LED Flashlight Again

I wrote about some LED flashlight that I built or purchased many years ago.   Recently the incandescent bulb on my Garrity flashlight burnt out.  I tried to find an LED replacement bulb.  At the price $3.5 a piece, it was disappointing: 0.3W@3V, 30 Lumens.  The LED bulb went from drawing 100mA at 3V to 7mA at 2.6V and is completely off at 2.4V.  That means it cannot be used with rechargeable NiMH batteries and the Li-Ion battery voltage is too high.  I decided to build my own LED replacement bulb (P13.5S type) again.  I want to run on both two NiMH cells or one Li-Ion cell (1.8-4.2V).  LTC3429 used previously can run on a Lithium-Ion battery but as a linear regulator with the efficiency in the 60s%.  

Here is a simpler solution.  We configure the circuit to be always boost.  We can take advantage of the built-in current limit of converters.  In LT1615, the switch current limit is 350mA.  We eliminate the Schottky diode; only an inductor and a capacitor are needed.  It works down to 1V (single alkaline cell).  Because LT1615 operates with a fixed off-time control, the switching frequency changes with the input voltage, from 200-300KHz. The average current through the LED varies from 85-140mA. The efficiency is around 80%.  It has as few components as it can possibly get.  The deficiency is that LT1615 is relatively expensive ($3 a piece at volume quantity) and its current limit is a little too low.

                                

Here is the actual implementation. 

 

Testing Antonki Thermometer/Hygrometer

 Antonki Thermometer/Hygrometer is sold at $8 for two.  It claims a temperature range of -50 to 70°C (-58 to 158°F) with accuracy +/-1°C and relative humidity range 10% to 99% with accuracy of +/-5%.  It is good value if it is indeed accurate.

We use SHT31 and HTU21 breakout boards with Arduino Nano and an OLED display module (0.96" OLED 128x64).  Arduino 3.3V output is used to power the sensors and the display.  Very quickly, we pull together the code needed to read from the sensors and display the data on the OLED module.  It shows the power of Arduino.   We update the reading every 3s.

 For temperature, we also have the analog thermometer, thermocouple, and PRT measured by multimeter, and Fluke non-contact IR thermometer.   The temperatures are measured at four settings, ambient (22°C), refrigerator (3°C), freezer (-18°C), and heater (50°C).

Here are the results.  It is pretty good at the room temperature, likely at which it is calibrated, but the readings deviate greater at the colder temperatures.

Sensor	Freezer	Refrigerator	Ambient	Heater
Antonki	-15.9	5.5	22.2	46.5
HTU21	-17.3	5.8	22.6	46.5
SHT31	-17.1	5.5	22.4	46.6
TC	-21.0	3.0	18.0	46.0
PRT	-17.4	6.0	22.8	48.5
Thermometer	-19.0	3.0	22.0	51.0
Fluke	-16.4	4.2	22.5	46.5

We compare a few humidity readings.  Since HTU21 and SHT 31 disagree by 10%, it is hard to assess the accuracy.

Sensor	Reading 1 (% RH)	Reading 2 (% RH)	Reading 3 (% RH)
Antonki	55.0	31.0	71.0
HTU21	46.5	26.2	59.0
SHT31	56.4	34.6	69.4

 It should also be noted that the response time of Antonki is slow; it takes time to stabilize the readings, especially the humidity.  

 In conclusion, Antonki Thermometer/Hygrometer is usable at normal room temperature range.