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
Cheap LED torch
3 v battery holder (3 cell)
8 pin socket
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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