It’s been awhile since I blogged this, so I may be saying what I’ve already said before. But I get comments from others about how to make measurements of the LED brightness. A recent one is in the comments of an older blog. Many Joule Thief builders don’t have the minimum tools needed for some electronic projects. But then Alessandro Volta, Michael Faraday, Georg Ohm and other pioneers of electricity did not have the tools and equipment we have today. They had to invent equipment to make measurements. It is surprising how a few simple things can be used to make measurements.
It is possible to make a good measurement of the light output of a LED using a grease spot on a piece of paper. More about this grease spot comparison can be found here and in other websites, but instead of candles you use LEDs.
This website shows the experiment being done on a table in a room. But the room is dark when the experiment is done, so many people can see at the same time. It would be better to set up the experiment inside a cardboard box about 2 feet or 60 cm long and a width and height big enough to accommodate the experiment. Once the box is calibrated, the lid can be closed, and all that’s needed is a hole to see the grease spot.
The grease spot paper is mounted on a block of wood that has two upright posts to hold the paper. The block sits flat on the bottom of the box. The posts can be sticks of wood, twigs, coat hanger wires, anything that will not let the paper move. The grease spot paper is mounted on the posts with adhesive tape. During calibration this assembly will be moved back and forth between the LEDs that will be mounted on the ends of the box.
A hole has to be made in each end of the box so that the LED light hits the grease spot paper with the brightest point of the LED light beam at the grease spot. Both holes should be very close to the same point on each end. The LEDs should be mounted in the holes so that the brightest point in their light output hits the grease spot. Some LEDs have a beam that is not even, so several LEDs may have to be tried. The LEDs should be mounted securely so they do not move. Hot glue should do a good job. The wires to the LEDs should be mounted securely to the box so that there is no tension on the LEDs.
The LEDs are connected in series so that the same current passes through both LEDs. This causes the LEDs to have equal light output.
The grease spot box will need a source of current for the LEDs. A simple source of 5 VDC is a phone charger. With a resistor in series with the 5 volts, and a white LED that has a voltage drop of 3.3 volts, that leaves 1.7 volts across the resistor. The current should be 20 milliamps or .02 amp. 1.7 divided by .02 is 85 ohms. A close common value is 82 ohms. If this is not obtainable, then a 100 ohm resistor will give 17 milliamps. A 560 ohm resistor in parallel with the 100 ohm resistor will give 20 mA.
But first we want to calibrate the grease spot. Both of the LEDs are connected in series and a 100 ohm resistor is connected in series with the LEDs. The remaining two ends of the wire are connected to a 9 volt battery. A power supply can be used too. This is only used during calibration and won’t be needed afterwards.
The light in the room should be low. If not then put a blanket over your head and the box. With the LEDs lit, the grease spot is moved back and forth between the LEDs until the grease spot looks the same as the paper around it – it seems to disappear.
This point should be marked where the wood block is on the box. The block should be about in the middle of the box. The block should be secured with some strong tape. The block should not have to be moved again. From now on, the grease spot can be seen through a hole or slit in the box.
The 9V battery is removed and the LEDs are disconnected. One LED is connected in series with the 82 ohm or 100 ohm resistor. The remaining ends may be connected to the 5VDC charger, and it will put 20 or 17 milliamps through the LED. But we really want to vary the current, from 20 mA down to just 1 or 2 mA. So we add a variable resistor or potentiometer between the 82 ohm resistor and the 5 volts. This pot will have a resistance of 1000 ohms or less, but not lower than 470 ohms. It will allow adjusting the current down to 2 mA for a 1000 ohm pot.
The remaining LED is connected to the Joule Thief. With the JT powering the LED, the other LED’s pot should be adjusted until the grease spot disappears. We know from the calibration that at this point both LEDs were the same brightness, and they had the same current passing through. So we can now measure the voltage across the 82 or 100 ohm resistor and calculate the current by dividing the measured voltage by the resistance. We measure 1.23 volts DC across the 82 ohm resistor. So 1.23 volts divided by 82 ohms gives .015 or 15 milliamps. Thus the JT is putting out the equivalent of 15 milliamps.