How to Switch Off a Joule Thief During Daylight

About: I am a retired analytical chemist living with my wife Cynthia in Cornwall, south west England. I have held the UK radio amateur call sign G3PPT since 1961. I have been interested in computing since the day...

The battery life of a Joule Thief circuit can be drastically increased if the device is switched off during daylight. This Instructable shows an extremely simple way of doing it.

Step 1: The Basic Circuit--1

In the picture we see the usual Joule Thief circuit which will be familiar to many as it has been reproduced so many times. There are a couple of components which are marked with red arrows which are often left out. These help enormously with circuit stability. The components are a ten Ohm resistor in series with the power supply line and a decoupling capacitor from the supply line to ground. The ten Ohm resistor has an addditional use in that by measuring the voltage across it we can measure the current consumption of the circuit using Ohms Law.

Step 2: The Basic Circuit--2

The picture shows the Joule Thief circuit working on a solderless breadboard.

The circuit is being powered by a Ni/MH cell giving 1.3 Volts and the voltage across the ten Ohm resistor reads 72.9 milli Volts and we divide this by ten to calculate the current which is 7.29 milli Amps.

Step 3: The Extra Components

Here we see the three components added to the basic circuit diagram and these that allow us to switch off the Joule Thief in daylight and they are marked with red arrows.

A second 2N3904 transistor has been added between the base of the oscillator transistor and ground. The base of the new transistor is connected to a 2 Volt solar cell via the 22 k resistor. When light falls on the solar cell it switches on the new transistor which in turn lowers the voltage on the oscillator transistor base and stops the oscillation.

Step 4: Run the Circuit in Daylight

Now it's time to make the circuit actually work and the picture shows it running with the extra components on the breadboard in daylight which has turned the circuit off. The 2 Volt solar cell is one salvaged from a garden light. The meter is reading 1.4 mV across the 10 Ohm resistor in the supply line and because the voltage is across 10 Ohms we divide by 10 to convert this to 140 microAmps of current and this is excellent for the circuit in the turned off state.

Step 5: Run the Circuit in "Darkness"

Here we simulate darkness by covering the solar cell.

The circuit has turned fully on, the LED is lit and the meter reads 64.6 mV which translates to 6.46 mA supply current and this is normal for the 1.2 Volt supply from a Ni/MH cell. (The current is slightly lower than that in step 2 as the battery has dropped a little.)
The daylight to darkness drop from 6.46 mA to 140 microAmps is an enormous saving and would result in much longer battery life.

Step 6: Last Thoughts

If the ambient light is insufficient to turn off the circuit such as in a north facing room you may need to put two 2 Volt solar cells in series or perhaps use a 4 Volt unit as shown in the picture.

There is a YouTube video of this contribution which can be found on:

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