Auto-Off Flashlight




Introduction: Auto-Off Flashlight

The Auto-Off Flashlight  is a battery powered flashlight that turns on by shaking and stays on at full brightness for seven minutes then automatically turns off.  The light can be reset by shaking the flashlight at any time.  Its based on a low power CMOS 555 timer and a MOSFET switch.  The current draw of the CMOS 555 timer is super low, just 30 uA while in standby mode, so it won't drain your batteries.  This is a totally analog project, no silly microcontrollers or complex programming languages needed.  Your friends will never know that you did not use an Ardunio!

The inspiration of this project came to me when I was repeatedly replacing flashlight batteries in my children’s flashlights.  They like to have a small flashlight on at bedtime.  The problem is they fall asleep with the light on and the batteries are dead by morning.  I  decided to design an auto-off flashlight to increase battery life.  Introducting the  Auto-Off Flashlight.

Step 1: Parts

So lets get started.  First assemble the following parts:

LM7555 CMOS timer
BUZ101A N Channel Mosfet
470uF cap
.01uF cap
1M resistor
Cheap LED torch
3 v battery holder (3 cell)
Tilt switch
Project box
8 pin socket
Prototype board

Step 2: The Design

Start by putting the circuit together on your circuit board.  I'm using a Radio Shack project board and point to point wiring.  I like to use 22 ga. wire from a cut-up CAT-5 network cable.  Do your self a favor and use stranded wire.  Make sure to install a socket for your LM7555 as the CMOS chip is very sensitive to voltage spikes.  The socket also allows you to give a final voltage check without the chip installed. 

The tilt switch is wired in between pin 2 on the 7555 and ground.  I made another flash light in which I replaced it with a normally open button.  The type of tilt switch I used in this project allows the light to be placed in an always-on state.  The switch is a metal ball which will roll onto two leads to complete a circuit.

Step 3: Construction

Attach the 4.5 volt battery pack to your power and ground points.  My LED torch is made up of 9 LEDs in parallel.  Hook the anodes to the 4.5v bus and the cathodes up to the MOSFET's drain pin.   The anodes in my flashlight were connected to the center  point on the underside of the LED torch.  The cathodes were all connected to the edge.

This design places the low power CMOS 7555 timer in monostable configuration.  Monostable is also known as one-shot, as this design has one stable state.  A low pulse on the trigger input will cause the output to go high for 7 minutes.  The 7555’s output pin, 3, is connected to the gate of an N channel MOSFET.  While the MOSFET conducts,  the LEDS connected to the MOSFET’s source are illuminated.  When the voltage on the external capacitor is greater than 2/3 VCC, the supply voltage, the timer’s output goes low and the MOSFET and LEDs are turned off.

The length of time the light is on is determined by the resistor and cap on pin 7.  The equation 1.1*R*C can be used to calculate the time.  I'm using a 1M resistor and a 470uF cap which gives me a on time of  517 seconds.  It is not recommended to use a resistor value greater than 10M and a cap value greater than 1000uF for a 555 timer. 

Step 4: Testing

Give the light a nice shake and set it down.  Wait 7 minutes and the flashlight will turn off!



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    15 Discussions

    Also you may be able to turn on the light without having to shake it with an on/off switch to battery common. Just put an electrolytic capacitor across the tilt sensor switch to common. The cap will pull pin 2 low when the battery common switch is turned on.

    7 replies

    How about keeping the circuit as-is and wiring the switch between pin 2 and ground? The bouncing of the switch might trigger the 555 timer every time.

    That would work too. I would prefer to have the switch shut off everything if I can build the circuit in the existing torch.

    The current draw of the CMOS version of the 555 is really low in the stable state, around 30ua.

    If the 555 is constantly triggered by the tilt switch even if the LED's don't light because of the switch, it will draw more current than when idle. I've played around with them before.

    I'm not liking that change. You don't really know if the 555 timer will trigger when you turn your switch on. If you are worried about the light turning on when you don't want it, just pull the reset line, pin 4, low and the light will never turn on.

    It was just a suggestion if you use an existing torch light. The schematic shows the existing circuit in the torch in white. I plan to try to build your circuit into the top AAA battery holder so it would be nice if pressing the existing switch would turn it on instead of having to shake it. I found a similar torch to try it on. May be a tight fit.

    you might want to look into using a small 8 pin microprocessor such as the attiny85, it only requires power, gnd and a bypass cap. The program would set a pin high for 6 minutes and go to sleep. The micro starts running when power is applied

    Correction: The LED's are connected to the Drain according to your schematic. Shame you could not fit it all into the original flashlight. Cool color.

    4 replies

    Thanks for catching the error. If you want to keep the original flashlight case, just get a 2nd flashlight, empty out the battery holder, and connect it to the first. Those $1 LED torches use some pretty cheap battery sockets though.

    If the original light had 3 batteries, I'm betting it will work with just 2 batteries pretty well. That means that you could put the circuit board where the first battery was and wire it as below. The second switch on the right is the one built into the flashlight that would supply common to the FET Source.



    Yep, the circuit will work at 3 volts. Lots of choices are made with design. My requirement was to make the light bright, use AA cells and be able to shine the light up at the ceiling while it is resting on a table or chair. I found a bright LED torch for $1 but it used AAA cells. I therefore chose to house the AA batteries and circuit in a $10 project case vice sourcing an expensive AA flashight.