- High voltage sense
Battery and power
A single Li-Ion cell will be managed by a charging and protection board. It will then be stepped up to 5V for the logic section and then 12V for the low voltage rail. The 12V will be boosted to 150V using a discrete board to do the high voltage test.
Logic and display
A small microcontroller board will handle the testing logic, the high voltage shut off, and the display. It should be small, cheap, low power and doesn’t need any unnecessary components like WIFI. This rules out ESP32 based boards.
Low voltage sense
I think the easiest and most bulletproof way to do this is via an LM317T in adjustable constant current mode. It can handle a decent amount of power with a heatsink which allows testing up to ~100mA, giving us around a watt of power if needed. The max current will be controlled with either a front pot or a selector switch. It will use a discrete sensor board for simplicity.
High voltage sense
The high voltage is a little more complicated. We want to prevent it from blowing up any LEDs under test but it has to be as automatic as possible. We will use nixie tube voltage booster combined with a two transistor current source to limit current and use the MCU’s ADC for sensing. Preventing damage to the LED will be done in firmware by restricting to very low current until it reads a forward voltage and then turning on a mosfet to either switch to higher current or display an message to switch to the low voltage tester.
Parts
The parts will consist of discrete modules and jellybean components. The modules were chosen for price, availability, re-usability and function weighted equally. Re-usability is key because the parts need to be ordered in some quantity for them to be cost effective. The jellybeans were chosen by what I have on hand.
Modules
Modules are listed with links to amazon page.
- STM8S003F3P6 MCU ($3.50) + STLink V2 programmer ($6)
- I2C LCD ($5)
- TP4056 Li-Ion charging board ($0.50)
- TPS63802 5V Buck-Boost ($2)
- MT3608 12V Boost ($1)
- 150V HV Boost ($11)
- INA219 Current sense ($3.50)
Jellybeans are usually available in kits and don’t really matter much as far as quality so ali-express deals should be fine.
- LM317T linear voltage regulator
- Power mosfet
- Diodes
- Resistors
- Capacitors
- SPST Switch
- Battery holder
- Wire
- Perfboard
- Multiturn pot
- Other transistors/mosfets
Besides these we will need a 5 position selector switch which can be salvaged from an old audio receiver or some other kind of selector box, or you can buy one on amazon for a few bucks.
Other
Ideas and critiques welcome. I will post more as the project proceeds with both wins and fails, depending on how it goes.